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I am trying to make a power-on reset circuit that can also be manually reset. The reset is active-low. When the capacitor is charging, I'm assuming 5v will be present on pin 10. Would this damage the IC?

circuit schematic

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  • \$\begingroup\$ That circuit will not work as you think - The upper HC14 has a totem pole output stage it will not allow the capacitor to predictably charge or discharge. \$\endgroup\$ Feb 25 '21 at 17:30
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No, that will not work. The output will drive the capacitor voltage high or low relatively quickly (maybe a millisecond) and the resistor will do nothing of value. You'd need a gate or to short the capacitor with a switch + small value resistor.

Ignoring the top inverter, this kind of Rube Goldberg R-C reset only works some of the time and has hazards that may not be immediately obvious- the capacitor can discharge into protection networks of the inverter under some conditions (such as a heavy load or short on Vcc), destroying the chip. That can be patched by adding a resistor and a diode but that still won't make it a reliable reset circuit.

I suggest using a proper reset chip that is guaranteed to work and give an adequate length reset pulse (eg. 200ms) under all conditions including brief interruptions of power, extremely slow rise or fall of power, etc.

To specify that you need to know the minimum voltage your circuit will operate correctly at (where can reset be released) and the maximum voltage at which you can be assured that no ill effects will result from reset not being asserted. And the minimum reset pulse width. If there is insufficient overlap to allow the use of a commercial device with its tolerances you may have to rethink clock speeds or device specifications etc. to allow operation at lower voltage.

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    \$\begingroup\$ Thanks for the answer. I'm going to take your advice and use the MAX1232 to generate a reset. \$\endgroup\$
    – Bailey
    Feb 25 '21 at 22:07
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    \$\begingroup\$ That's an all-sing, all-dance solution. Where fewer features (like PB input) are necessary, the ADM8xx series is good and multiple-sourced. \$\endgroup\$ Feb 25 '21 at 22:09
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It may not be damaged immediately, but it also a bad circuit and makes no sense, as the chip will have to charge and discharge the capacitor which is quite large in value.

So the answer is, that is not OK to have capacitors directly on logic outputs.

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