[time-nuts] Frequency divider design critique request

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jul 16 01:48:34 EDT 2008

John Miles wrote:
> Right; when the thumbswitches are toggled, the RC integrators will slow down
> the edges into pins 9-11.  Sometimes CMOS parts will latch up or otherwise
> fail to reliably with slow edges -- it probably comes down to the
> "complementary" thing, where both halves of a totem pole can turn on
> erratically during a slow transition.
> I have seen large capacitors used on TTL MUXes for EMI suppression, but
> never on CMOS.  I have a feeling 100 pF would be safer and still adequate.
> Probably no big deal, but if you want to be anal about it, it could pay to
> check the edge-time specs for the HC family.
> -- john, KE5FX
What about the common RC switch debouncer using a CMOS Schmitt trigger 
and an RC filter?

Capacitors of 100nF or more are often used in such circuits.
When using Phillips HCMOS (there is an internal 100 ohm polysilicon 
resistor between the input pin and the protection devices.) an external 
diode (anode to HCMOS input cathode to Vcc) usually suffices to protect 
the input during turnoff. With other brands you may have to add an 
external 100 ohm resistor in series with the input pin.


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