piclist 2002\08\24\195058a >
Thread: Wondering about the avr...
face BY : mike.....@spam_OUTwhitewing.co.uk

On Sat, 24 Aug 2002 14:25:35 -0700, you wrote:

>Mike Harrison <spam_OUTmikeEraseMEspamspamBeGoneWHITEWING.CO.UK> wrote:
>> >AVR Cons
>> >--------
>> High power consumption at 5V
>> Non-consistent interrupt response time (can be a problem with timer ints)
>What causes that?
Variable-length instructions. For example, if you set up a timer interrupt on a PIC, it occurs with
rock-solid regularity. With an AVR there will be jitter - I think up
to 3 cycles, as the interrupt latency is longer if the foreground task
is executing a longer instruction. As AVR instructions can take 1, 2,
3 or 4 cycles. This will only be an issue in certain types of
application, where regular timing off interrupts (e.g. generating
external waveforms) is important.
Although some PIC instructions take 2 cycles, the pipelining behaviour
ensure that interrupt latency from internal-clock-generated interrupts
is consistent. .of course the AVR's multiple interrupt vectors and better
context-saving is MUCH MUCH nicer than the PIC for handling multiple
interrupt sources.
>Several years ago, I tried to prototype an application on a 90S1200 in
>which I needed to take precisely-timed samples of the output of the analog
>comparator. I could never get this to work right and eventually gave up on
>trying to use the AVR. Which is too bad, because in many other respects it
>was the ideal chip for the job.
>-- Dave Tweed

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