Introduction: Interrupt Routine in Assembly Language Using 8-bit PIC

About: Hey there! I'm a Venezuelan DIY Enthusiast. i hope my projects can be used by any to make their life easier. Have fun always people.

Hello reader. This is one of the most basic and important instruction ever made for Microcontrollers low level programming. Talking about the external interrupts routine in Assembly Language for PIC. Today I'll present a blueprint for the development of an experimental basic circuit and show you the coding to learn how to use this simple but usefull routine.

Firstable, i'll assume you all know basics about PIC, Assembly and the whole working enviroment (MPLAB, PicKit 2 v-2.55). If you dont, just ask.

Step 1: Step 1: Circuit Schems and Materials.

Circuit is pretty simple: I use a 8bit/28pin PIC 16F870 but i recommend 16F84A because its unexpensive and enough to learners. Also, i use 2x LED's 1/4W connected to RB5 & RB6, 1x 20MHz Crystal XT Oscillator and 3x 330 OHM's Resistors: two to control the LED's current and one for the external impulse.

Step 2: Assembly Coding

Next step: The coding thing. Some people consider a good practice to perform a "Blink" to test the PIC Outputs. Asuming that everything is ok and we're having an stable 5VDC source, let's "Burn" that microcontroller with the code attached.

<p>list p=16F870<br>include	""</p><p>		ORG		00h
 		GOTO		INICIO</p><p>		ORG		04h			;Comienzo de los registros para almacenar Instrucciones
		RETFIE</p><p>		ORG		30h			;Final de los registros para almacenar Instrucciones</p><p>INICIO	BSF		STATUS,5		;Cambio de Banco a 1
		BCF		STATUS,6		;Cambio de Banco a 1	</p><p>		BCF		TRISB,5			;RB5 Salida
		BCF		TRISB,4			;RB4 Salida
		BSF		TRISB,0			;RB0 Entrada		
		BCF		STATUS,5		;Devuelve al Banco 0</p><p>		BSF		INTCON,7		;Habilita las interrupciones
		BSF		INTCON,4		;Habilita Interrupciones externas RB0/INT </p><p>		BCF		PORTB,5			; LED APAGADO

This interruption routine works with and external high impulse set on RB0 pin. When the INT its performed, both of the LED's are going to change their status.

Let's breakdown the code:

Configuration Routine just setup the ports we're going to use as Inputs and Outputs

		BSF		STATUS,5		;Bank 1º						BCF		STATUS,6		;Bank 1
		BCF		TRISB,5			;RB5 Output
		BCF		TRISB,4			;RB4 Output
		BSF		TRISB,0			;RB0 Input
		BCF		STATUS,5		;Bank 0

Then enable the INTCON Register and set the portb status.

		BSF		INTCON,7		;Enable Global Interrupt Routines
		BSF		INTCON,4		;Enable External interrupts RB0/INT			BCF		PORTB,5			; LED OFF						BSF		PORTB,4			; LED ON

Finally the INT routine which change PORTB status and clear the INT External interrupt Flag.

<p>	BCF		PORTB,4<br>	BSF		PORTB,5

Try it, change it to perform any other action and enjoy!