PINOUT DESCRIPTION

Most pins of the PIC16F887 microcontroller are multi-functional as seen in figure above. For example, designator RA3/AN3/Vref+/C1IN+ for the fifth pin of the microcontroller indicates that it has the following functions:

• RA3 Port A third digital input/output
• AN3 Third analog input
• Vref+ Positive voltage reference
• C1IN+ Comparator C1 positive input

Such pin functionality is very useful as it makes the microcontroller package more compact without affecting its operation. These various pin functions cannot be used simultaneously, but can be changed at any point during operation. The following tables refer to the PDIP 40 microcontroller.

CENTRAL PROCESSOR UNIT (CPU)

We are not going to bore you with the operation of the CPU at this stage. However, we will just state that the CPU is manufactured with RISC technology as it is an important factor when deciding which microcontroller to use. RISC stands for Reduced Instruction Set Computer, which gives the PIC16F877 two great advantages:

• The CPU only recognizes 35 simple instructions. Just to mention that in order to program other microcontrollers in assembly language it is necessary to know more than 200 instructions by heart.
• The execution time is the same for almost all instructions, and lasts for 4 clock cycles. The oscillator frequency is stabilized by a quartz crystal. The execution time of jump and branch instructions is 2 clock cycles. It means that if the microcontroller’s operating speed is 20MHz, the execution time of each instruction will be 200nS, i.e. the program will execute 5 million instructions per second!

MEMORY

The PIC16F887 has three types of memory ROM, RAM and EEPROM. All of them will be separately discussed since each has specific functions, features and organization.

ROM MEMORY

ROM memory is used to permanently save the program being executed. This is why it is often called ‘program memory’. The PIC16F887 has 8Kb of ROM (in total of 8192 locations). Since the ROM memory is made with FLASH technology, its contents can be changed by providing a special programming voltage (13V). However, it is not necessary to explain it in detail as being automatically performed by means of a special program on the PC and a simple electronic device called the programmer.

EEPROM MEMORY

Similar to program memory, the contents of EEPROM is permanently saved, even when the power goes off. However, unlike ROM, the contents of EEPROM can be changed during the operation of the microcontroller. This is why this memory (256 locations) is perfect for permanently saving some of the results created and used during the operation.

RAM MEMORY

This is the third and the most complex part of microcontroller memory. In this case, it consists of two parts: general-purpose registers and special-function registers (SFR). All these registers are divided in four memory banks to be explained later in the chapter. Even though both groups of registers are cleared when power goes off and even though they are manufactured in the same manner and act in a similar way, their functions do not have many things in common.

GENERAL-PURPOSE REGISTERS

General-purpose registers are used for storing temporary data and results created during operation. For example, if the program performs counting (products on the assembly line), it is necessary to have a register which stands for what we in everyday life call ‘sum’. Since the microcontroller is not creative at all, it is necessary to specify the address of some general purpose register and assign it that function. A simple program to increment the value of this register by 1, after each product passes through a sensor, should be created. Now the microcontroller can execute the program as it knows what and where the sum to be incremented is. Similarly, each program variable must be preassigned some of the general- purpose registers.

SPECIAL FUNCTION REGISTERS (SFRS)

Special-function registers are also RAM memory locations, but unlike general-purpose registers, their purpose is predetermined during manufacturing process and cannot be changed. Since their bits are connected to particular circuits on the chip (A/D converter, serial communication module, etc.), any change of their contents directly affects the operation of the microcontroller or some of its circuits. For example, the ADCON0 register controls the operation of A/D converter. By changing its bits it is determined which port pin is to be configured as converter input, the moment conversion is to start as well as the speed of conversion. Another feature of these memory locations is that they have their names (both registers and their bits), which considerably simplifies the process of writing a program. Since high-level programming languages can use the list of all registers with their exact addresses, it is enough to specify the name of a register in order to read or change its contents.

RAM MEMORY BANKS

The RAM memory is partitioned into four banks. Prior to accessing any register during program writing (in order to read or change its contents), it is necessary to select the bank which contains that register. Two bits of the STATUS register are used for bank selection to be discussed later. In order to simplify the operation, the most commonly used SFRs have the same address in all banks, which enables them to be easily accessed. Handling banks may be difficult only if you write a program in assembly language. When using higher programming languages such as C and compilers such as mikroC PRO for PIC, all you have to do is to specify the register name. On the basis of that, the compiler selects necessary bank and appropriate instructions used for bank selection will be built in the code during the process of compilation. You have been using only assembly language so far and this is the first time you use the C compiler, haven’t you? Isn’t this a wonderful news?

STACK

A part of RAM used as stack consists of eight 13-bit registers. Before the microcontroller starts to execute a subroutine (CALL instruction) or when an interrupt occurs, the address of the first next instruction to execute is pushed onto the stack, i.e. one of its registers. Thanks to that the microcontroller knows from where to continue regular program execution upon a subroutine or an interrupt execution. This address is cleared after returning to the program because there is no need to save it any longer, and one location of the stack becomes automatically available for further use. It is important to bear in mind that data is always circularly pushed onto the stack. It means that after the stack has been pushed eight times, the ninth push overwrites the value that was stored with the first push. The tenth push overwrites the second push and so on. Data overwritten in this way is not recoverable. In addition, the programmer cannot access these registers for write or read and there is no Status bit to indicate stack overflow or stack underflow conditions. For this reason, it is necessary to take special care of it during program writing. Let's do it in mikroC...

/* When entering or exiting an assembly instruction in the program, the compiler
doesn’t save data on the currently active RAM bank. It means that in this program
section, bank selection depends on the SFR registers in use. When switching back
to the program section written in C, the control bits RP0 and RP1 must return the
state they had before assembly language code execution. In this example, the problem
is solved by using the saveBank auxiliary variable which saves the state of
these two bits. */

    saveBank = STATUS & 0b01100000; // Save the state of bits RP0 and RP1
                                    // (bits 5 and 6 of the STATUS register)
    asm {                           // Start of assembly sequence
    ...
    ...                             // Assembly code
    ...
    }                               // End of assembly sequence
    STATUS &= 0b10011111;           // Bits RP0 and RP1 return their original state
    STATUS |= saveBank;
    ...
    ...

INTERRUPT SYSTEM

The first thing the microcontroller does when an interrupt request arrives is to execute the current instruction and then stops the regular program execution. As a result, the current program memory address is automatically pushed onto the stack and the default address (predefined by the manufacturer) is written to the program counter. The location from where the program proceeds with execution is called an interrupt vector. For the PIC16F887 microcontroller, this address is 0004h. As seen in figure below, the location containing the interrupt vector is passed over during regular program execution. A part of the program to be executed when an interrupt request arrives is called an interrupt routine. Its first instruction is located at the interrupt vector. How long will it take to execute this subroutine and what it will be like depends on the skills of the programmer as well as on the interrupt source itself. Some of the microcontrollers have more interrupt vectors (every interrupt request has its vector), but in this case there is only one. Consequently, the first part of the interrupt routine consists in interrupt source detection. Finally, when the interrupt source is recognized and the interrupt routine is executed, the microcontroller reaches the RETFIE instruction, pops the address from the stack and proceeds with program execution from where it left off. mikroC recognizes an interrupt routine to be executed as the void interrupt() function. The body of that function, i.e. interrupt routine, should be written by the user.

void interrupt() { // Interrupt routine
    cnt++ ;        // Interrupt causes variable cnt to be incremented by 1
}

In Short: How to Use SFRs

You have bought the microcontroller and have a good idea how to use it... There is a long list of SFRs and their bits. Each of them controls some process. All in all, it looks like a big control table with a lot of instruments and switches. Now you are concerned about whether you will manage to learn how to use them all? You will probably not, but don’t worry, you don’t have to! Such powerful microcontrollers are similar to supermarkets: they offer so many things at low prices and it is up to you to choose those you need. Therefore, select the field you are interested in and study only what you need to know. When you completely understand hardware operation, study SFRs which are in control of it (there are usually a few of them). As all devices have a sort of control system, the microcontroller has its ‘levers’ which you have to be familiar with in order to be able to use it properly. Of course, we are talking about SFRs from which the process of programming begins and where it ends.