Critique my circuit for chips with RESET pins

[JerryH]

I'm using a 74LS192 DECADE counter which has a RESET pin on it. I've come up with a simple circuit that seems to work just fine for now:



Should I be concerned that my switch effectively shorts out the capacitor? We're only talking 5 volts at most here, which pretty low capacitance, but still....

[MrAl]

Hi,

The 10uf cap will cause somewhat large surge currents through the switch when it is closed. This would most likely shorten the life of the switch.
Do you really need the 10uf cap though? When the reset switch is closed even if it bounces the first 'high' logic level that reaches the chip will reset it and it will get multiple reset 'high' pulses as the switch is released but who cares. The chip functions normally once the switch stops bouncing.

The only time this might matter is when a relatively high frequency clock is fed to the counter chip, where it would start to count in between switch bounces. However, with the cap on the input like that that also means that the input to the chip goes through a region of input voltages where the noise immunity is going to be very bad so there is still the chance that the chip will count and reset, count and reset, etc., when it is not supposed to (depending on clock frequency and availability).

If this does happen to be a critical application, then the best thing to do is to debounce the switch, but do it in a more reliable way such as use a flip flop and a SPDT switch or even two SPST switches. If that's not possible, then perhaps a Schmitt trigger logic inverter on that input with cap and resistor on the input of that with input pulled high. A small resistor in series with the switch limits current also.

[psycho]

I do not know if you are willing to go a different route altogether, but the following may be of interest to you:
http://www.maxim-ic.com/app-notes/index.mvp/id/287

Hope it helps,
Kevin

[Dean Huster]

Be darned sure that you disable the parallel load input. Although you can sometimes depend upon an unconnected TTL input to float HIGH (a poor practice), make sure that you disable them with firm connections to +Vcc or GND. I've been burned many times, specifically with the 74LS192 and 74LS193 when those asynchronous inputs were left floating accidentally. You could get away with that crap with the ancient 7490 -- not the little-bit newer '192/193.