On Sun, 9 Apr 2006, Brooke Clarke wrote:

> Hi:
>
> I have a 16F54 that's being used to divide a precision 10 MHz frequency 
> source down to a precision 1 PPS output by using the 10 MHz as the PIC clock 
> and managing the number of instructions executed. 
> http://www.leapsecond.com/tools/PPSDIV.ASM
>
> If the DC is applied then the clock input connected it works the way the code 
> would suggest.  But if the clock is connected before the DC power is applied 
> strange things happen (LEDs not in the states the code would suggest).  Then 
> manually grounding MCLR gets things back to normal, but I don't want to add a 
> START switch if there's a way around the phantom power problem.

Couple the clock with a small value capacitor and resistor (e,g, 100pF 
and 100 ohms in series) and shunt Vdd and Vss of the pic with 1kohm. 
This assumes that the clock has a suitable amplitude (nearly 5Vpp). The 
dc recovery is done by the bulk diodes in the PIC.

Peter
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