[Coco] Questions about the Coco3 GIME Timer

[jdaggett at gate.net]

On 13 Feb 2007 at 20:48, Robert Gault wrote:

> Gene Heskett wrote:
> ><snip>
> > Do you recall the tick resolution of this timer, John?  If its not
> > in 16ms increments, but finer, as in hsync or even Eclocks, this
> > would be a great way for George to get some higher speeds out of his
> > coco while driving the steppers.  But of course there is only one
> > timer, and there are at least 3 axis's to independently control.  So
> > it looks as if that's ruled out.  Darn.
> > 
>  ><snip>
> 
> Two speeds, either 279.4 nsec (1/8 crystal speed = color burst) or 63
> usec. The service manual is incorrect claiming 70 nsec for the fast
> speed.
> 
>   There is a limit to how fast you want the timer interrupt to
>   function. 
> It can easily exceed any program routine which is part of the IRQ or
> FIRQ routine and if that is true, the technique is useless as
> interrupts will be missed.
>   Put another way, the Coco is not fast enough for many purposes even
>   at 
> 2 MHz.

A 12 bit timer yields 4096 counts. At ~63.7uS per clock period, the total time 
before countdown is complete will be ~261mS. At high speed that is 466,905 
machine cycles. Half that at slow speed. 

With 4096 counts and 279nS you get a time out every 1.142mS. ~2042 machine 
cycles in high speed mode. Again half that in slow speed. 

I would think that SECB would not take that long to process and determine who 
generated an IRQ or FIRQ. One could plow through the SECB code to determine 
how long the IRQ/FIRQ from the GIME takes to determine which source it is and 
what else is done. 

james