> -----Original Message-----
> From: D Yates [mailto:piclist@NERDULATOR.NET]
> Sent: 05 December 2003 07:09
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC] : Choice of PIC
>
> Sounds like any chip with more than 8 I/O pins could do it in
> some form or
> fashion.  200khz of bandwidth (4x50khz) takes up only 4% of
> the available
> instruction cycles on a pic running at 20mhz, leaving quite a
> few cycles to
> sample your input, do some math, then generate PWM on the output.

That sounds very optimistic to me, not to mention a somewhat novel method of
calulating CPU usage.  You have just 20us to run an 8 bit counter at a rate
of 256 * 50Kz = 12.8MHz and perform the comparisons for each channel and set
clear the appropriate port pins.

At 20MHz this is 100 instructions.  A 4% CPU usage suggests you can execute
the whole 4 channle PWM in just 4 instructions.

In fact, even running one 8 bit channel at 50KHz purely through software is
simply not possible.  The CCP module tops out at 78kHz with a 20MHz clock
for 8 bit resolution which is OK, so I'd suggest that a PIC with 4 CCP
channels is required for this task (or two smaller PIC's with two channels
each).

Regards

Mike





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