AVR-Chat

On Mar 24, 2009, at 1:36 PM, David VanHorn wrote:

> I frequently end up using a bunch of timers in a 1mS ISR, that are
> decremented to zero by the ISR, and set by some other task.
>
> The code would end up looking like this:
>
> if (0 < Timer1) Timer1--;
> if (0 < Timer2) Timer2--;
> and so on.
>
> Is there a more efficient way to handle those?
> Generally they will be living in SRAM, but I wouldn't rule out them
> living in registers for small apps.

I write same thing, but differently. Guessing the compiler generates  
the same code as you have written:

if( timer1 )
	timer1--;
if( timer2 )
	timer2--;

Have seen others write this way but I'm certain its slower unless you  
don't know in advance how many timers are needed:

for( i = 0 ; i < number_of_active_timers ; i++ )
	if( timer[i] )
		timer[i]--;

When I need a coarser but longer running timer:

if( timer_100ms && (--internal_timer_100ms == 0) ) {
	timer_100ms--;
	internal_timer_100ms = 100;
}

The "if" is evalated left to right and stops once the expression must  
be true or false. Perhaps clearer to write this way which should  
generate exactly the same code:

if( timer_100ms ) {
	if( --internal_timer_100ms == 0 ) {
		timer_100ms--;
		internal_timer_100ms = 100;
	}
}

When timer_100ms reaches 0 then internal_timer_100ms should be 100,  
initialized for the next use.

But more often I need to count off minutes or seconds, so I make an  
internal timer that counts down to seconds, and then place my  
software seconds timers in that group, Perhaps I need minutes as well  
so I put another internal timer counting 60 seconds before handling  
my minutes timers.

It looks like it takes a lot of time. Perhaps it does if one must  
count in ms resolution.

If one needs fine resolution consider running a hardware timer with a  
divisor and count interrupts on overflow. When your event occurs you  
can read the hardware timer and the software overflow interrupt  
counter to know the time very precisely.

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.