;************TIMER0 INTERRUPT************
; by: Eng'r Roland Michael A. dela Peņa *
;****************************************

; This is a program to increment variables ROLAND0,ROLAND1,ROLAND2 using
; the  TMR0 Interrupt.
; At every overflow count of TMR0, the variables are incremented
;
;
; For 8MHz crystal
; 8,000,000 cycles/sec = 4 cycles/t
; t = 4/8,000,000 sec
; t = 500 nsec
; pre-scaler => 1:2 | 1:4 | 1:8 | 1:16 | 1:32 | 1:64 | 1:128 | 1:256
; If TIMER0 = 0
; 256 counts to overflow and interrupt occurs
; 500 nsec x 256 counts = 128 usec (1)
; If pre-scaler is 1:256
; 128 usec x 256 = 32.768 msec (2)
; max interrupt delay time is 32.768 msec 
;
; If i want 10 msec 
; determine the pre-scaler and value of TIMER0
; let pre-scaler = 1:256
; T x 256 = 10 msec (2)
; T = 39.0625 usec
; 500 nsec x C = 39.0625 usec (1)
; C = 78.125 or 78 counts
; therefore 1:256 @ 78 counts = 10 msec

        LIST            P=16F84
	ERRORLEVEL      -302
	#include p16f84.inc

;************************************************************
; REGISTER DEFINITIONS
;************************************************************
	CBLOCK	0CH		; VARIABLES
		ROLAND0		
		ROLAND1		
		ROLAND2		
	ENDC
;************************************************************************
;THIS PART INITIALIZES THE PORTS AND OTHER PHERIPERALS IN MICROCONTROLLER
;************************************************************************
	ORG 	0000h 		; SET POWER ON RESET VECTOR.
	CLRF	PORTA
	CLRF	PORTB
	GOTO INIT
	ORG 	0004h 		; SET INTERRUPT VECTOR.
	GOTO ISR
	ORG	0005h
INIT
	CLRF	ROLAND0
	CLRF	ROLAND1
	CLRF	ROLAND2
	BSF 	STATUS,RP0	; SET TO BANK = 1.
	BCF	OPTION_REG,5	; SET CLOCK SOURCE TO INTERNAL INSTRUCTION CYCLE CLOCK.
	BCF	OPTION_REG,3	; SET PRESCALER TO TMR0.
	BCF	OPTION_REG,0	
	BCF	OPTION_REG,1
	BCF	OPTION_REG,2
	BCF	OPTION_REG,7	; ENABLE BUILT-IN PULL-UP RESISTORS.
        MOVLW 	h'FF'
        MOVWF   TRISB           ; SET PORTB TO ALL INPUT.
	MOVLW	b'00000'
	MOVWF	TRISA		; SET PORTA TO ALL OUTPUT.
        BCF 	STATUS,RP0	; SET BANK = 0.
;**************************************************************
; ENABLING THE INTERRUPT AT TIMER0
;**************************************************************
	MOVLW	d'250'		; LOAD TIMER0 INTERVAL VALUE.
	MOVWF	TMR0		; SET TMR0 = F0h or 15 MORE COUNTS TO OVERFLOW.
	BSF	INTCON,5	; ENABLE TMR0.
        BSF	INTCON,GIE	; TIMER0 IS NOW READY FOR INTERRUPT.
				; TMR0  STARTS TO DECREMENT AFTER GIE = 1.

MAIN
	; reserved for program lines
	;
	;
	;
	GOTO MAIN
;****************************************************************
; THIS ROUTINE EXECUTES AFTER TIMER0 REACHES 256 COUNTS
;****************************************************************
ISR	
	BCF	INTCON,7	; DISABLE GIE SO THAT NO INTERRUPT ALWAYS OCCUR.
	BCF	INTCON,2	; SET TIMER0 INTERRUPT TO DID NOT OCCUR.
	MOVLW	d'250'		; RELOAD TIMER0 VALUE.
	MOVWF	TMR0
	INCF	ROLAND0		
	INCF	ROLAND1
	INCF	ROLAND2
	RETFIE			; RETFIE RELOADS PC WITH THE SP AND AUTOMATICALLY
				; ENABLE GIE.

	END