Hi,
We have a PCB with a micro on it. It receives a digital input from a cable from a different PCB, which gets plugged in to the micro's PCB.
How may we implement ESD protection for the micro?

 

tvs diode

 

A TVS diode may not have the accuracy necessary to protect the input.

A small resistance (e.g 1kΩ) in series with the input, along with small Schottky diodes, one from the micro's input (anode) to the micro's V+, and one from the micro's input (cathode) to ground should work.
That should protect against both plus and minus transient voltages.