Time10s - Timer set up and Interrupt based time delay

The main program can be broken up into five main areas:

Start - This part of the program sets up the Input Output Ports as in Flash1. Note that in this case PORTS A bits 0-4 are inputs and Port B are all inputs except bits 1 and 2. Also the weak pullup feature of PORT B is used. This enables a switch to be connected between the port and ground with no other hardware. If the switch is open circuit, the internal resistor "pulls up" the port to HI (or a 1 input).
Setup Timer - This part of the program sets up the internal timer and prescaler to divide the microprocessor clock frequency by 128. The timer then uses this reduced frequency to count. When the timer expires a Time0 interrupt is enabled, which causes a jump to the interrupt service routine, regardless of where in the program the processor is. The Timer is also pre-loaded with Decimal 255 to cause it to expire almost immediately the first time.
Main - This is the main loop. The program sits here in a continuous time delay, doing nothing unless interrupted.
Interrupt - The most important thing is to turn off interrupts and to keep the Interrupt Service Routine (ISR) short (so it doesn't get interrupted itself). The ISR Counts and tests the Ones Counter and turns LED1 on on the odd numbers and off on the even numbers. Every time Ones get to Decimal 100 the Hundreds are incremented and tested and the same goes for LED2. Finally interrupts are re-enabled and the program returns to exactly where it was interrupted from. A word of warning when using Interrupt Service Routines. Normally you should save the Accumulator and Status word and any registers that could be altered in the ISR or the results of the main program will be unpredictable.
LED Routines - These are very straight forward. It makes things a little easier to call LED1ON throughout the program than to set the bits all the time and risk making errors. It also means if you change the hardware, you can globally adjust this in software by changing just one routine.


;**************************************************************************
;
; This Program developed by Peter Hayles for PIC 16C711
; Flashes the LEDs on B1 and B2
; at 2 and 200 timer intervals using the timer
; 
;

;**************************************************************************

#include 	"p16c711.inc"
        	LIST P=16C711

;--------------------------------------------------------------------------
; Variables
;--------------------------------------------------------------------------

RAM		EQU	0x10	

Loop1       	EQU   	RAM+0
Loop2	      	EQU   	RAM+1
Loop3		EQU	RAM+2
Ones		EQU	RAM+3
Hunds		EQU	RAM+4


;--------------------------------------------------------------------------
; Program Code
;--------------------------------------------------------------------------

            	ORG   	0       
            	GOTO  	Start
		ORG	4
		GOTO	Interrupt

;--------------------------------------------------------------------------
; Main Program
;--------------------------------------------------------------------------

            	ORG   	H'10'

Start

; Set up I/O Ports

		MOVLW	B'00011111'		; Port A 0-4=Input
		BSF	STATUS,RP0
		MOVWF	TRISA
				
		MOVLW	B'11111001'		; Port B 1,2=Output 0,3-7=Input	
		BSF	STATUS,RP0
		MOVWF	TRISB
		BSF	STATUS,RP0		; Enable weak pullups
		BCF	OPTION_REG,NOT_RBPU	; Set PortB Pulled up
		BCF	STATUS,RP0
		MOVLW	B'11111001'		
		MOVWF	PORTB

; Set up Timer

		CLRWDT				;Clear WDT and prescaler
		BSF	STATUS,RP0		;Assign prescaler and set it up
		BCF	OPTION_REG,T0CS		;Internal Clock
		BCF	OPTION_REG,PSA		;Prescaler ->Timer0
		MOVLW	B'000000111'		;1:128 Prescaler
		IORWF	OPTION_REG,1
		BCF	STATUS,RP0

; Set initial values	

		CLRF	Ones
		CLRF	Hunds

; Setup for quick Interrupt entry the first time, and enable interrupts		

		MOVLW	H'FF'			;Dummy Value !!
		MOVWF	TMR0
		MOVLW	B'10100000'		;Enable Timer0 Interrupt INTCON,GIE
		MOVWF	INTCON	

Main		NOP	
		GOTO	Main


; This routine gets entered every 1 second

Interrupt	CLRF	INTCON			;Disable Interrupts during ISR
		
		INCF	Ones,1
		BTFSC	Ones,0
		GOTO	ON1
		CALL	LED1OFF
		GOTO	Next1		
ON1		CALL	LED1ON


Next1		
		MOVLW	D'100'
		SUBWF	Ones,0
		BTFSS	STATUS,Z		;If Zero, then =100
		GOTO	Next2
		INCF	Hunds,1
		CLRF	Ones
		BTFSC	Hunds,0
		GOTO	ON2
		CALL	LED2OFF
		GOTO	Next2		
ON2		CALL	LED2ON

Next2		

		MOVLW	H'00'			;Enable Interrupt after a fixed number of cycles
		MOVWF	TMR0
		MOVLW	B'10100000'		;Enable Timer0 Interupt	INTCON,GIE
		MOVWF	INTCON	
		RETFIE



; LED Routines

LED1ON	BCF	PORTB,1
		RETLW	0

LED1OFF	BSF	PORTB,1
		RETLW	0

LED2ON	BCF	PORTB,2
		RETLW	0

LED2OFF	BSF	PORTB,2
		RETLW	0

		END

Contacts:

Email Peter Hayles, Author