On Tue, Jun 17, 2008 at 5:33 PM, Tom=E1s =D3 h=C9ilidhe <toe@lavabit.com> w=
rote:
>
>
> James Nick Sears wrote:
>> And Olin, have YOU even looked at the datasheet?  There is some info
>> yes, but it's not immediately apparent how to select RC values or the
>> range of appropriate frequencies with an RC to someone who doesn't
>> have some prior knowledge of how RC oscillators work.
>
> I know about RC circuits, about the time constant, about exponential
> decay and all that. What I don't know is how the RC values actually
> yield a specific processor speed. I just need an equation, something like:
>
>    Processor Speed =3D 7.56 * Time Constant
>
> And then my next question would be what capacitor value I should use,
> given that I can have a really small one with a really big resistance,
> or a really big one with a really small resistance.

OK, to quote Olin, this is in the datasheet, specifically look in the
text at the bottom of Figure 4-5 "External RC Modes"

I believe (this isn't clearly stated in the datasheet AFAIK) that Fosc
=3D 1/RC, where Fosc(max) for the chip is 20MHz.  The actual instruction
clock is 1/4 this speed, meaning at most you can run 5 million
instructions / second.

However in that figure, MChip recommends C > 20pF and R > 3k, which
yields a max frequency of 16.666MHz.  And as others have mentioned,
pushing the limits of this may not be the most stable arrangement,
hence the recommendations of external oscs, crystals, and ceramic
resonators.

But hey, if you've got the parts, give it a shot and see if it works.

-n.



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