hello
i need some help in constructing pwm program.. i need to vary duty cycle from 0% to 100%.. interruption will occur before the duty cycle is changed.. can any one fix this program???..

; oscillator : 4 mHZ


List p=16f8773
#include <p16f873.inc>
;***declare registeraion

ORG 20H
COUNT RES 1
AN0 RES 1
AN1 RES 1

;***port initialiization

MAIN BCF STATUS,RP0
CLRF PORTA ;clear content of port A
CLRF PORTB ;clear content of port B
CLRF PORTC ;clear content of port C
BSF STATUS,RP0 ;change to bank 1
MOVLW B'11111111'
MOVWF TRISA ;set port A as input
CLRF TRISB ;set port B as output

;***A/D conversion

BCF STATUS,RP0 ;change to bank 0
MOVLW B'01000001'
MOVWF ADCON0 ;fosc/8, channel 0=RA0/AN0, A/D converter=on
BSF STATUS,RP0 ;change to bank 1
MOVLW B'00000000'
MOVWF ADCON1 ;set port A as analog input port

;***PWM initialization
;**step 1: set the PWM period by writting to the PR2 register

MOVLW D'49'
MOVWF PR2 ;PR2=49

;**step 2: set the PWM duty cycle by writting duty cycle by writing to the CCPR1RL register and CCP1CON<5:4> bits

BCF STATUS,RP0
MOVLW B'10110100'
MOVWF CCPR1L ;load 180 in W to CCPR1L
BSF CCP1CON,CCP1Y ;set bit 1 of the duty cycle
BSF CCP1CON,CCP1X ;set bit 0 of the duty cycle


;**step 3: make the CCP1 pin output by clearing the TRISC<2> bit

MOVLW B'11111011'
ANDWF TRISC ;clear TRISC<2>

;**step 4: set the TMR2 prescale value and enable Timer2 by writting to T2CON

CLRF TMR2
BSF T2CON,TMR2ON

;**step 5: configure the CCP1 module for PWM operation

BSF CCP1CON,CCP1M3
BSF CCP1CON,CCP1M2

BTFSS PIR1,TMR2IF
GOTO $-1
BCF PIR1,TMR2IF

;***1st loop

LOOP1 BTFSS INTCON,T0IF
GOTO LOOP1

BCF INTCON,T0IF

BCF INTCON,T0IF
BSF ADCON0,GO_DONE
BTFSC ADCON0,GO_DONE
GOTO $-1

MOVF ADRESH,W
MOVWF AN0
MOVWF PORTB

MOVLW B'01001001'
MOVWF ADCON0

;***2nd loop

LOOP2 BTFSS INTCON,T0IF
GOTO LOOP2

BCF INTCON,T0IF
BCF INTCON,T0IF

BSF ADCON0,GO_DONE
BTFSC ADCON0,GO_DONE
GOTO $-1

MOVF ADRESH,W
MOVWF AN1
MOVWF PORTB

SUBWF AN0,0
BTFSS STATUS,Z
MOVWF CCPR1L
GOTO MAIN

END

 

Have you turned the watchdog timer off? There are no config settings so it probably takes default settings from MPLAB.
I've also spotted a couple of things.

List p=16f8773 <------- change to 16f873
#include <p16f873.inc>
;***declare registeraion

ORG 20H <-------- This sets the program start to
COUNT RES 1 20H, not the variable space
AN0 RES 1
AN1 RES 1