PIC16F628A Test Setup.
Fig. 1
Click here for larger image.

PIC16F628 Pulse Width Modulation Controls Brightness of LED

The following code snippet sets up the PWM hardware in the PIC16F628A. The output at RB3 is connected to an LED.


	BANKSEL 1 ; Bank 1
	MOVLW B'11111001'
	MOVWF PR2
	BANKSEL 0 ; Back Bank 0
	MOVLW B'00000101'
	MOVWF T2CON
	MOVLW 0 ; duty cycle
	MOVWF  CCPR1L ; duty cycle 0-255
	MOVLW B'00001100'
	MOVWF CCP1CON

Code example from 16F628A_PWM.asm contains PWM setup. The RB3 LED goes from off to bright in steps, then reverses back to off.


main

; Demo 2
; CCPR1L is 8-bit PWM duty cycle reg. 0-255.
; RB3 is PWM output to LED2 to GRD.
; LED2 on pin RB3 goes from off to bright in steps.
; Clear PWM register CCPR1L, LED2 = OFF
; Flip state of LED1 RB1 with XORWF
; W = 50, call delay Wx10ms = 500mSec. 
; 2 loops = 1 Hz. rate on LED1 RB1
; Every loop 20 ADDed to CCPR1L duty cycle reg.
; LED2 gets brighter
; Every 1Hz 40 is ADDed to PWM CCPR1L
; Check STATUS bit C
; Exit loop if C = 1, C = 0 back aa
; Repeat until STATUS C bit = 1.
	clrf 	CCPR1L
aa	movlw 0x02
	xorwf PORTB, f
	movwf d'50' 
	call Wx10ms ; LED ON 0.5 sec.
	movlw d'20'
	addwf CCPR1L, f
	btfss STATUS, C ; check carry
	goto aa

; CCPR1L is 8-bit PWM duty cycle reg. 0-255.	
; RB3 is PWM output to LED2 to GRD.
; LED2 on pin RB3 goes from bright to off in steps.
; PWM register CCPR1L = 255, LED2 on RB3 is bright
; Flip state of LED1 RB1 with XORWF
; W = 50, call delay Wx10ms = 500mSec. 
; 2 loops = 1 Hz. rate on LED1 on RB1
; Every loop 20 SUBed from PWM CCPR1L reg.
; LED2 get dimmer
; Every 1Hz 40 is SUBed from PWM CCPR1L
; Check STATUS bit C for borrow
; Exit loop if C = 0, C = 1 back  bb
; Repeat until STATUS C bit = 0.
	movlw d'255'
	movwf CCPR1L
bb	movlw 0x02
	xorwf PORTB, f
	movwf d'50' 
	call Wx10ms ; LED ON 0.5 sec.
	movlw d'20'
	subwf CCPR1L, f
	btfsc STATUS, C ; check borrow
	goto bb	
	
	goto	main

For how to calculate frequency and duty cycle for PIC PWM
see Working with Pulse-Width Modulation and the PIC Microcontroller

See How I got into Electronics