Wednesday, June 02, 2010

atmel atmega32 avr-libc interrupt example

Edit: Fixed a bug in the example code where I tried to do _BV(foo|bar), which doesn't work at all. Changed them to (_BV(foo) | _BV(bar)). Also added a #define for _BV in case you don't already have it. I think this code is correct, but I haven't tried it.

I wanted to read a pair of quadrature encoders with my atmega32, so I needed to enable the INT0 and INT1 pins. Incidentally, if you don't mind throwing away half the resolution of your encoder, you can get away with using only one interrupt per encoder. (You put one of the pair of sensors on an interrupt, and the other one on a regular GPIO pin. You get interrupts half as often, and the other sensor tells you which way you moved).

Here's the code I used. The compiler complained about using obsolete avr-libc identifiers, but it seems to work fine.

#include <avr/interrupt.h>
#include <avr/signal.h>

// Just count the number of interrupts we see
int interrupt_counter = 0;

// I got lots of random resets until I added this handler.  So apparently you shouldn't enable
// an interrupt unless you have a handler installed.

And then in my init code:

#define _BV(x) (1 << (x))

// setup for servicing INT0, INT1 interrupt pins

// generate an interrupt on any logical change
MCUCR |= _BV(ISC10);
MCUCR &= ~_BV(ISC11);

MCUCR |= _BV(ISC00);
MCUCR &= ~_BV(ISC01);

// enable the interrupt pins
GICR |= _BV(INT1) | _BV(INT0);

// Set d2 and d3 to be inputs
DDRD &= ~(_BV(PD2) | _BV(PD3));

// Pullups off
PORTD &= ~(_BV(PD2) | _BV(PD3));

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