difficulty making external rc osc work

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difficulty making external rc osc work

Peter P.
I made the RC osc work on 16f57. It requires a minimum of 18k as R to work at all. I chose 22k for reliability. With 18k and 47pF at 5V and 30C it ran at about 100kHz (at CLK2 == CLK1/4). I would really like to not thank mchip for not wasting the 3 minutes it takes to put in some decent limits for the R and the C. The 18k minimum R is significantly different from previously listed minimums (about 10 times higher than the one implied for 16c57, the last chip of this class for which I found data in its datasheet).

With lead frame capacitance given as ~5pF for DIP I infer that the RC osc can go about 8 times faster than with 47pF external cap, namely about 800kHz maximum core clock (6.4MHz at CLK1).

-- Peter

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ivp
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Re: difficulty making external rc osc work

ivp
> put in some decent limits for the R and the C

It does seem odd that (1) they'd make that little bit of silicon
on the die so different from 'usual' and (2) omit the R and C
limits diagram text found in other datasheets. Unless the two
points are related - ie the silicon IS different and so the text
had to be changed but they never got around to it

Joe
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Re: difficulty making external rc osc work

Neil
In reply to this post by Peter P.
I used the RC oscillator reliably on hundreds of 16F872-based boards
using a 4.7k resistor and 56pf resistor.  It's been a while, but IIRC
the frequency was around 1Mhz (but don't hold me to that).  Also never
understood why ballpark values weren't put in the datasheet, and
would've liked something more "known", but RC was great for debugging --
I could drop in a much larger cap and slow down the PIC to a crawl, so
LED's would serve as a crude logic analyzer.

Cheers,
-Neil.


On 8/22/2012 10:29 AM, Peter P. wrote:
> I made the RC osc work on 16f57. It requires a minimum of 18k as R to work at all. I chose 22k for reliability. With 18k and 47pF at 5V and 30C it ran at about 100kHz (at CLK2 == CLK1/4). I would really like to not thank mchip for not wasting the 3 minutes it takes to put in some decent limits for the R and the C. The 18k minimum R is significantly different from previously listed minimums (about 10 times higher than the one implied for 16c57, the last chip of this class for which I found data in its datasheet).
>
> With lead frame capacitance given as ~5pF for DIP I infer that the RC osc can go about 8 times faster than with 47pF external cap, namely about 800kHz maximum core clock (6.4MHz at CLK1).
>
> -- Peter
>

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