Re: [avr-gcc-list] Timer0 interrupt on Mega128

[Jan Menzel]

Hi!
        Many thanks for this hint. Yes, the code was not ideal in that point.
	I now setup a completely new project with just the lines below and - as 
expected - the flags are set correctly. Then I started putting back the 
other code I previously had and found, the the sequence of changing the 
clock divider to 1:2 causes the problem: after XDIV = 0xff the timer0 
flags are not raised anymore. Ok, I carefully studied the datasheet 
again and found: "When the system clock is divided, Timer/Counter0 can 
be used with Asynchronous clock only." (page 37) Is there any workaround 
for this? (the datasheet does not say anything...)

        Best regards
                Jan

On 20.04.2009 18:06, Jan Waclawek wrote:
> Although I don't understand the mechanism why it does not work - is this the 
> only code you have? -  you don't want to do this:
> TIFR |= _BV(OCF0);
>
> Instead, you want to do this:
> TIFR = _BV(OCF0);
>
> The reason is explained in Note 2 under Register Smmary table in the datasheet 
> (and elsewhere):
> 2. Some of the status flags are cleared by writing a logical one to them. Note 
> that the CBI and SBI instructions will operate on
> all bits in the I/O register, writing a one back into any flag read as set, 
> thus clearing the flag. The CBI and SBI instructions
> work with registers $00 to $1F only.
>
> JW
>
>
> ----- Original Message ---------------
> > Hi all!
> > 	I'm trying to generate interrupts using timer0 on a Mega128 but 
> > unfortunately can't see any change in overflow and capture/compare 
> > flags. This is the code I tried (compiled using WinAVR 20081205):
> >
> > TCCR0 = _BV(CS02);
> >
> > cli();
> > while (1)
> > {
> >         if (TIFR & _BV(OCF0))
> >         {
> >                 DEBUG_PORT ^= _BV(DEBUG_PIN);
> >                 TIFR |= _BV(OCF0);
> >         }
> > }
> >
> > I tried the same with TOV0 flag without success. If I read the counter 
> > register TCNT0 I can see the counter counting from 0x00 to 0xff. I 
> > verified in the list file that all registers and bits are correct. 
> > Surprisingly the code works well if changed to timer2.
> >         Can anyone give me a hit why timer0 is not raising this flags in my 
> > code?