Setting up a 16F87x for PWM Output

Tony Nixon [sales at picnpoke.com] of ICmicro's says:

Assume PIC runs at 4MHz 1000Hz wanted at 50% duty cycle
These PWM enable steps are taken from the 16F87X data sheet
Set the PWM period (1000Hz = 0.001S) by writing to the PR2 register.
PWM Period = [PR2 + 1]*4*Tosc*TMR2 PreScale value
Use 4 for TMR2 prescale. You can try other values but make sure it works with the duty cycle calcs as well.
Tosc = 1/4000000 = 2.5*10-7
Rearranging the equation gives...
PR2 = (Period/(4 * Tosc * TMR2 Prescale)) - 1
Therefore...
PR2 = (0.001/(4 * 2.5 * 10-7 * 4)) - 1 = 249

   bsf STATUS,RP0
   movlw d'249'    ; when TMR2 = 249 = end of Period
   movwf PR2
   bcf STATUS,RP0
Set the PWM Duty Cycle by writing to the CCPR1L register and CCP1CON<5:4> bits.
CCPR1L contains the upper 8 bits of the 10 bit Duty Cycle value CCP1CON<5:4> contain the lower 2 bits
PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>)*Tosc*TMR2 Prescale Value

CCPR1L:CCP1Con<5:4> = PWM Duty Cycle / (Tosc * TMR2 Prescale)

PWM Duty Cycle = 50% of Period

PWM Duty Cycle = 50% of 0.001 = 0.0005

CCPR1L:CCP1Con<5:4> = 0.0005 / (2.5 * 10-7 * 4)

CCPR1L:CCP1Con<5:4> = 500

500 in 10 bit binary = 0111110100

CCPR1L = 01111101    CCP1Con<5:4> = 00
Write the 10 bit value
   movlw b'01111101'  ; set bits 9 - 2
   movwf CCPR1L
   bcf CCP1CON,CCP1X  ; set bit 1
   bcf CCP1CON,CCP1Y  ; set bit 0
Make the CCP1 pin an output by clearing the TRISC<2> bit
   bsf STATUS,RP0
   movlw b'11111011'
   andwf TRISC
   bcf STATUS,RP0
Set The TMR2 prescale value and enable TMR2 by writing to T2CON.
We decided previously to set the prescale value to 4.
TMR2 prescale are bits 1 and 0 in T2CON
TMR2 enable is bit 2
   movlw b'00000101'   ; TMR2 = on, prescale = 1:4
   movwf T2CON
Configure the CCP1 module for PWM
   movf CCP1CON,W
   andlw b'00110000'  ; mask all but previously set Duty Cycle bits
   iorlw b'00001111'  ; and enable PWM mode
   movwf CCP1CON
A 1000 Hz square wave with a 50% duty cycle should now be coming out of RC2.

Liew Danny Jiefu says:

how do you use the pwm after setting it up, for example like on the leds? could u write a sample program?

Comments:

Thnk yuo very much for the explanation. I have red a lot of papers, including Micfochips Datasheet (pic16F877. But no ones explain as clear as you.

Regards
Jaime
thanks for pwm answers..
hi my name is sylvester, an elect/elect student. i'm presently working on a school mini project using the hitech picc compiler & microcontrollers. Really appreciate this doc, helped me alot. thankz
Quite possible that my advice is thoroughly outdated, but as of Liew's question, first you have to decide an application for the PWM output (like driving a switch in a Switch Mode converter)...as far as using it on LED's is concerned, if you connect the LED, with suitable series resistence, to output RC2, you would indeed notice the difference in light intensity with duty ratio; however, the switching would be impossible to notice by naked eye at a frequnecy of 1kHz, but you can always look at the waveform on O-scope.
rseaf@dsamail.com.sg
Searching for so long on internet on how to program PWM for 16f877 and finally I found this! Thanks alot Tony. The datasheet should have consider adding your tutorial to it in the furture. Btw...
```
movf CCP1CON,W
   andlw b'00110000'  ; mask all but previously set Duty Cycle bits
   iorlw b'00001111'  ; and enable PWM mode
   movwf CCP1CON
```
What is the rationale behind this? I know it work but how?