piclist 2001\02\16\165136a >
Thread: ETS on a PIC ??? Yes - it can be done.
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face BY : Dan Michaels email (remove spam text)



Drew wrote:
>>Sounds about right 20:4 = 5:1 = 50khz:10khz. You would need to select
>>AD clock = 8 Tosc.
>>=============
>
>Yes sir, works well.  Interestingly, I'm curious as to which frequency I
>would select at 20MHz.  The datasheets say that 32Tosc violates the minimum
>A/D time, but the only option for 20MHz, then, is RC, in which case the Tad
>is 2-6us (4us typ.)!  I'm not sure what's happening here, and I haven't
>tried it at 20MHz to find out.  Any info?
>

Umm, my datasheets for both 'C76 and 'F876 show 32Tosc is ok at 20 Mhz.
8Tosc does violate min Tad. Forget RC, you can find a way to do it using
the clock.

BTW, using 'C76, you can get reliably sampling right at a 50 khz [~20
usec] rate with 20 mhz xtal - although there may be a slight LSB resolution
loss - at least according to the datasheet. Looking at it on the screen
looks fine however.

OTOH, you cannot go quite this fast on the 'F876, due to its having
a 10-bit A/D. However, you can slow down to 40 Khz [~25 usec] sampling,
and all is ok.
=============


>>Ha - noooooo way with a PIC76, 2.5 Mhz is tops, although 18Cxxx w/10mhz
>
>Exactly what's the limiting factor here?
>

How fast you can execute an instruction. You will notice, with a 20
mhz xtal, sampling at 2.5 mhz allows you "exactly" 2 instructions to
do everything --> ie, linear code.
===========

>>xtal with PLL enabled might do 5Mhz. Scenix will do 20 Mhz all day long.
>
>Hmm, sounds like a project for the WFT (World's Fastest Testbench) :)
>

WFST - World's Fastest Smallest TestBench. Been there, done that,
too :).

- dan

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