How to Set Up a Timer in PIC16F877A

We have Three Timers in PIC16F877A , and here we have a look to how to setup the TIMER0 in PIC16F877A using HiTech C programm 




• To set up the timer, one must first disable interrupts so that an interrupt doesn’t occur when the timer expires.
• Then, enable the timer and assign the prescaler to the timer.
• Establish the prescaler value, and finally, load the timer register.

 Whenever the timer expires, the T0IF bit in the INTCON register will be set.

• We must clear this bit, reload the timer register, and then execute the code that is to be done at this time.

In code, the setup portion might look something like:

########################## IN ASSEMBLY PROGRAM ###########################

banksel INTCON

bcf INTCON,T0IE ; Mask timer interrupt

banksel OPTION_REG

bcf OPTION_REG,T0CS ; Enable timer

bcf OPTION_REG,PSA ; Prescaler to timer

bcf OPTION_REG,PS2 ; \

bsf OPTION_REG,PS1 ; >- 1:16 prescale

bsf OPTION_REG,PS0 ; /

movlw D’100’ ; Timer will count

movwf TMR0 ; 156 (256-100) counts

######################## IN HITECH C PROGRAM #######################################

OPTION_REG= 0x0F;

  TMR0=0x0F;

  while(TMR0IF=0);

  TMR0IF=0;

##################################################

• Clearly, the individual bits in the option register could all be set with a single store.

• If we didn’t care about the RB0 interrupt, the weak pullups, or the transition of RA4, then instead of five bit manipulations we could have said:

• movlw B’10000011’ ; Set up prescaler and

• movwf OPTION_REG ; timer

The execution loop might look something like:

main:

    btfss INTCON,T0IF ; Did timer overflow?

    goto main ; No, hang around some more 

    movlw D’100’ ; Timer will count

    movwf TMR0 ; 156 (256-100) counts

    bcf INTCON,T0IF ; reset overflow flag

    call DoCode ; Execute main code

    goto main ; Go back and wait