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: 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 "p16f870.inc"</p><p> ORG 00h GOTO INICIO</p><p> ORG 04h ;Comienzo de los registros para almacenar Instrucciones BCF PORTB,4 BSF PORTB,5 BCF INTCON,1 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 BSF PORTB,4 ; LED ENCENDIDO</p><p>ARRIBA BTFSS PORTB,5 nop GOTO ARRIBA END</p>
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 BCF INTCON,1 RETFIE</p>
Try it, change it to perform any other action and enjoy!