AVR-Chat

Thanks for the reply.

I wanted a tick every (about) 10ms so I got 78 ticks, subtracted that from the overflow value of 255 and came up with the 177 which I used as my start value.

So I think what you're suggesting is that I set 78 in a compare register and then have my isr get called on compare.  That's fine too.

Am I on the right track as far as the registers I've set?

Thanks.

----- Original Message -----
From: "Jim Wagner" <wagnerj@proaxis.com>
To: AVR-Chat@yahoogroups.com
Sent: Saturday, September 24, 2011 3:23:19 PM
Subject: Re: [AVR-Chat] Setting timer0 on Atmega48

Hello, Phillipe

First, signal is deprecated for use in gcc.

Second, 8MHz divided by 1024 gives a tick every 128uS. You cannot get  
1ms from that,  because it would take 7.8125 clock "ticks".

Third, by far the easier way to count a specific number of clock ticks  
is to set the compare value in a compare register and have the counter  
reset. You can then count, additionally, on the overflow interrupt. No  
counter reloading is needed.

Jim Wagner
Oregon Research Electronics

On Sep 24, 2011, at 3:05 PM, Philippe Habib wrote:

> This should be a simple thing but I'm having trouble getting it to  
> work.
>
> I want a regular interrupt every 10ms while using the internal 8Mhz  
> oscillator. I'm using AVR studio with the default (GCC?) compiler.
>
> Can someone let me know what I'm doing wrong?
>
> I try to set the various timer registers like this:
>
> TCCR0A = 0x00;
> TCCR0B = 0x05;
> TCNT0 = 177;
> TIMSK = 0x01;
>
> What I'm hoping for here is a timer that will use the div1024 start  
> at 177 and set off the timer int when it overflows at 255.
>
> Then I turn on interrupts
>
> SEI();
>
> Here is my ISR:
>
> #if __GCC__
> SIGNAL(SIG_OVERFLOW0)
> #else
> #pragma vector=TIMER0_OVF_vect
> __interrupt void TIMER0_OVF(void)
> #endif
> {
> uccounter++;
> TCNT0 = 177;	// reset the timer
> return;
> }
>
> uccounter is a global unsigned char. I just use it to toggle a port  
> when its 0.
>
> Am I missing something and is there a library call that makes  
> setting all this easier?
>
> Thanks.
> 



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