[time-nuts] Frequency divider design critique request

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jul 15 19:21:09 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
>
>   
Maximum recommended transition time for most HCMOS inputs is 500ns.
However this is more intended to reduce the time for which both p 
channel and n channel devices are simultaneously conducting.
When both p and n channel devices conduct the power supply current is 
increased (to around 1mA for HCMOS gates with a 5V power supply), 
however this current is to low to cause latchup.
There should not be an oscillation problem with slow input transition 
times when using  devices that don't incorporate feedback (eg gates, 
multiplexers).
However if there is excessive noise on the a gate input near threshold 
it may switch on noise spikes leading to an apparent oscillation problem.
For flipflops slow clock transition times may cause oscillation problems 
especially if the clock inputs dont incorporate an internal Schmitt 
trigger circuit.
In synchronous systems employing multiple  flipflop or divider packages 
the maximum clok rise time should be about 2x the individual device 
clock to output propagation delay to avoid incorrect operation due to 
clock threshold variations from device to device.

If you are concerned about the transition times at the multiplexer 
inputs then use a 74HC14 hex inverting Schmitt trigger after the RC 
filters and use pullup resistors instead of pulldown resistors.
Schmitt trigger devices are recommended whenever the inpu transition 
times exceed 500ns.

The RC filter network actually attenuates the dc mux inputs by about 
10%, its better to use the alternate circuit that I posted earlier that 
doesnt attenuate the dc levels at the multiplexer inputs.

The only major concern with large capacitors connected from the 
multiplexer inputs to ground is that should the power supply slew rate 
at turn on or turn off be too large the the multiplexer input protection 
diode current rating will be exceeded and the protection diodes will 
fail. If this is likely then use a pair of external clamp diodes 
connected to Vcc and ground and use a connect a small resistor between 
the capacitor and the CMOS input.

Bruce




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