Fan Controller - PIC16F871

[Omar.M]

Hello,
I was building a small fan controller for my computer's case using PIC16F871, writing the code in ASM. I pieced together bits and pieces of code from the net, but suddenly it stopped working-- I must have made a change that killed it.

I backtracked all the way down to the bare minimum code, still unable to find the problem. I am sure it is a silly little mistake that I just cannot pick up. Any help is appreciated!

I am using LEDs to test it, outputting the three fan signals to PORTB0, 1 and 2. The interrupt should be occurring at 250 Hz, however, each LED is on for a few seconds, then the next LED turns on, and so on. This leads me to believe that the TMR1 was setup incorrectly. But I've double checked! I tried a different chip and even new XT OSC. No luck!

						;16F871, 4mHz EXT OSC	
	list P=16F871
	#include <p16F871.inc>
	
	cblock		20h
		pwmwar, dutycycle1, dutycycle2, dutycycle3
	endc
	
	org	0x0000
	goto	Init

	ORG	0x0004
							;Turn off interrupts
		BCF		INTCON, 7
		BCF		PIR1, 0

							;If fan1 is less than the value of PWM Var, keep it on. Else, set it off.
 		BSF     STATUS, C
        MOVF    dutycycle1, W
        SUBWF   pwmwar, W

        BTFSS   STATUS, C
        BSF     PORTB, 0
        BTFSC   STATUS, C
        BCF     PORTB, 0

							;If fan2 is less than the value of PWM Var, keep it on. Else, set it off.
        BSF     STATUS, C
        MOVF    dutycycle2, W
        SUBWF   pwmwar, W

        BTFSS   STATUS, C
        BSF     PORTB, 1
        BTFSC   STATUS, C
        BCF     PORTB, 1
				
							;If fan3 is less than the value of PWM Var, keep it on. Else, set it off
        BSF     STATUS, C
        MOVF    dutycycle3, W
        SUBWF   pwmwar, W

        BTFSS   STATUS, C
        BSF     PORTB, 2
        BTFSC   STATUS, C
        BCF     PORTB, 2

							;Increase PWM Variable. If it has reached 100, reset it.
        INCF    pwmwar, F
        MOVF    pwmwar, W
        SUBLW   d'100'
        BTFSC   STATUS, Z
        CLRF    pwmwar
		BSF		INTCON, 7
	retfie

Init
							;Set TMR1 interrupt. Approximately 4000uS.
	MOVLW	0x5F
	MOVWF	TMR1L
	MOVLW	0xF0
	MOVWF	TMR1H
	MOVLW	0x1
	MOVWF	T1CON
	MOVLW	0xC0
	MOVWF	INTCON

	bsf		STATUS,RP0		;bank 1
	clrf	TRISB			;Set PORTB as outputs
	BSF		PIE1, 0
	bcf		STATUS,RP0

							;Set PWM Variable to 1
	MOVLW	0x1
	MOVWF	pwmwar

start
							;Setting dutycycles for each of the fans.
	MOVLW	d'99'
	MOVWF	dutycycle1
	MOVLW	d'1'
	MOVWF	dutycycle2
	MOVLW	d'50'
	MOVWF	dutycycle3
	goto	start
	end

-Omar