I have a microcontroller with an active low reset pin. This pin is connected to a timer that periodically triggers a low edge and resets the microcontroller. What would be the best circuit that would allow me to also manually trigger the reset pin through an external circuit while the pin is connected to the timer? Shorting the two together would obviously not work.
I assume your timer output is a push-pull circuit?
If you can configure it to be open-drain, then you just need a pull-up resistor on the reset line, and use a switch to short the reset input to ground.
If you can't configure the Timer output to be open-drain, then you just need a series resistor between the timer output (say 1k) and the reset input. Again, the reset switch shorts the reset input to ground.
SW1 can be pressed in order to generate the reset independent of timer. Whenever the
SW1 is pressed first transistor turns on and
RST# net will be pulled low to ground. Whenever the Timer is driving low, the first transistor will turn on and
RST# line will go low and a low level voltage will be presented to the MCU
When either switch or TimerOutput is low, Q3 will have a base current of about 8 mA (assuming VCC as 5 V, Vbe as 0.8 and diode drop of 0.4 V), which will sufficiently to saturate Q3. Q4 too will be saturated so that, the Vol will be less than 0.1 V.