Unused pins and ESD?

[Jim]

What is the best practice for protecting against ESD for unused pins, should I leave them floating or ground them?

I noticed in other posts here that it has been recommend by Noridc to leave the pins floating and disconnect the input buffer using the PIN_CNF register, but it has not been explained what implication this has on ESD.

devzone.nordicsemi.com/.../circuit-design-unused-i-o-pins
devzone.nordicsemi.com/.../unused-gpio-pins

Kind regards,
James

[Kenneth]

Hi James,

I am not aware of any specific nordic recommendations on this topic. I have heard of very little issues in general, both due to internal chip design, but also the fact that the pcb is typically wrapped in plastic casing that prevent direct path externally. In terms of ESD I think it is a good idea to set pins as output low and ground them when possible, this will create a low impedance path to ground, which should be better than leave them floating. In general try to have solid and good grounding, with lots of vias and follow reference layout and pcb guidelines in the product specifications. Then I don't expect any problems.

Kenneth

[Jim]

Hi Keneth

This is exactly what I would normally do for ESD protection; i.e. grounding any unused pins. But this contradicts what Nordic have recommended to other people in the Q&A. I would like a confirmation from Nordic ideally before deciding what to with my unused pins.

[Kenneth]

I see your point, you can safely ground unused pin for improved ESD handling.

Best regards,
Kenneth

[Jim]

My gut feeling is to ground the unused pins and disconnect them in from the buffer in firmware using the PIN_CNF register.

If I do what Nordic have recommend before i.e. leaving the pins floating and disconnecting the pins in firmware, the unused pins could still be connected to other pins via the internal ESD protection diode clamps to GND and 3.3V. Floating pins are more susceptible to ESD affects and thus more likely to transfer noise to other pins via the diode clamp connections possibly.

It would be great if Nordic could confirm this?

[Kenneth]

Jim said:

unused pins could still be connected to other pins via the internal ESD protection diode clamps to GND and 3.3V.

Yes, that is correct, and also why pins should never exceed VDD+0.3V in the maximum ratings table (even if they are configured to be disconnected).

Best regards,
Kenneth