MikroElektronika books

4.5 Example 3

 

TMR0 as a counter, declare new variables, use symbols, use a relay ...

In the previous two examples the microcontroller executes the program without being affected by its surrounding. Practically, microcontroller-based devices operating in this manner are very rare (for example, a simple neon sign controller). Input pins are also used in this example. The schematic is given in figure below, while the program is on the next page. It’s still very simple. Timer TMR0 is used as a counter. The counter input is connected to a push button in such a way that any button pressure causes the TMR0 timer to count one pulse. When the number of pulses matches the number stored in the TEST register, a logic one (5V) will appear on the PORTD.3 pin. This voltage is used to activate an electromechanical relay, and this bit is therefore called ‘RELAY’ in the program. Here, the TEST register stores number 5. Of course, it can be any number obtained either by computing or defined as a constant. Besides, the microcontroller can run some other device instead of relay, while a sensor can be used instead of the push button. This example illustrates one of the most common applications of the microcontroller in the industry; when something is performed as many times as required, then something else should be turned on or off....
pic-microcontrollers-programming-in-basic-chapter-04-image-32
' Header******************************************************
program example_3 ' Program name
symbol RELAY = PORTD.3 ' Pin PORTD.3 is named RELAY
dim TEST as byte ' Variable TEST is of byte type
main: ' Start of program
TEST = 5 ' Constant TEST = 5
ANSEL = 0 ' All I/O pins are configured as digital
ANSELH = 0
PORTA = 0 ' Reset PORTA
TRISA = 0xFF ' All portA pins are configured as inputs
PORTD = 0 ' Reset PORTD
TRISD = %11110111 ' Pin RD3 is configured as an output, while other pins are
 ' configured as inputs
OPTION_REG.5 = 1 ' Counter TMR0 receives pulses through the RA4 pin
OPTION_REG.3 = 1 ' Prescaler rate is 1:1
TMR0 = 0 ' Reset timer/counter TMR0

while 1
if TMR0 = TEST then ' Does the number in timer match constant TEST?
 RELAY = 1 ' Numbers match. Set the RD3 bit (output RELAY)
end if
wend ' Remain in endless loop

end. ' End of program
Only one symbol (RELAY) is used here. It is assigned the third pin of PORTD in declaration. 
symbol RELAY = PORTD.3 ' Symbol RELAY = PORTD.3
If several port D pins are connected to relays, the expression above could be written in this way as well: