Appendix B. Examples
The purpose of this chapter is to provide basic information about microcontrollers that one needs to know in order to be able to use them successfully in practice. This chapter, therefore, does not contain any super interesting program or device schematic with amazing solutions. Instead, the following examples are more proof that program writing is neither a privilege nor a talent issue but the ability of simply putting puzzle pieces together using directives. Rest assured that design and development of devices mainly consists of the following method "test-correct-repeat". Of course, the more you are in it the more complicated it becomes since the puzzle pieces are put together by both children and first-class architects...
BASIC CONNECTING
As seen in the figure below, in order to enable the microcontroller to operate properly it is necessary to provide:
- Power Supply;
- Reset Signal; and
- Clock Signal.
Clearly, it is about simple circuits, but it does not have to always be like that. If the target device is used for controlling expensive machines or maintaining vital functions, everything gets more and more complicated! However, this solution is sufficient for the time being...
POWER SUPPLY
Even though the PIC16F887 can operate at different supply voltages, why to test "Murphy's low"?! A 5DC power supply is shown above. The circuit, uses a cheap integrated three-terminal positive regulator, LM7805, provides high-quality voltage stability and quite enough current to enable microcontroller and peripheral electronics to operate normally (enough in this case means 1Amp).
RESET SIGNAL
In order that the microcontroller can operate properly, a logic one (VCC) must be applied on the reset pin it explains the connection pin-resistor 10K-VCC. The push-button connecting the reset pin MCLR to GND is not necessary. However, it is almost always provided because it enables the microcontroller safe return to normal operating conditions if something goes wrong. By pushing this button, 0V is brought to the pin, the microcontroller is reset and program execution starts from the beginning. The 10K resistor is there to allow 0V to be applied to the MCLR pin, via the push-button, without shorting the 5VDC rail to ground.
CLOCK SIGNAL
Even though the microcontroller has a built in oscillator, it cannot operate without external components which stabilize its operation and determine its frequency (operating speed of the microcontroller). Depending on which elements are in use as well as their frequencies, the oscillator can be run in four different modes:
- LP - Low Power Crystal;
- XT - Crystal / Resonator;
- HS - High speed Crystal / Resonator; and
- RC - Resistor / Capacitor.
Why are these modes so important? Owing to the fact that it is almost impossible to make a stable oscillator which operates over a wide frequency range, the microcontroller must know which crystal is connected in order that it can adjust the operation of its internal electronics to it. This is why all programs used for chip loading contains an option for oscillator mode selection. See above figure.
Quartz resonator
When a quartz crystal is used for frequency stabilization, the built in oscillator operates at a very precise frequency which is isolated from changes in temperature and voltage power supply as well. This frequency is normally labelled on the microcontroller package.
Apart from the crystal, capacitors C1 and C2 must be also connected as per the schematic below. Their capacitance is not of great importance, therefore, the values provided in the table should be considered as a recommendation rather than a strict rule.
Ceramic resonator
Ceramic resonator is cheaper, but very similar to quartz by its function and the way of operating. This is why the schematics illustrating their connection to the microcontroller are identical. However, the capacitor value is a bit different in this case due to different electric features. Refer to the table.
These oscillators are used when it is not necessary to have extremely precise frequency.
RC oscillator
If the operating frequency is not of importance then there is no need for additional expensive components for stabilization. Instead, a simple RC network, as shown in the figure below, will be enough. Since only the input of the local oscillator input is in use here, the clock signal with frequency Fosc/4 will appear on the OSC2 pin. Furthermore, that frequency becomes operating frequency of the microcontroller, i.e. the speed of instruction execution.
External oscillator
If it is required to synchronize the operation of several microcontrollers or if for some reason it is not possible to use any of the previous schematics, a clock signal may be generated by an external oscillator. Refer to the figure below.
ADDITIONAL COMPONENTS
Regardless of the fact that the microcontroller is a product of modern technology, it is of no use without being connected to additional components. Simply, the appearance of voltage on the microcontroller pins mean nothing if not used for performing certain operations (turn something on/off, shift, display etc.).
This section intentionally covers only the most commonly used additional components in practice such as resistors, transistors, LED diodes, LED displays, LCD displays and RS232 communication circuits.
SWITCHES AND PUSH-BUTTONS
There is nothing simpler than switches and push-buttons! This is definitely the simplest way of detecting the appearance of a voltage on the microcontroller input pin and there is no need for additional explanation of how these components operate. Nevertheless, it is not so simple in practice... Then, what is it all about?
It is about contact bounce- a common problem with mechanical switches. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. Generally, it mostly occurs due to vibrations, slight rough spots and dirt between contacts. This effect is usually unnoticeable when using these components in everyday life because the bounce happens too quickly to affect most equipment, but causes problems in some analogue and logic circuits that respond fast enough to misinterpret the on-off pulses as a data stream. Anyway, the whole process does not last long (a few micro- or milliseconds), but long enough to be registered by the microcontroller. When using only a push-button as a pulse counter, errors occurs in almost 100% of cases!
This problem may be easily solved by connecting a simple RC circuit to surpress quick voltage changes. Since the bounce period is not defined, the values of components are not precisely determined. In most cases, it is recommended to use the values as shown in figure below.
If complete stability is needed then radical measures should be taken! The output of the circuit, shown in figure below (RS flip-flop), will change its logic state only after detecting the first pulse triggered by contact bounce. This solution is more expensive (SPDT switch), but the problem is definitely solved!
In addition to these hardware solutions, there is also a simple software solution. When a program tests the state of an input pin and detects a change, the check should be done one more time after a certain delay. If the program confirms the change, it means that a switch/push-button has changed its position. The advantages of such solution are obvious: it is free of charge, effects of noises are eliminated and it can be applied to the poorer quality contacts as well.
RELAY
A relay is an electrical switch that opens and closes under the control of another electrical circuit. It is therefore connected to output pins of the microcontroller and used to turn on/off high-power devices such as motors, transformers, heaters, bulbs, etc. These devices are almost always placed away from the boards sensitive components. There are various types of relays, but all of them operate in the same way. When a current flows through the coil, the relay is operated by an electromagnet to open or close one or many sets of contacts. Similar to optocouplers, there is no galvanic connection (electrical contact) between input and output circuits. Relays usually demand both higher voltage and current to start operation but there are also miniature ones that can be activated by a low current directly obtained from a microcontroller pin.
This figure shows the most commonly used solution.
In order to prevent the appearance of high voltage self-induction caused by a sudden stop of current flow through the coil, an inverted polarized diode is connected in parallel to the coil. The purpose of this diode is to "cut off" the voltage peak.
LED DIODES
You probably know all you need to know about LED diodes, but we should also think of the younger generations...How to destroy a LED?! Well...Very simple.
Quick burning
Like any other diode, LEDs have two ends an anode and a cathode. Connect it properly to a power supply voltage. The diode will happily emit light. Turn it upside down and apply the same power supply voltage (even for a moment). It will not emit light- NEVER AGAIN!
Slow burning
There is a nominal, i.e. maximum current determined for every LED which should not be exceeded. If it happens, the diode will emit more intensive light, but not for a long time!
Something to remember
Similar to the previous example, all you need to do is to discard a current limiting resistor shown below. Depending on power supply voltage, the effect might be spectacular!
LED DISPLAY
Basically, LED display is nothing more than several LEDs moulded in the same plastic case. There are many types of displays composed of several dozens of built in diodes which can display different symbols. The most commonly used is so called 7-segment display. It is composed of 8 LEDs- 7 segments are arranged as a rectangle for symbol displaying and there is an additional segment for decimal point displaying. In order to simplify connection, anodes or cathodes of all diodes are connected to the common pin so that there are common anode displays and common cathode displays, respectively. Segments are marked with the letters from a to g, plus dp, as shown in figure below. On connecting, each diode is treated separately, which means that each must have its own current limiting resistor.
Here are a few important things that one should pay attention to when buying LED displays:
- Depending on whether anodes or cathodes are connected to the common pin, there are common anode displays and common cathode displays. The figure above shows a common anode display. Looking at physical features, there is no difference between these displays at all so it is recommended to check carefully prior installation which of them is in use;
- For each microcontroller pin, there is a maximum current limitation it can receive or give. Because of this, if several displays are connected to the microcontroller it is rec ommended to use so called Low current LEDs using only 2mA for operation; and
- Display segments are usually marked with the letters from a to g, but there is no fast rule indicating to which micro controller pins they should be connected. For this reason it is very important to check connecting prior to commencing program writing or designing a device.
Displays connected to the microcontroller usually occupy a large number of valuable I/O pins, which can be a big problem especially when it is needed to display multi-digital numbers. The problem is more than obvious if, for example, it is needed to display two 6-digit numbers (a simple calculation shows that 96 output pins are needed in this case)! This problem has a solution called MULTIPLEXING.
Here is how an optical illusion based on the same operating principle as a film camera is made. Only one digit at a time is active, but they change their state so quickly that one gets impression that all digits of a number are active simultaneously.
Here is an explanation on the figure above. First a byte representing units is applied on a microcontroller port and a transistor T1 is activated simultaneously. After a while, the transistor T1 is turned off, a byte representing tens is applied on a port and transistor T2 is activated. This process is being cyclically repeated at high speed for all digits and corresponding transistors.
A disappointing fact which indicates that the microcontroller is just a kind of miniature computer designed to understand only the language of zeros and ones is fully expressed when displaying any digit. Namely, the microcontroller does not know what units, tens or hundreds are, nor what ten digits we are used to look like. Therefore, each number to be displayed must go through the following procedure:
First of all, in a particular subroutine a multi-digital number must be split into units, tens etc. Then, these must be stored in special bytes each. Digits get recognizable format by performing "masking". In other words, a binary format of each digit is replaced by a different combination of bits using a simple subroutine. For example, the digit 8 (0000 1000) is replaced by binary number 0111 1111 in order to activate all LEDs displaying digit 8. The only diode remaining inactive in this case is reserved for the decimal point.
If a microcontroller port is connected to the display in a way that bit 0 activates segment "a", bit 1 activates segment "b", bit 2 segment "c" etc., then the table below shows the mask for each digit.
| DIGITS TO DISPLAY |
DISPLAY SEGMENTS |
|
dp |
a |
b |
c |
d |
e |
f |
g |
| 0 |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
0 |
| 1 |
0 |
0 |
1 |
1 |
0 |
0 |
0 |
0 |
| 2 |
0 |
1 |
1 |
0 |
1 |
1 |
0 |
1 |
| 3 |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
1 |
| 4 |
0 |
0 |
1 |
1 |
0 |
0 |
1 |
1 |
| 5 |
0 |
1 |
0 |
1 |
1 |
0 |
1 |
1 |
| 6 |
0 |
1 |
0 |
1 |
1 |
1 |
1 |
1 |
| 7 |
0 |
1 |
1 |
1 |
0 |
0 |
0 |
0 |
| 8 |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
| 9 |
0 |
1 |
1 |
1 |
1 |
0 |
1 |
1 |
In addition to digits from 0 to 9, there are some letters- A, C, E, J, F, U, H, L, b, c, d, o, r, t- that can be also displayed by means of the appropriate masking.
In the event that the common anode displays are used, all ones contained in the previous table should be replaced by zeros and vice versa. Additionally, NPN transistors should be used as drivers as well.
OPTOCOUPLER
An optocoupler is a device commonly used to galvanically separate microcontroller electronics from any potentially dangerous current or voltage in its surroundings. Optocouplers usually have one, two or four light sources (LED diodes) on their input while on their output, opposite to diodes, there is the same number of elements sensitive to light (phototransistors, photo-thyristors or photo-triacs). The point is that an optocoupler uses a short optical transmission path to transfer a signal between elements of circuit, while keeping them electrically isolated. This isolation makes sense only if diodes and photo-sensitive elements are separately powered. In this way, the microcontroller and expensive additional electronics are completely protected from high voltage and noises which are the most common cause of destroying, damaging or unstable operation of electronic devices in practice. The most frequently used optocouplers are those with phototransistors on their outputs. Additionally, optocouplers with internal base-to-pin 6 connection (there are also optocouplers without it), the base may be left unconnected.
The R/C network represented by the broken line in the figure above denotes optional connection which lessens the effects of noises by eliminating very short pulses.
LCD DISPLAY
This component is specifically manufactured to be used with microcontrollers, which means that it cannot be activated by standard IC circuits. It is used for displaying different messages on a miniature liquid crystal display. The model described here is for its low price and great capabilities most frequently used in practice. It is based on the HD44780 microcontroller (
Hitachi) and can display messages in two lines with 16 characters each. It displays all the letters of alphabet, Greek letters, punctuation marks, mathematical symbols etc. In addition, it is possible to display symbols made up by the user. Other useful features include automatic message shift (left and right), cursor appearance, LED backlight etc.
Along one side of a small printed board there are pins used for connecting to the microcontroller. There are in total of 14 pins marked with numbers (16 if the backlight is built in). Their function is described in the table bellow:
| FUNCTION |
PIN NUMBER |
NAME |
LOGIC STATE |
DESCRIPTION |
| Ground |
1 |
Vss |
- |
0V |
| Power supply |
2 |
Vdd |
- |
+5V |
| Contrast |
3 |
Vee |
- |
0 - Vdd |
| Control of operating |
4 |
RS |
0
1 |
D0 – D7 are interpreted as commands
D0 – D7 are interpreted as data |
| 5 |
R/W |
0
1 |
Write data (from controller to LCD)
Read data (from LCD to controller) |
| 6 |
E |
0
1
From 1 to 0 |
Access to LCD disabled
Normal operating
Data/commands are transferred to LCD |
| Data / commands |
7 |
D0 |
0/1 |
Bit 0 LSB |
| 8 |
D1 |
0/1 |
Bit 1 |
| 9 |
D2 |
0/1 |
Bit 2 |
| 10 |
D3 |
0/1 |
Bit 3 |
| 11 |
D4 |
0/1 |
Bit 4 |
| 12 |
D5 |
0/1 |
Bit 5 |
| 13 |
D6 |
0/1 |
Bit 6 |
| 14 |
D7 |
0/1 |
Bit 7 MSB |
LCD screen
The LCD screen consists of two lines with 16 characters each. Every character consists of 5x8 or 5x11 dot matrix. This book covers the 5x8 character display, which is indeed the most commonly used.
Display contrast depends on the power supply voltage and whether messages are displayed in one or two lines. For this reason, varying voltage 0-Vdd is applied on the pin marked as Vee. Trimmer potentiometer is usually used for that purpose. Some LCD displays have built in backlight (blue or green diodes). When used during operation, a current limiting resistor should be serially connected to one of the pins for backlight (similar to LED diodes).
If there are no characters displayed or if all of them are dimmed when the display is switched on, the first thing that should be done is to check the potentiometer for contrast adjustment. Is it properly adjusted? The same applies if the mode of operation has been changed (writing in one or two lines).
LCD Memory
LCD display contains three memory blocks:
- DDRAM - Display Data RAM;
- CGRAM - Character Generator RAM; and
- CGROM - Character Generator ROM.
DDRAM Memory
DDRAM memory is used for storing characters that should be displayed. The size of this memory is sufficient for storing 80 characters. Some memory locations are directly connected to the characters on display.
It works quite simply: it is enough to configure the display to increment addresses automatically (shift right) and set the starting address for the message that should be displayed (for example 00 hex).
After that, all characters sent through lines D0-D7 will be displayed as a message format we are used to- from left to right. In this very case, displaying starts from the first field of the first line because the address is 00 hex. If more than 16 characters are sent then all of them will be memorized, but only the first sixteen characters will be visible. In order to display the rest of them, a shift command should be used. Virtually, everything looks as if the LCD display is a window which shifts left-right over memory locations containing different characters. In reality, this is how the effect of message shifting on the screen has been created.
If the cursor is on, it appears at the location which is currently addressed. In other words, when a character appears at the cursor position, it will automatically move to the next addressed location.
This is a sort of RAM memory so data can be written to and read from it, but its contents is irretrievably lost upon the power goes off.
CGROM Memory
CGROM memory contains the default character map with all characters that can be displayed on the screen. Each character is assigned to one memory location:
The addresses of CGROM memory locations match the characters of ASCII. If the program being currently executed encounters a command "send character P to port" then the binary value 0101 0000 appears on the port. This value is the ASCII equivalent to the character P. It is then written to LCD, which results in displaying the symbol from the 0101 0000 location of CGROM. In other words, the character "P" is displayed. This applies to all letters of the alphabet (capitals and small), but not to the numbers!
As seen on the previous map, addresses of all digits are pushed forward by 48 relative to their values (digit 0 address is 48, digit 1 address is 49, digit 2 address is 50 etc.). Accordingly, in order to display digits correctly it is necessary to add a decimal number 48 to each of them prior to sending them to LCD.
What is ASCII? From their inception till today, computers can recognize only numbers, but not letters. It means that all data a computer swaps with a peripheral device has a binary format even though the same is recognized by the man as letters (The keyboard is an excellent example)! It’s as simple as that- every character matches the unique combination of zeroes and ones. ASCII is character encoding based on the English alphabet. ASCII code specifies a correspondence between standard character symbols and their numerical equivalents.
LCD Basic Commands
All data transferred to LCD through the outputs D0-D7 will be interpreted as a command or a data, which depends on the pin RS logic state:
RS = 1 - Bits D0 - D7 are addresses of the characters to be displayed. LCD processor addresses one character from the character map and displays it. The DDRAM address specifies the location on which the character is to be displayed. This address is defined prior character transfer or the address of the previously transferred character is automatically incremented.
RS = 0 - Bits D0 - D7 are commands which determine display mode.
The commands recognized by the LCD are listed in table below:
| COMMAND |
RS |
RW |
D7 |
D6 |
D5 |
D4 |
D3 |
D2 |
D1 |
D0 |
EXECUTION TIME |
| Clear display |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
1.64mS |
| Cursor home |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
x |
1.64mS |
| Entry mode set |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
I/D |
S |
40uS |
| Display on/off control |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
D |
U |
B |
40uS |
| Cursor/Display Shift |
0 |
0 |
0 |
0 |
0 |
1 |
D/C |
R/L |
x |
x |
40uS |
| Function set |
0 |
0 |
0 |
0 |
1 |
DL |
N |
F |
x |
x |
40uS |
| Set CGRAM address |
0 |
0 |
0 |
1 |
CGRAM address |
40uS |
| Set DDRAM address |
0 |
0 |
1 |
DDRAM address |
40uS |
| Read "BUSY" flag (BF) |
0 |
1 |
BF |
DDRAM address |
- |
| Write to CGRAM or DDRAM |
1 |
0 |
D7 |
D6 |
D5 |
D4 |
D3 |
D2 |
D1 |
D0 |
40uS |
| Read from CGRAM or DDRAM |
1 |
1 |
D7 |
D6 |
D5 |
D4 |
D3 |
D2 |
D1 |
D0 |
40uS |
I/D 1 = Increment (by 1) R/L 1 = Shift right
0 = Decrement (by 1) 0 = Shift left
S 1 = Display shift on DL 1 = 8-bit interface
0 = Display shift off 0 = 4-bit interface
D 1 = Display on N 1 = Display in two lines
0 = Display off 0 = Display in one line
U 1 = Cursor on F 1 = Character format 5x10 dots
0 = Cursor off 0 = Character format 5x7 dots
B 1 = Cursor blink on D/C 1 = Display shift
0 = Cursor blink off 0 = Cursor shift
What is Busy flag ?
Compared to the microcontroller, the LCD is an extremely slow component. Because of this, it was necessary to provide a signal which would, upon command execution, indicate that the display is ready for the next piece of data. That signal, called the
busy flag, can be read from the line D7. When the voltage on this line is 0V (BF=0), the display is ready to receive new data.
LCD Connecting
Depending on how many lines are used for connecting the LCD to the microcontroller, there are 8-bit and 4-bit LCD modes. The appropriate mode is selected at the beginning of the operation in this process called "initialization". 8-bit LCD mode uses outputs D0-D7 to transfer data as explained on the previous page.
The main purpose of 4-bit LED mode is to save valuable I/O pins of the microcontroller. Only 4 higher bits (D4-D7) are used for communication, while others may be unconnected. Each piece of data is sent to the LCD in two steps- four higher bits are sent first (normally through the lines D4-D7) and four lower bits are sent afterwards. Initialization enables the LCD to link and interpret received bits correctly.
Data is rarely read from the LCD (it is mainly transferred from the microcontroller to LCD) so it is often possible to save an extra I/O pin by simple connecting R/W pin to the Ground. Such saving has its price. Messages will be normally displayed, but it will not be possible to read the busy flag since it is not possible to read the display as well. Fortunately, there is a simple solution. After sending a character or a command it is important to give the LCD enough time to do its job. Owing to the fact that the execution of the slowest command lasts for approximately 1.64mS, it will be sufficient to wait approximately 2mS for LCD.
LCD Initialization
The LCD is automatically cleared when powered up. It lasts for approximately 15mS. After this, display is ready for operation. The mode of operation is set by default. It means that:
- Display is cleared.
- Mode
DL = 1 Communication through 8-bit interface
N = 0 Messages are displayed in one line
F = 0 Character font 5 x 8 dots
- Display/Cursor on/off
D = 0 Display off
U = 0 Cursor off
B = 0 Cursor blink off
- Character entry
ID = 1 Displayed addresses are automatically incremented by 1
S = 0 Display shift off
Automatic reset is mostly done without any problems. Mostly, but not always! If for any reason power supply voltage does not reach full value within 10mS, display will start performing completely unpredictably. If voltage supply unit is not able to meet that condition or if it is needed to provide completely safe operation, the process of initialization is applied. Initialization, among other things, causes a new reset enabling display to operate normally.
Refer to the figure below for the procedure on 8-bit initialization:
It is not a mistake! In this algorithm, the same value is transferred three times in a row.
In case of 4-bit initialization, the procedure is as follows:
EXAMPLE 1
Writing header and configuring I/O pins
The only purpose of this program is to turn on a few LED diodes on port B. It is nothing special. Anyway, use this example to study what a real program looks like. The figure below shows a connection scheme, while the program is on the next page.
When switching on, every other LED diode on the port B emits light. That is enough to indicate that the microcontroller is properly connected and operates normally.
This example gives the description of a correctly written header and a few initial directives. They represent a part of the program used in all programs described in this book. To skip repetitiveness, it will not be written in the following examples, but is considered to be at the beginning of every program (marked as a "Header").
The purpose of the header and initial directives is briefly described below.
Header:
The header is placed at the beginning of the program and gives basic information in the form of comments (name of the program, release date etc.). Don't be deluded into thinking that after a few months you will know what that program is about and why it is saved in your computer.
Initial directives:
list p=16f887
This directive defines processor to execute a program.
#include <p16f887.inc>
It enables the compiler to access the document p16f887.inc (If you have MPLAB installed, it is placed by default on C:\Program files\Microchip\MPASM Suite). Every SFR register contained in this document, as well as every bit, has its own name and address. If the program reads for example:
bsf INTCON, GIE
It means that the GIE bit of the INTCON register should be set. Instruction, as such, makes no sense to the compiler. It has to access the ".inc" document in order to know that the seventh bit of the SFR at the address 000B hex should be set.
errorlevel -302
This is a "cosmetic" directive which disables the irritating message "Register in operand not in ..." to appear at the end of every compiling process. It is not necessary, but useful.
__config
This directive is used to include config word in the program upon compiling. It is not necessary because the same operation is performed by software for loading program into chip. However, do you have any idea which software will be used by the end user? What options will be set by default? You are the end user?! Do you know which program you will be using for MCU programming next year? Make life easier for yourself, take this directive as a necessary one and include it in your program.
EXAMPLE 2
Using program loop and internal oscillator LFINTOSC
This is a continuation of the previous example, but deals with a bit more complicated problem... The idea is to make the LED diodes on the port B blink. A simple thing at first glance! It is enough to periodically change logic state on the port B. In this case, numbers 01010101 and 10101010 are selected to change in the following way:
- Set binary combination 01010101 on port B;
- Remain in loop1;
- Replace existing bits combination on port B with 10101010;
- Remain in loop2; and
- Return to the step 1 and repeat the whole procedure.
Do you know how fast this should be done? It would be possible to observe changes on port B only if, apart from the delays provided in loop1 and loop2, the whole process is slowed down approximately 250 times more. Because of this, the microcontroller uses internal oscillator LFINTOSC with the frequency of 31kHz instead of the external oscillator with quartz crystal (8MHz).
You have noticed that the clock signal source has changed "on the fly". If you want to make sure of it, remove quartz crystal prior to switching the microcontroller on. What will happen? The microcontroller will not start operating because the config word loaded with the program requires the use of the crystal on switching on. If you remove the crystal later during the operation, it will not affect the microcontroller at all!
Sorce Code
;************************************************************************
; Header
;************************************************************************
;************* DEFINING VARIABLES ***************************************
cblock 0x20 ; Block of variables starts at address 20h
counter1 ; Variable "counter1" at address 20h
endc
;************************************************************************
org 0x0000 ; Address of the first program instruction
banksel OSCCON ; Selects memory bank containing
; register OSCCON
bcf OSCCON,6 ; Selects internal oscillator LFINTOSC with
bcf OSCCON,5 ; the frequency of 31KHz
bcf OSCCON,4
bsf OSCCON,0 ; Microcontroller uses internal oscillator
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
banksel PORTB ; Selects bank containing register PORTB
loop
movlw B'01010101' ; Binary number 01010101 is written to W
movwf PORTB ; Number is moved to PORTB
movlw h'FF' ; Number hFF is moved to W
movwf counter1 ; Number is moved to variable "counter1"
loop1
decfsz counter1 ; Variable "counter1" is decremented by 1
goto loop1 ; If result is 0, continue. If not,
; remain in loop1
movlw B'10101010' ; Binary number 10101010 is moved to W
movwf PORTB ; Number is moved to PORTB
movlw h'FF' ; Number hFF is moved to W
movwf counter1 ; Number is moved to variable "counter1"
loop2
decfsz counter1 ; Variable "counter1" is decremented by 1
goto loop2 ; If result is 0, continue. If not,
; remain in loop2
goto loop ; Go to label loop
end ; End of program
EXAMPLE 3
Using nested loop
The connection scheme is again the same. To make this a bit more interesting, a different combination of port B bits change each other. And, that’s not all of course. As seen from the previous two examples, the microcontroller is very fast and often, it needs to be slowed down. The use of the built-in oscillator LF, as in example 2, is the last measure that should be applied. The problem is more often solved by using nested loops in a program. In this example, the variable "counter1" is decremented 255 times by 1 in the shorter loop1. Prior to leaving this loop, the program will countdown 255 times from 255 to 0. It means that between only two LED diode’s blink on the port, there are 255x255 pulses coming from the quartz oscillator. Precisely speaking, the number of pulses amounts to approximately 196 000 since it also takes some time to execute jump instructions and decrement instructions. Yes, it’s true, the microcontroller mostly waits and does nothing...
Source Code
;******************* Header ***********************************************
;************* DEFINING VARIABLES *****************************************
cblock 0x20 ; Block of variables starts at address 20h
counter1 ; Variable "counter1" at address 20h
counter2 ; Variable "counter2" at address 21h
endc
;**************************************************************************
org 0x0000 ; Address of the first program instruction
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; Clears TRISB
banksel PORTB ; Selects bank containing register PORTB
loop
movlw B'11110000' ; Binary number 11110000 is moved to W
movwf PORTB ; Number is moved to PORTB
movlw h'FF' ; Number hFF is moved to W
movwf counter2 ; Number is moved to variable "counter2"
loop2
movlw h'FF' ; Number hFF is moved to W
movwf counter1 ; Number is moved to "counter1"
loop1
decfsz counter1 ; Decrements "counter1" by 1. If result is 0
goto loop1 ; skip next instruction
decfsz counter2 ; Decrements "counter2" by 1. If result is 0
goto loop2 ; skip next instruction
movlw B'00001111' ; Binary number 00001111 is moved to W
movwf PORTB ; Number is moved to PORTB
movlw h'FF' ; Number hFF is moved to W
movwf counter2 ; Number is moved to variable "counter2"
loop4
movlw h'FF' ; Number hFF is moved to W
movwf counter1 ; Number is moved to variable "counter1"
loop3
decfsz counter1 ; Decrements "counter1" by 1. If result is 0
; skip next instruction
goto loop3
decfsz counter2 ; Decrements "counter2" by 1. If result is 0
goto loop4 ; skip next instruction
goto loop ; Jump to label loop
end ; End of program
Example 4
Using timer TMR0 and Interrupts
If you have read the previous example, you would have noticed a disadvantage of providing delays using loops. In all these cases, the microcontroller is "captive" and does nothing. It simply waits for some time to pass. Such wasting of time is an unacceptable luxury and some other method should be applied.
Do you remember the story about the timers? About interrupts? This example makes links between them in a practical way. The schematic is still the same as well as the challenge. It is necessary to provide delay long enough to notice changes on a port. This time, the timer TMR0 with the assigned prescaler is used for that purpose. Interrupt occurs on every timer register overflow and interrupt routine increments the number in port B by 1. The whole procedure is performed "behind the scenes" of the whole process, which enables the microcontroller to do other things.
Pay attention to a few details:
- Even though it is unnecessary in this case, the contents of the most important registers (W, STATUS and PCLATH) must be saved at the beginning of the interrupt routine;
- Interrupt causes the appropriate flag bit to be automatically set and the GIE bit to be automatically cleared. At the end of the interrupt routine, do not forget to return these bits to the state they had prior to the interrupt occurring; and
- At the end of the interrupt rutine, important registers should be given the original content.
Source Code
;********************** Header **********************************************
;**************** DEFINING VARIABLES ****************************************
cblock 0x20 ; Block of variables starts at address 20h
w_temp ; Variable at address 20h
pclath_temp ; Variable at address 21h
status_temp ; Variable at address 22h
endc
;************************ START OF PROGRAM **********************************
org 0x0000 ; Address of the first program instruction
goto main ; Go to label "main"
;************************ INTERRUPT ROUTINE *********************************
org 0x0004 ; Interrupt vector
movwf w_temp ; Saves value in register W
movf STATUS ; Saves value in register STATUS
movwf status_temp
movf PCLATH ; Saves value in register PCLATH
movwf pclath_temp
banksel PORTB ; Selects bank containing PORTB
incf PORTB ; Increments register PORTB by 1
banksel INTCON ; Selects bank containing INTCON
bcf INTCON,TMR0IF ; Clears interrupt flag TMR0IF
movf pclath_temp,w ; PCLATH is given its original content
movwf PCLATH
movf status_temp,w ; STATUS is given its original content
movwf STATUS
swapf w_temp,f ; W is given its original content
swapf w_temp,w
bsf INTCON,GIE ; Global interrupt enabled
retfie ; Return from interrupt routine
;************************ MAIN PROGRAM **************************************
main ; Start of the main program
banksel ANSEL ; Bank containing register ANSEL
clrf ANSEL ; Clears registers ANSEL and ANSELH
clrf ANSELH ; All pins are digital
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
banksel OPTION_REG ; Bank containing register OPTION_REG
bcf OPTION_REG,T0CS ; TMR0 counts pulses from oscillator
bcf OPTION_REG,PSA ; Prescaler is assign to timer TMR0
bsf OPTION_REG,PS0 ; Prescaler rate is 1:256
bsf OPTION_REG,PS1
bsf OPTION_REG,PS2
banksel INTCON ; Bank containing register INTCON
bsf INTCON,TMR0IE ; TMR0 interrupt overflow enabled
bsf INTCON,GIE ; Global interrupt enabled
banksel PORTB ; Bank containing register PORTB
clrf PORTB ; Clears port B
loop
goto loop ; Remain here
end ; End of program
Example 5
Using subroutine, using push-buttons
In the previous examples the microcontroller executes the program without being influenced in any way its surrounding. In practice, devices operating in this way are very rare (for example, simple neon signs). You guess, among other components, input pins will also be used in this example. There is a schematic in the figure below, while the program is on the next page. Everything is still very simple.
At the beginning of the program, immediately upon defining variables, the microcontroller pins* are configured by using registers TRISA and TRISB.
In the main program, one bit on port B is set first. Then the contents of this register is constantly moved by one place to the left (instruction rlf PORTB). It gives us the impression that the lit LED diodes is moving. To make it visible, the whole process must be slow enough. Press on the push-button "STOP" stops the movement and the program remains in loop3. Delay is provided by means of a nested loop. This time, it is placed in a short subroutine "DELAY".
* It is not necessary for PORTA pins since they are automatically configured as inputs after every reset.
Source Code
;****************************************************************************
; Header
;****************************************************************************
;************* DEFINING VARIABLES *******************************************
cblock 0x20 ; Block of variables starts at address 20h
counter1 ; Variable "counter1" at address 20h
counter2 ; Variable "counter2" at address 21h
endc ; Block of variables ends
;************************ MAIN PROGRAM **************************************
org 0x0000 ; Address of the first program instruction
banksel ANSEL ; Selects bank containing register ANSEL
clrf ANSEL ; Clears registers ANSEL and ANSELH to
clrf ANSELH ; configure all inputs as digital
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
movlw B'00000010'
movwf TRISA ; Pin RA1 is input
banksel PORTB ; Selects bank containing register TRISB
movlw B'00000001' ; Writes 1 to register W
movwf PORTB ; Number is moved to PORTB
loop
rlf PORTB ; Port B bits rotates by one place left
call DELAY ; Calls subroutine "DELAY"
loop3
btfss PORTA,1 ; Tests the firs port A bit
goto loop3 ; "0" is applied to pin.Go to label "loop3"
goto loop ; "1" is applied to pin.Go to label "loop"
;************************ SUBROUTINES ***************************************
DELAY
clrf counter2 ; Clears variable "counter2"
loop1
clrf counter1 ; Clears variable "counter1"
loop2
decfsz counter1 ; Decrements variable "counter1" by 1
goto loop2 ; Result is not 0. Go to label loop2
decfsz counter2 ; Decrements variable "counter2" by 1
goto loop1 ; Result is not 0. Go to lab loop1
return ; Return from subroutine "DELAY"
end ; End of program
EXAMPLE 6
TMR0 as a counter, defining new variables, using relay
This time, TMR0 is used as a counter. The idea is to connect the counter input to one pushbutton so that it counts one pulse at a time upon every button press. When the number of counted pulses becomes equal to the number in register TEST, logic one voltage (5V) will be applied to the PORTD, 3 pin. Since this voltage activates an electro-mechanical relay, this bit is called the same- "Relay".
In this example, the TEST register contains number 5. Naturally, it could be any number and could be calculated or entered via the keyboard. Instead of a relay, the microcontroller can activate some other device and instead of push-buttons it can use sensors. This example illustrates one of the most common uses of the microcontroller in industry. When something is done as many times as needed, then something else should be switched on or off...
Source Code
;****************************************************************************
; Header
;****************************************************************************
;************* DEFINING VARIABLES *******************************************
TEST equ B'00000101' ; Binary number 00000101 = TEST
#define RELAY PORTD,3 ; Pin PORTD,3 = RELAY
;************************ MAIN PROGRAM **************************************
org 0x0000 ; Address of the first program instruction
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
clrf TRISD ; All port D pins are configured as outputs
movlw B'00010000' ; This number is written to W register
movwf TRISA ; Only the forth pin of port A is input
banksel OPTION_REG ; Bank containing OPTION_REG register
bsf OPTION_REG,T0CS ; Pin RA4 is supplied with pulses
bsf OPTION_REG,PSA ; Prescaler rate is 1:1
banksel PORTB ; Selects bank containing PORTB register
clrf TMR0 ; Clears timer register
bcf PORTD,3 ; Pin PORTD,3 = 0
loop
movfw TMR0 ; Timer register is moved to W register
movwf PORTB ; W register is moved to PORTB
xorlw TEST ; Operation exclusive OR between
; W register and number TEST (00000101)
btfsc STATUS,Z ; If numbers are equal, result is 0 and
bsf PORTD,3 ; bit STATUS,Z = 1. Bit PORTD,3 is set
goto loop ; and jump to label loop is executed
end ; End of program
EXAMPLE 7
Using macros in the program, using debounce routine
You have probably noticed in the previous example that the microcontroller does not always operate as expected. Namely, by pressing the push-button, the number on port B is not always incremented by 1. Mechanical push-buttons make several short successive contacts when they have been activated. You guess, the microcontroller registers and counts all that...
There are several ways to solve this problem. This program uses program delay known as
debounce. Basically, it is a simple procedure. Upon input change detection (button press), a short program delay is provided and the program waits for another change (button release). Only after this, the program comes to a conclusion that the button is activated.
In this very case, the push-button is tested by means of macro called
button. Besides, this macro contains a program delay which is provided by means of another macro
pausems.
The main program is relatively simple and enables the variable "cnt" to be incremented and decremented by using two push-buttons. This variable is thereafter copied to port B and affects the LED (logic one (1) turns LED diode on, while logic zero (0) turns LED diode off).
Source Code
;********************** Header **********************
;************* DEFINING VARIABLES *************************
cblock 0x20 ; Block of variables starts at address 20hex
HIcnt
LOcnt
LOOPcnt
cnt
endc ; End of block of variables
;**********************************************************************
ORG 0x000 ; Reset vector
nop
goto main ; Go to program start (label "main")
;**********************************************************************
include "pause.inc"
include "button.inc"
;**********************************************************************
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL ; All pins are digital
clrf ANSELH
banksel TRISB
bsf TRISA, 0
bsf TRISA, 1
clrf TRISB
banksel PORTB
clrf cnt
Loop
button PORT,0,0,Increment
button PORT,1,0,Decrement
goto Loop
Increment
incf cnt, f
movf cnt, w
movwf PORTB
goto Loop
Decrement
decf cnt, f
movf cnt, w
movwf PORTB
goto Loop
end ; End of program
Macro "pausems"
;**********************************************************************
pausems MACRO arg1
local Loop1
local dechi
local Delay1ms
local Loop2
local End
movlw High(arg1) ; Higher byte of argument is moved
; to HIcnt
movwf HIcnt
movlw Low(arg1) ; Lower byte of argument is moved
; to LOcnt
movwf LOcnt
Loop1
movf LOcnt, f ; Decrements HIcnt and LOcnt while
btfsc STATUS, Z ; needed and calls subroutine Delay1ms
goto dechi
call Delay1ms
decf LOcnt, f
goto Loop1
dechi
movf HIcnt, f
btfsc STATUS, Z
goto End
call Delay1ms
decf HIcnt, f
decf LOcnt, f
goto Loop1
Delay1ms: ; Delay1ms provides delay of
movlw .100 ; 100*10us=1ms
movwf LOOPcnt ; LOOPcnt<-100
Loop2:
nop
nop
nop
nop
nop
nop
nop
decfsz LOOPcnt, f
goto Loop2 ; Execution time of Loop2
return ; is 10 us
End
ENDM
;**********************************************************************
Macro "button"
;****************************************************
button MACRO port,pin,hilo,label
local Pressed1 ; All labels are local
local Pressed2
local Exit1
local Exit2
IFNDEF debouncedelay ; Enables debounce time to be defined
; in main program
#define debouncedelay .10
ENDIF
IF (hilo == 0) ; If pull-up used
btfsc port, pin ; If "1", push-button is pressed
goto Exit1
pausems debouncedelay ; Wait for 10ms debounce
Pressed1
btfss port, pin
goto Pressed1
pausems debouncedelay ; Wait until released and
goto label ; jump to specified address
Exit1
ELSE ; If pull-down used
btfss port, pin
goto Exit2 ; If "0", push-button is released
pausems debouncedelay ; Wait for 10ms debounce
Pressed2
btfsc port, pin
goto Pressed2
pausems debouncedelay ; Wait until released and
goto label ; jump to specified address
Exit2
ENDIF
ENDM
;**********************************************************************
EXAMPLE 8
Using timer TMR1 and using interrupt
16-bit timer TMR1 is used in this example. By occupying its registers TMR1L and TMR1H, an interrupt occurs and the number on port B is incremented. This has already been seen in the previous examples. The difference is in the program delay which is a bit longer this time because the prescaler rate is 1:8.
Source Code
;*************************** Header *****************************************
;************* DEFINING VARIABLES *******************************************
cblock 0x20 ; Block of variables starts at address 20h
w_temp ; Variable at address 20h
pclath_temp ; Variable at address 21h
status_temp ; Variable at address 22h
endc
;************************ PROGRAM START ************************
org 0x0000 ; Address of the first program instruction
goto main ; Jump to label "main"
;************************ INTERRUPT ROUTINE **********************************
org 0x0004 ; Interrupt vector
movwf w_temp ; Save register W
movf STATUS ; Save register STATUS
movwf status_temp
movf PCLATH ; Save register PCLATH
movwf pclath_temp
banksel PORTB ; Selects bank containing PORTB
incf PORTB ; Register PORTB is incremented by 1
movf pclath_temp,w ; PCLATH is given its original content
movwf PCLATH
movf status_temp,w ; STATUS is given its original content
movwf STATUS
swapf w_temp,f ; W is given its original content
swapf w_temp,w
banksel PIR1 ; Selects bank containing PIR1
bcf PIR1,TMR1IF ; Clears interrupt flag TMR1IF
bsf INTCON,GIE ; Global interrupt enabled
retfie ; Return from interrupt routine
;************************ MAIN PROGRAM ***************************************
main ; Start of main program
banksel ANSEL ; Selects bank containing register ANSEL
clrf ANSEL ; Clears registers ANSEL and ANSELH
clrf ANSELH ; All pins are digital
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
banksel T1CON ; Selects bank containing register T1CON
bcf T1CON,TMR1CS ; TMR1 counts pulses generated by oscillator
bsf T1CON,T1CKPS0 ; Prescaler rate is 1:8
bsf T1CON,T1CKPS1
bsf T1CON,TMR1ON ; Turns on timer TMR1
banksel PIE1 ; Selects bank containing register PIE1
bsf PIE1,TMR1IE ; TMR1 interrupt overflow enabled
bsf INTCON,PEIE ; Peripheral modules interrupt enabled
; Timer TMR1 belongs to peripheral modules
bsf INTCON,GIE ; Global interrupt enabled
banksel PORTB ; Selects bank containing register PORTB
clrf PORTB ; Clears port B
loop
goto loop ; Remain here
end ; End of program
EXAMPLE 9
Using timer TMR2, configuring quartz oscillator
This example illustrates the use of timer TMR2. The microcontroller uses internal oscillator HFINTOSC with the frequency of 500 kHz. The whole program works as follows: After the period of time defined by register PR, prescaler and postscaler has expired, an interrupt occurs. Interrupt routine decrements the content of the PR register and simultaneously increments the content of port B. Since the number in register PR, which determines when interrupt is to occur is constantly decremented, interrupt will occur for shorter and shorter periods of time. In other words, counting will be carried out faster. A new cycle of accelerated counting starts after every register PR overflow.
Source Code
;********************** Header ***************************************************
;************* DEFINING VARIABLES ************************************************
cblock 0x20 ; Block of variables starts at address 20h
w_temp ; Variable at address 20h
pclath_temp ; Variable at address 21h
status_temp ; Variable at address 22h
endc
;************************ PROGRAM START ******************************************
org 0x0000 ; Address of the first program instruction
goto main ; Jump to label "main"
;************************ INTERRUPT ROUTINE **************************************
org 0x0004 ; Interrupt vector
movwf w_temp ; Save register W
movf STATUS ; Save register STATUS
movwf status_temp
movf PCLATH ; Save register PCLATH
movwf pclath_temp
banksel PORTB ; Selects bank containing PORTB
incf PORTB ; Increments PORTB register by 1
banksel PR2 ; Selects bank containing PR2
decf PR2 ; PR2 is decremented by 1
movf pclath_temp,w ; PCLATH is given its original state
movwf PCLATH
movf status_temp,w ; STATUS is given its original state
movwf STATUS
swapf w_temp,f ; W is given its original state
swapf w_temp,w
banksel PIR1 ; Selects bank containing PIR1
bcf PIR1,TMR2IF ; Clears interrupt flag TMR2IF
bsf INTCON,GIE ; Global interrupt enabled
retfie ; Return from interrupt routine
;************************ MAIN PROGRAM *******************************************
main ; Start of the main program
banksel OSCCON ; Selects bank containing register OSCCON
bcf OSCCON,6 ; Selects internal oscillator HFINTOSC with
bsf OSCCON,5 ; frequency of 500KHz
bsf OSCCON,4
bsf OSCCON,0 ; Microcontroller uses internal oscillator
banksel ANSEL ; Selects bank containing register ANSEL
clrf ANSEL ; Clears registers ANSEL and ANSELH
clrf ANSELH ; All pins are digital
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
clrf PR2
banksel T2CON ; Selects bank containing register T2CON
movlw H'FF' ; Sets all control register bits
movwf T2CON ; prescaler=1:16, postscaler=1:16 TMR2=ON
clrf PORTB
banksel PIE1 ; Selects bank containing register PIE1
bsf PIE1,TMR2IE ; TMR2 interrupt enabled
bsf NTCON,PEIE ; Peripheral modules interrupt enabled
; Timer TMR2 belongs to peripheral modules
bsf INTCON,GIE ; Global interrupt enabled
loop
goto loop ; Remain here
end ; End of program
EXAMPLE 10
Module CCP1 as PWM signal generator
Since the CCP modules have a wide range of possibilities they are commonly used in practice. This example illustrates the use of CCP1 module in PWM mode. Bits of the CCP1CON register determine that the module operates as a single-output PWM. The same bits determine the PWM frequency to be 4.88 kHz. To make things more interesting, the duration of the output P1A (PORTC,2) pulses may be changed by means of push-buttons symbolically called "DARK" and "BRIGHT". Push-buttons are tested in interrupt routine initiated by the timer TMR1. Any change affects the LED diode so that it changes light intensity. Note that port B does not use external resistors because internal pull-up resistors are enabled. The whole process of generating PWM signal is performed "behind the scenes", which enables the microcontroller to do other things.
Source Code
;********************** Header **********************************************
;************* DEFINING VARIABLES *******************************************
cblock 0x20 ; Block of variables starts at address 20h
w_temp ; Variable at address 20h
pclath_temp ; Variable at address 21h
status_temp ; Variable at address 22h
endc
#define DARK PORTB,0 ; Push-button "DARK" is connected
; to PORTB,0 pin
#define BRIGHT PORTB,1 ; Push-button "BRIGHT" is connected
; to PORTB,1 pin
;************************ PROGRAM START *************************************
org 0x0000 ; First program instruction address
goto main ; Jump to label "main"
;************************ INTERRUPT ROUTINE *********************************
org 0x0004 ; Interrupt vector
movwf w_temp ; Save register W
movf STATUS ; Save register STATUS
movwf status_temp
movf PCLATH ; Save register PCLATH
movwf pclath_temp
banksel CCPR1L
btfss DARK ; Tests push-button "DARK"
decf CCPR1L ; Push-button is pressed - decrement CCP1L by 1
btfss BRIGHT ; Testing push-button "BRIGHT"
incf CCPR1L ; Push-button is pressed - increment CCP1L by 1
movf pclath_temp,w ; PCLATH is given its original content
movwf PCLATH
movf status_temp,w ; STATUS is given its original content
movwf STATUS
swapf w_temp,f ; W is given its original content
swapf w_temp,w
banksel PIR1 ; Selects bank containing PIR1
bcf PIR1,TMR1IF ; Clears interrupt flag TMR1IF
bsf TMR1H,7 ; Accelerates timer TMR0 counting
bsf TMR1H,6 ;
bsf INTCON,GIE ; Global interrupt enabled
retfie ; Return from interrupt routine
;************************ MAIN PROGRAM **************************************
main ; Start of the main program
banksel ANSEL ; Selects bank containing register ANSEL
clrf ANSEL ; Clears registers ANSEL and ANSELH
clrf ANSELH ; All pins are digital
banksel OPTION_REG ; Selects bank containing register ANSEL
bcf OPTION_REG,7 ; Pull-up resistors enabled
bsf WPUB,0 ; Pull-up resistors enabled
bsf WPUB,1 ; on port B pins 0 and 1
banksel TRISC ; Selects bank containing register TRISC
clrf TRISC ; All port C pins are configured as outputs
banksel T1CON ; Selects bank containing register T1CON
bcf T1CON,TMR1CS ; TMR1 operates as a timer
bcf T1CON,T1CKPS0 ; Prescaler rate is 1:8
bcf T1CON,T1CKPS1
bsf T1CON,TMR1ON ; Activates timer TMR1
banksel PIE1 ; Selects bank containing register PIE1
bsf PIE1,TMR1IE ; Interrupt TMR1 is enabled
bsf INTCON,PEIE ; Peripheral modules interrupts are
; enabled
bsf INTCON,GIE ; Global interrupt enabled
movlw B'11111101' ; Prescaler TMR2 = 1:4
banksel T2CON
movwf T2CON
movlw B'11111111' ; Number in register PR2
banksel PR2
movwf PR2
banksel CCP1CON
movlw B'00001100' ; Bits to configure CCP1 module
movwf CCP1CON
loop
goto loop ; Remain here
end ; End of program
EXAMPLE 11
Using A/D converter
PIC16F887 A/D converter is used in this example. Everything is quite simple. A variable analog signal is applied on the AN2 pin while the result of conversion is shown on port B as a binary number. In order to simplify the program as much as possible, only 8 lower bits of the result of conversion are shown. GND is used as a negative voltage reference Vref-, while positive voltage reference is applied on the AN3 pin. It enables voltage measurement scale to "stretch and shrink".
To make this clear, the A/D converter always generates a 10-bit binary result, which means that it detects a total of 1024 voltage levels (210=1024). The difference between two voltage levels is not always the same. The less the difference between Vref+ and Vref, the less the difference will be between two of 1024 levels. Accordingly, the A/D converter is able to detect slight changes in voltage.
Source Code
;*********************** Header *********************************************
;************************ PROGRAM START *************************************
org 0x0000 ; Address of the first program instruction
banksel TRISB ; Selects bank containing register TRISB
clrf TRISB ; All port B pins are configured as outputs
movlw B'00001100'
movwf TRISA ; Pins RA2 and RA3 are configured as inputs
banksel ANSEL ; Selects bank containing register ANSEL
movlw B'00001100' ; Inputs AN2 and AN3 are analog while
movwf ANSEL ; all other pins are digital
clrf ANSELH
banksel ADCON1 ; Selects bank including register ADCON1
bsf ADCON1,ADFM ; Right justification of result
bcf ADCON1,VCFG1 ; Voltage Vss is used as Vref
bsf ADCON1,VCFG0 ; RA3 pin voltage is used as Vref+
banksel ADCON0 ; Selects bank containing register ADCON0
movlw B'00001001' ; AD converter uses clock Fosc/2, AD channel
movwf ADCON0 ; on RA2 pin is used for conversion and
; AD converter is enabled
loop
banksel ADCON0
btfsc ADCON0,1 ; Tests bit GO/DONE
goto loop ; Conversion in progress, remain in
; loop
banksel ADRESL
movf ADRESL,w ; Lower byte of conversion result is
; copied to W
banksel PORTB
movwf PORTB ; Byte is copied to PORTB
bsf ADCON0,1 ; Starts new conversion
goto loop ; Jump to label "loop"
end ; End of program
EXAMPLE 12
Using EEPROM memory
This example demonstrates write to and read from built-in EEPROM memory. The program works as follows. The main loop constantly reads EEPROM memory location at address 5 (decimal). This number is displayed on port D. The same loop tests the state of three push-buttons connected to port A. The push-buttons "INCREMENT" and "DECREMENT" have the same purpose like in example 7 - increment and decrement the variable "cnt" which is thereafter displayed on port B. The push-button "MEMO" enables that variable to be written to EEPROM memory. In order to check it, it is enough to press this push-button and switch off the device. On the next switch on, the program displays the value of the variable on port D (at the moment of writing, this value was displayed on port B).
Source Code
;********************** Header **********************************************
;*********** Defining variables in program **********************************
cblock 0x20 ; Block of variables starts at address 20h
HIcnt
LOcnt
LOOPcnt
cnt
endc ; End of block
;****************************************************************************
ORG 0x000 ; Reset vector
nop
goto main ; Go to start of the program (label "main")
;****************************************************************************
include "pause.inc"
include "button.inc"
;****************************************************************************
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL
clrf ANSELH ; All pins are digital
banksel TRISB
bsf TRISA, 0 ; Input pin
bsf TRISA, 1 ; Input pin
bsf TRISA, 2 ; Input pin
clrf TRISB ; All port B pins are outputs
clrf TRISD ; All port D pins are outputs
banksel PORTB
clrf PORTB ; PORTB=0
clrf PORTD ; PORTD=0
clrf cnt ; cnt=0
Loop
banksel PORTA
button PORTA,0,0,Increment
button PORTA,1,0,Decrement
button PORTA,2,0,Save
banksel EEADR
movlw .5 ; Reads EEPROM memory location
movwf EEADR ; at address 5
banksel EECON1
bcf EECON1,EEPGD
bsf EECON1,RD ; Reads data from EEPROM memory
banksel EEDATA
movfw EEDATA ; Moves data to W
banksel PORTD
movwf PORTD ; Data is moved from W to PORTD
goto Loop
Increment ; Increments number on port B
incf cnt, f
movf cnt, w
movwf PORTB
goto Loop
Decrement ; Decrements number on port B
decf cnt, f
movf cnt, w
movwf PORTB
goto Loop
Save ; Copies data from port B to EEPROM
banksel EEADR ; memory location at address 5
movlw .5
movwf EEADR ; Writes address
banksel PORTB
movfw PORTB ; Copies port B to register W
banksel EEDAT
movwf EEDAT ; Writes data to temporary register
banksel EECON1
bcf EECON1,EEPGD
bsf EECON1,WREN ; Write enabled
bcf INTCON,GIE ; All interrupts disabled
btfsc INTCON,GIE
goto $-2
movlw 55h
movwf EECON2
movlw H'AA'
movwf EECON2
bsf EECON1,WR
btfsc EECON1,WR ; Wait for write to complete
goto $-1
bsf INTCON,GIE ; Interrupt enabled
bcf EECON1,WREN
goto Loop ; Tests push-buttons again
end ; End of program
EXAMPLE 13
Two-digit LED counter, multiplexing
In this example, the microcontroller operates as a two-digit counter. Concretely, the variable Dval is decremented (slow enough to be visible) and its value is displayed on twodigit LED display (99-0). The challenge is to enable binary number to be converted in decimal one and split it in two digits (tens and ones). Besides, since the LED display segments are connected in parallel, it is necessary to ensure that they change fast in order to make impression of simultaneous light emission (time-division multiplexing). Remember that in electronics, multiplexing allows several analog signals to be processed by one analog-todigital converter (ADC). In this very case, time-division multiplexing is performed by the timer TMR0, while binary to decimal number conversion is performed in macro "digbyte". Counter may be reset to its starting value (99) at any moment by pressing the pushbutton "COUNTER RESET".
Source Code
;************************* Header *******************************************
;****************************************************************************
; DEFINING VARIABLES IN PROGRAM
w_temp EQU 0x7D ; Variable for saving W register
status_temp EQU 0x7E ; Variable for saving STATUS register
pclath_temp EQU 0x7F ; Variable for saving PCLATH register
CBLOCK 0x20 ; Block of variables starts at address 20h
Digtemp
Dig0 ; Variables for displaying digits - LSB
Dig1
Dig2
Dig3 ; Variables for displaying digits - MSB
Dval ; Counter value
One ; Auxiliary variable which determines which
; display is to be switched on
ENDC ; End of block of variables
poc_vr EQU .99 ; Initial counter value is 99
include "Digbyte.inc"
;****************************************************************************
ORG 0x0000 ; First instruction address
goto main ; Jump to label "main"
;****************************************************************************
ORG 0x0004 ; Interrupt vector address
movwf w_temp ; Move w register to w_temp register
movf STATUS,w ; Move STATUS register to status_temp
movwf status_temp ; register
movf PCLATH,w ; Move PCLATH register to pclath_temp
movwf pclath_temp ; register
; Start of interrupt routine...
BANKSEL TMR0
movlw .100
movwf TMR0
bcf INTCON, T0IF
bcf PORTA, 0
bcf PORTA, 1
btfsc One, 0
goto Lsdon
goto Msdon
Lsdon
incf One, f
movlw HIGH (Bcdto7seg)
movwf PCLATH
digbyte Dval
movf Dig1, w
call Bcdto7seg ; Place L1 mask on the PORTD
movwf PORTD
bsf PORTA, 1
goto ISR_end
Msdon
incf One, f
movlw HIGH (Bcdto7seg)
movwf PCLATH
digbyte Dval
movf Dig0, w
call Bcdto7seg ; Place LO mask on the PORTD
movwf PORTD
bsf PORTA, 0
goto ISR_end
; End of interrupt routine...
ISR_end
movf pclath_temp,w ; PCLATH register is given its original
movwf PCLATH ; state
movf status_temp,w ; STATUS register is given its original
movwf STATUS ; state
swapf w_temp,f ; W register is given its original
; state
swapf w_temp,w
retfie ; Return from interrupt routine
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL ; All pins are digital
clrf ANSELH
BANKSEL TRISA
movlw b'11111100' ; RA0 and RA1 are configured as outputs and
; used for 7-segment display multiplexing
; RA2 is input push-button for initializa
; tion
movwf TRISA
clrf TRISD
BANKSEL OPTION_REG
movlw b'10000110' ; TMR0 is incremented each 32us (Fclk=8MHz)
movwf OPTION_REG
BANKSEL PORTA
movlw poc_vr
movwf Dval ; Dval contains counter value
movlw b'00000001' ; Initializes variable specifying display
movwf One ; to switch on
movwf PORTA
movlw .100
movwf TMR0 ; TMR0 interrupt appr.every 10ms
bsf INTCON, GIE ; Global interrupt enabled
bsf INTCON, T0IE ; Timer TMR0 interrupt enabled
bcf INTCON, T0IF
Loop
btfss One, 3 ; Falling edge encountered?
goto Dec ; Yes! Go to Dec
btfss PORTA, 2 ; Counter reset button pressed?
goto Reset ; Yes! Go to Reset
goto Loop
; Decrement Dval counter by 1
Dec
btfss One, 3
goto Dec
movf Dval, f
btfsc STATUS, Z ; Is Dval equal to 0?
goto Loop ; If it is, go to loop and wait for T2
decf Dval, f ; If Dval not equal to 0, decrement it by 1
goto Loop
Reset
btfss PORTA, 2 ; Wait for rising edge
goto Reset
movlw poc_vr
movwf Dval ; Write initial value to counter
goto Loop
;****************************************************************************
ORG 0x0300 ; Lookup table is at the top of third page, but
; can be placed at some other place, it is impor
; tant to have it all on one page
Bcdto7seg
addwf PCL, f
DT 0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f
;****************************************************************************
END ; End of program
Macro "digbyte":
digbyte MACRO arg0
LOCAL Exit0
LOCAL Exit1
LOCAL Exit2
clrf Dig0
clrf Dig1
clrf Dig2
clrf Dig3
movf arg0, w
movwf Digtemp
movlw .100
Exit2
incf Dig2, f
subwf Digtemp, f
btfsc STATUS, C
goto Exit2
decf Dig2, f
addwf Digtemp, f
Exit1
movlw .10
incf Dig1, f
subwf Digtemp, f
btfsc STATUS, C
goto Exit1
decf Dig1, f
addwf Digtemp, f
Exit0
movf Digtemp, w
movwf Dig0
ENDM
Macro digbyte is used to convert the number from digital to decimal format. Besides, digits of such decimal number are stored into special registers in order to enable them to be displayed on LED displays.
EXAMPLE 14
Sound generating, using macros
The generation of sound is a task commonly assigned to the microcontroller. Basically, it all comes to generating a pulse sequence on one output pin. While doing so, the proportion of logic zero (0) to logic one (1) duration determines the tone pitch and by changing different tones, different melodies arise.
In this example, any press on push-buttons T1 and T2 generates a sound. The appropriate instructions are stored in macro "beep" containing two arguments.
Source Code
;*********************** Header ***************************************
;******************* Defining variables in program ********************
cblock 0x20
HIcnt ; Auxiliary variables for macro pausems
LOcnt
LOOPcnt
PRESCwait
Beep_TEMP1 ; Belongs to macro "BEEP"
Beep_TEMP2
Beep_TEMP3
endc
#define BEEPport PORTD, 2 ; Speaker pin
#define BEEPtris TRISD, 2
expand
;**********************************************************************
ORG 0x0000 ; RESET vector address
goto main ; Jump to program start (label - main)
;**********************************************************************
; remaining code goes here
include "pause.inc"
include "button.inc"
include "beep.inc"
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL ; All outputs are digital
clrf ANSELH
banksel TRISD
movlw b'11111011' ; PORTA D initialization
movwf TRISD
banksel PORTD
BEEPinit ; Macro "Beep"
Loop
button PORTD,0,0,Play1 ; Push-button 1
button PORTD,1,0,Play2 ; Push-button 2
goto Loop
Play1 ; First tone
BEEP 0xFF, 0x02
BEEP 0x90, 0x05
BEEP 0xC0, 0x03
BEEP 0xFF, 0x03
goto Loop
Play2 ; Second tone
BEEP 0xBB, 0x02
BEEP 0x87, 0x05
BEEP 0xA2, 0x03
BEEP 0x98, 0x03
goto Loop
;**********************************************************************
END ; End of program
Macro "beep":
BEEPinit MACRO
bcf STATUS, RP0
bcf STATUS, RP1
bcf BEEPport
bsf STATUS, RP0
bcf STATUS, RP1
bcf BEEPtris
movlw b'00000111' ; TMR0 prescaler rate 1:256
movwf OPTION_REG ; OPTION <- W
bcf STATUS, RP0
bcf STATUS, RP1
ENDM
BEEP MACRO freq, duration
bcf STATUS, RP0
bcf STATUS, RP1
movlw freq
movwf Beep_TEMP1
movlw duration
movwf Beep_TEMP2
call BEEPsub
ENDM
;**********************************************************************
; Subroutines
BEEPsub
clrf TMR0 ; Counter initialization
bcf INTCON, T0IF
bcf BEEPport
BEEPa
bcf INTCON, T0IF ; Clears TMR0 Overflow Flag
BEEPb
bsf BEEPport
call B_Wait ; Logic one "1" duration
bcf BEEPport
call B_Wait ; Logic zero "0" duration
btfss INTCON, T0IF ; Check TMR0 Overflow Flag,
goto BEEPb ; skip next if set
decfsz Beep_TEMP2, f ; Is Beep_TEMP2 = 0 ?
goto BEEPa ; Go to BEEPa again
return
B_Wait
movf Beep_TEMP1, w
movwf Beep_TEMP3
B_Waita
decfsz Beep_TEMP3, f
goto B_Waita
return
EXAMPLE 15
Using LCD display
This example illustrates the use of the alphanumeric LCD display. The program itself is very simple because macros are used (usually the effort of creating Macros pays off in the end).
Two messages written on two lines change on display. The second message is intended to display the current temperature. Since no sensor is installed, the measurement is not really carried out, the variable "temp" appears on the display instead of the measured temperature.
In reality, the current temperature or some other measured value would be displayed.
Source Code
;*********************** Header ***********************************
; DEFINING VARIABLES IN PROGRAM
CBLOCK 0x20 ; Block of variables starts at address 20h
HIcnt ; Belongs to macro "pausems"
LOcnt
LOOPcnt
LCDbuf ; Belongs to functions "LCDxxx"
LCDtemp
LCDportBuf ; LCD Port Buffer
Digtemp ; Belongs to macro "digbyte"
Dig0
Dig1
Dig2
Dig3
temp
ENDC ; End of block
LCDport EQU PORTB ; LCD is on PORTB (4 data lines on RB0-RB3)
RS EQU 4 ; RS line connected to RB4
EN EQU 5 ; EN line connected to RB5
;**********************************************************************
ORG 0x0000 ; Reset vector address
nop
goto main ; Go to beginning of the program (label "main")
;**********************************************************************
include "LCD.inc"
include "digbyte.inc"
include "pause.inc"
;**********************************************************************
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL ; All pins are digital
clrf ANSELH
bcf STATUS,RP0 ; Bank0 active only
bcf STATUS,RP1
movlw .23
movwf temp ; Move arbitrary value to variable
; is to be displayed on LCD
lcdinit ; LCD initialization
Loop
lcdcmd 0x01 ; Instruction to clear LCD
lcdtext 1, "mikroelektronika" ; Write text from the begin
; ning of the first line
lcdtext 2, "Beograd" ; Write text from the beginning of
; the second line
pausems .2000 ; 2 sec. delay
lcdcmd 0x01 ; Instruction to clear LCD
lcdtext 1, "Temperatura1" ; Write text from the begin
; ning of the first line
lcdtext 2, "temp=" ; Write text from the beginning of
; the second line
lcdbyte temp ; Write variable (dec.)
lcdtext 0, " C" ; Write text after cursor
pausems .2000 ; 2 sec. delay
goto Loop
;**********************************************************************
end ; End of program
LCD.inc
;**********************************************************************
; Initialization must be done by using macro lcdinit before access
; ing LCD
;**********************************************************************
lcdinit MACRO
bcf STATUS, RP0 ; Bank0
bcf STATUS, RP1
clrf LCDportBuf
movf LCDportBuf, w
movwf LCDport
bsf STATUS, RP0 ; Bank1
bcf STATUS, RP1
clrf TRISB ; LCDport with output LCD
bcf STATUS, RP0 ; Bank0
bcf STATUS, RP1
; Function set (4-bit mode change)
movlw b'00100000'
movwf LCDbuf
swapf LCDbuf, w
movwf LCDportBuf
bcf LCDportBuf, RS
movf LCDportBuf, w
movwf LCDport
bsf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
bcf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
call Delay1ms ; 1 ms delay
; Function set (display mode set)
lcdcmd b'00101100'
call Delay1ms ; 1 ms delay
; Display ON/OFF Control
lcdcmd b'00001100'
call Delay1ms ; 1 ms delay
; Entry Mode Set
lcdcmd b'00000110'
call Delay1ms ; 1 ms delay
; Display Clear
lcdcmd b'00000001'
pausems .40 ; 40 ms delay
; Function set (4-bit mode change)
movlw b'00100000'
movwf LCDbuf
swapf LCDbuf, w
movwf LCDportBuf
bcf LCDportBuf, RS
movf LCDportBuf, w
movwf LCDport
bsf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
bcf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
call Delay1ms ; 1 ms delay
; Function set (display mode set)
lcdcmd b'00101100'
call Delay1ms ; 1 ms delay
; Display ON/OFF Control
lcdcmd b'00001100'
call Delay1ms ; 1 ms delay
; Entry Mode Set
lcdcmd b'00000110'
call Delay1ms ; 1 ms delay
; Display Clear
lcdcmd b'00000001'
pausems .40 ; 40 ms delay
ENDM
;**********************************************************************
; lcdcmd sends command to LCD (see the table on the previous page)
; lcdclr is the same as lcdcmd 0x01
;**********************************************************************
lcdcmd MACRO LCDcommand ; Send command to LCD
movlw LCDcommand
call LCDcomd
ENDM
LCDcomd
movwf LCDbuf
bcf LCDportBuf, RS
movf LCDportBuf, w
movwf LCDport
goto LCDwr
LCDdata
movwf LCDbuf
bsf LCDportBuf, RS
movf LCDportBuf, w
movwf LCDport
goto LCDwr
LCDwr
swapf LCDbuf, w
call SendW
movf LCDbuf, w
call SendW
return
SendW
andlw 0x0F
movwf LCDtemp
movlw 0xF0
andwf LCDportBuf, f
movf LCDtemp, w
iorwf LCDportBuf, f
movf LCDportBuf, w
movwf LCDport
call Delay1ms
bsf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
bcf LCDportBuf, EN
movf LCDportBuf, w
movwf LCDport
call Delay1ms
return
;**********************************************************************
; lcdtext writes text containing 16 characters which represents a
; macro argument. The first argument select selects the line in which
; text writing is to start. If select is 0, text writing starts from
; cursor current position.
;**********************************************************************
lcdtext MACRO select, text ; This macro writes text from cursor
; current position. Text is specified
; in argument consisting of 16 charac
; ters
local Message
local Start
local Exit
local i=0
goto Start
Message DT text ; Create lookup table from arguments
DT 0
Start
IF (select == 1)
lcdcmd b'10000000'
ELSE
IF (select == 2)
lcdcmd b'11000000'
ENDIF
ENDIF
WHILE (i<16) ; Repeat conditional program compiling 16 times
call Message+i ; Read lookup table and place value in W
addlw 0
bz Exit ; until 0 is read
call LCDdata ; Call routine displaying W on LCD
i=i+1
ENDW
Exit
ENDM
;**********************************************************************
; This macro writes value in size of 1 byte on LCD
; excluding leading zeros
;**********************************************************************
lcdbyte MACRO arg0
digbyte arg0 ; A hundred is in Dig2,
; A ten is in Dig1 and one in Dig0
movf Dig2, w
addlw 0x30
call LCDdata
movf Dig1, w ; If digit is 0 move cursor
addlw 0x30
call LCDdata
movf Dig0, w ; If digit is 0 move cursor
addlw 0x30
call LCDdata
ENDM
;**********************************************************************
; 1ms Delay
Delay1ms:
movlw .200
movwf LOOPcnt
Delay10us:
nop ;1us
nop ;1us
nop ;1us
nop ;1us
nop ;1us
nop ;1us
nop ;1us
decfsz LOOPcnt, f ;1us
goto Delay10us ;2us
return
EXAMPLE 16
RS232 serial communication
This example illustrates the use of the microcontroller's EUSART module. Connection to the PC is enabled through RS232 standard. The program works in the following way: Every byte received via the serial communication is displayed using LED diodes connected to port B and is automatically returned to the transmitter thereafter. If an error occurs on receive, it will be signalled by switching the LED diode on. The easiest way to test device operation in practice is by using a standard Windows program called
Hyper Terminal.
Source Code
;*********************** Header ***************************************
; DEFINING VARIABLES IN PROGRAM
w_temp EQU 0x7D ; Variable for saving W register
status_temp EQU 0x7E ; Variable for saving STATUS register
pclath_temp EQU 0x7F ; Variable for saving PCLATH w register
cblock 0x20 ; Block of variables starts at address 20 h
Port_A ; Variable at address 20 h
Port_B ; Variable at address 21 h
RS232temp ; Variable at address 22 h
RXchr ; Variable at address 23 h
endc ; End of block of variables
;**********************************************************************
ORG 0x0000 ; Reset vector
nop
goto main ; Go to beginning of program (label "main")
;**********************************************************************
ORG 0x0004 ; Interrupt vector address
movwf w_temp ; Save value of W register
movf STATUS,w ; Save value of STATUS register
movwf status_temp
movf PCLATH,w ; Save value of PCLATH register
movwf pclath_temp
;**********************************************************************
; This part of the program is executed in interrupt routine
banksel PIE1
btfss PIE1, RCIE
goto ISR_Not_RX232int
banksel PIE1
btfsc PIR1, RCIF
call RX232_int_proc
ISR_Not_RX232int
movf pclath_temp,w
movwf PCLATH ; PCLATH is given its original value
movf status_temp,w
movwf STATUS ; STATUS is given its original value
swapf w_temp,f
swapf w_temp,w ; W is given its original value
retfie ; Return from interrupt routine
;**********************************************************************
RX232_int_proc ; Check if error has occurred
banksel RCSTA
movf RCSTA, w
movwf RS232temp
btfsc RS232temp, FERR
goto RX232_int_proc_FERR
btfsc RS232temp, OERR
goto RX232_int_proc_OERR
goto RX232_int_proc_Cont
RX232_int_proc_FERR
bcf RCSTA, CREN ; To clear FERR bit, receiver is first
; switched off and on afterwards
nop ; Delay ...
nop
bsf RCSTA, CREN
movf RCREG, w ; Reads receive register and clears FERR bit
bsf Port_A, 0 ; Switches LED on ( UART error indicator)
movf Port_A, w
movwf PORTA
goto RS232_exit
RX232_int_proc_OERR
bcf RCSTA, CREN ; Clears OERR bit
nop ; Delay ...
nop
bsf RCSTA, CREN
movf RCREG, w ; Reads receive register and clears FERR bit
bsf Port_A, 1 ; Switches LED on ( UART error indicator)
movf Port_A, w
movwf PORTA
goto RS232_exit
RX232_int_proc_Cont
movf RCREG, W ; Reads received data
movwf RXchr
movwf PORTB
movwf TXREG ; Sends data back to PC
RS232_exit
return ; Return from interrupt routine
;**********************************************************************
; Main program
main
banksel ANSEL ; Selects bank containing ANSEL
clrf ANSEL ; All inputs are digital
clrf ANSELH
;---------------------------------------
; Port configuration
;---------------------------------------
banksel TRISA
movlw b'11111100'
movwf TRISA
movlw b'00000000'
movwf TRISB
;---------------------------------------
; Setting initial values
;---------------------------------------
banksel PORTA
movlw b'11111100'
movwf PORTA
movwf Port_A
movlw b'00000000'
movwf PORTB
movwf Port_B
;---------------------------------------
; USART - setting for 38400 bps
;---------------------------------------
banksel TRISC
bcf TRISC, 6 ; RC6/TX/CK = output
bsf TRISC, 7 ; RC7/RX/DT = input
banksel BAUDCTL
bsf BAUDCTL, BRG16
banksel SPBRG
movlw .51 ; baud rate = 38400
; ( Fosc/(4*(SPBRG+1)) ) Error +0.16%
movwf SPBRG
clrf SPBRGH
banksel TXSTA
bcf TXSTA, TX9 ; Data is 8-bit wide
bsf TXSTA, TXEN ; Data transmission enabled
bcf TXSTA, SYNC ; Asynchronous mode
bsf TXSTA, BRGH ; High-speed Baud rate
banksel RCSTA
bsf RCSTA, SPEN ; RX/DT and TX/CK outputs configuration
bcf RCSTA, RX9 ; Select mode for 8-bit data receive
bsf RCSTA, CREN ; Receive data enabled
bcf RCSTA, ADDEN ; No address detection, ninth bit may be
; used as parity bit
movf RCSTA, W
movf RCREG, W
;---------------------------------------
; Interrupts enabled
;---------------------------------------
banksel PIE1
bsf PIE1, RCIE ; USART Rx interrupt enabled
bsf INTCON, PEIE ; All peripheral interrupts enabled
bsf INTCON, GIE ; Global interrupt enabled
;---------------------------------------
; Remain here
;---------------------------------------
goto $
end ; End of program
