piclist 2001\02\16\165136a >
Thread: ETS on a PIC ??? Yes - it can be done.
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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