At 04:48 AM 1/8/02 +0100, you wrote:
>I am wondering, have anyone tested max speed for a polled program
>decoding an rotary encoder ? Would an interrupt driven program
>do better ?
>
>PIC16F877 @ 20 MHz. Approx 50 instructions in the active loop
>Encoder is 100 pulses/rotation, quadrature decoding (400 flanks/rotation)
>I have reached around 2000-2500 rpm. 3000 rpm gives overspeed error.

Let's see, 3000 rpm is 60 rps, with 400 counts per revolution, that is
about 24k counts per second.  With some code tweaks, I think you could
easily be doing more than 5x better than that as I got at least that many
counts per second out of a 4MHz part before tweaking my code a bit more and
using it on a 20MHz part - but I didn't test its performance after
that.   I think polled mode has a _slight_ edge if you aren't doing much
else, I do recommend doing interrupts on encoders if accuracy is important,
just make sure that all your other interrupts are kept to a minimum.

Here are several examples you can look over to compare with your code:
http://www.piclist.com/techref/microchip/qenc.htm

Jeff

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