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Re: F21c


On Sun, 8 Jun 1997, Robert J. Brown wrote:

> >>>>> "Penio" == Penio Penev <penev@venezia.rockefeller.edu> writes:
> 
>     Penio> On Sat, 7 Jun 1997, Peter Jakacki wrote:
>     >> The timer circuit which interrupts the CPU at programmed
>     >> intervals is the basic building block of embedded systems, it
>     >> is good to see F21 with this timer.
> 
>     Penio> It is not necessary, though.  It you have a tight enough
>     Penio> multitasking loop, you can guarantee that a certain code
>     Penio> will be executed no later than a certain time it was
>     Penio> previously executed.  Then you can treat this as a "clock
>     Penio> interrupt".  Even more, you can think of it as an
>     Penio> "interrupt" in general -- you can attach whatever I/O
>     Penio> polling code you want there.
> 
> Only if the timing is invariant with respect to all possible paths
> thru all subtasks in the loop can you rely on this for sampling analog
> inputs of A to D converters, or strobing outputs to D to A converters.
> If the sampling is not uniform, some very bad distortions can result
> at input, signal processing, or output.  The timing must be uniform
> and known.  The timer interrupt makes this easy.

Strobing needs a timer, that's true.  This is what the analog and/or video
coprocessors are about. 

I was refering more to asynchronos (but time-critical) I/O events and
multitasking.  For those one does not need a timer. 


--
Penio Penev <Penev@pisa.Rockefeller.edu> 1-212-327-7423