Saving a pin with an inverter buffer and a RC circuit

Up to now, I’ve been connecting the ThinkPad keyboard’s TrackPoint RESET line straight to the microcontroller. This takes up a pin and has been preventing me from adding features to the adapter, like Bluetooth or maybe some external serial communication.

Using an RC circuit to create a RESET is not new, but I just thought of it recently and finally got time to try it out. The test circuit below includes some insight into how the circuit works. The capacitor’s value of 20uF is the minimum, it can be higher, or the resistor value can be increased. Either way, increasing either will increase the time required to charge/discharge the RC circuit.

Inverter RC Circuit

Here’s the circuit in action. As you can see, during normal data transfer, RESET line remains LOW. There’s a small jitter after the TrackPoint is reset, but increasing the values of the resistor-capacitor circuit would fix this easily.

Logic Analyzer Test

During planning, I used various online calculators for charging and discharging time. Why bother doing manual calculations. The calculated time is ideal. It can be slightly higher or slightly lower. In this case, the charging and discharging time is slightly higher. Instead of 600us, the time taken to charge is measured to be 633us.

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