> I've been looking at the Weeder Frequency counter on the PIClist archives.
I called Weeder today and they no longer do the kit or any part of it.  2.
It looks like the original ran on a 4MHz crystal;

            >if we used a 20MHz clock, couldn't we increase the max
frequency capability?

Careful - within reason the achievable maximum speed is set by the maximum
toggle rate of the prescaler/divider. This is a hardware spec which is
essentially inependent of cpu clock speed capability. AFAIR, in some earlier
processors LOWER spec CPUs were capable of higher prescaler clocking rates.

AFAIR the original 16C54's used for this application were capable of 50 MHz
while some chips with the enhanced core had a lower max prescaler rate.

A quick look at an arbitrarily old 16F84 data sheet shows a Thi and Tlo with
prescaler input of 30 ns and 20 ns for a max frequency of 20 Mhz if your
input wave was suitably asymmetric or more safely 1 / ( 2 x 30 nS) =~ 16
MHz.





                            Russell McMahon

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