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Atmega168 Interrupt help Answered

I'm trying to write code so I get an interrupt on an atmega168 when timer0 overflows. Right now I've got it set up so it blinks an LED every 8th interrupt(~1 second @16mHz /w div 8 prescaler I think), but it doesn't work, so this is where I came. Here's the code:

#include <avr/io.h>
#include <avr/interrupt.h>
volatile int flag=0;

PORTB = (1<<0);

int main(void){
TIMSK0 |= (1<<OCIE0A);
DDRB |= (1<<0);
PORTB |= (1<<0);

} //End while loop
return 0;
} //End Main

4 Replies

dlkarm123456Best Answer (author)2009-10-07

First of all, if you are running the ATmega68 at 16Mhz with the default configuration fuse settings, the clock to the Timer0 prescaler will be 2Mhz and with a prescaler value of 8 will give you a 250Khz rate into Timer0 and an overflow interrupt rate of 976.5625Hz (it overflows every 256 clocks). Therefore, to get an about 1 second rate, you'll have to use 976 for your 'flag' counter in the ISR. (<em>If you have cleared the CLKDIV8 fuse in the Fuse Low configuration fuses, everything will be running 8 times faster.</em>)<br /><br />Secondly, to enable the Timer0 overflow interrupt, you need to <strong>set </strong>the <strong>TOIE0 </strong>bit in the TIMSK0 register, <strong>NOT </strong>the OCIE0A bit.<br /><br />Finally, and more importantly, you appear to have your LED connected to PB0, but in your ISR, you are always <strong>only setting that pin</strong>, when I think you want to be toggling it (or alternately setting and clearing it)! There's more than one way to toggle a port pin, but the easiest is to do: <br /><strong> PINB |= (1 << 0);</strong><br />Note the use of <strong>PINB </strong>instead of <strong>PORTB </strong>- see the section in the datasheet under I/O-Ports / Ports as General Digital I/O / Toggling the Pin. (<em>It's section <strong>11.2.2 </strong>in the manual I have</em>.)<br />

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steveastrouk (author)2009-09-01

Not my processor family, but do AVRs reset the interrupt service flag on entry to the ISR ? COMMENT your code !

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geeklord (author)steveastrouk2009-09-04

i don't know if they do, but it doesn't sound like anything ive had to deal with..

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steveastrouk (author)geeklord2009-09-05

Get the handbook for the chip. The AVRs are descendants of the 8052, and there are circumstances when their timers need flags manually resetting in ISRs

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