Force the state of floating digital input to particular level

by fest   Last Updated December 06, 2018 15:25 PM

Given: a microcontroller in QFP package. A pin is configured as digital input, no pullups/pulldowns. The physical pin is not connected to anything else, besides the pad on PCB.

Is there a way to force the state of the digital input (high/low) without physically touching the pin? Electrostatic field? Heating the IC to increase leakage current in clamping diodes?

To those wondering why: suppose you are working on a complex system. In the previous HW revision, there was a button + pullup/pulldown resistor. The button and resistor was removed in the next hardware revision. The firmware team did not remove the code which was processing the input, just initialized the system to desired state. Since the software still reacted on long button press (not just rising/falling edge) and did not have sufficient logging, this led to very hard to debug issues down the road (requiring multiple man-weeks of debugging to find the root cause).

What I'm looking for is a relatively quick test to see if there is no software reaction to "supposedly unused" input pins changing state. Auditing the full software running on the MCU is non-trivial task (>1M LOC).

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