TM4C123BH6PZ: ESD Clamp on Fail Safe Pin Behavior

Hi Bryan,

Thanks for the details here about what and how you are testing and what the results are, very clear to understand the questions at hand and I'll try and shed as much light here as I can to help guide on what to expect regarding the ESD clamps on the device.

Bryan Grant1 said:

Is there any more documentation on the expected functionality of the ESD clamp?

We don't have any more detailed documentation to share on that, but I think I can explain sufficiently on this thread to help you understand this better.

Bryan Grant1 said:

Is toggling the output the best or only way to reset the pin output state? Is there an ESD clamp register or flag we can use?

So the way these ESD clamps are built into the device is to trigger and prevent the VDD rail from being exposed to the high voltage by clamping the I/O pad to GND:

The reason that you are needing to pull the pin to a low state is that you need to connect the I/O path from the internals of the device to ground to be able to reset the ESD clamp and restore the connection to the I/O pad and the external pin.

There aren't any registers or flags that can used for this, but by setting the I/O low you are releasing the ESD clamp at that point and then the pin can toggle between VDD and GND again.

The other way to reset the pin would be to run a POR. This is actually how we do it when testing the ESD clamps for the devices. When doing a POR you would then need to reconfigure the I/O as they will default to inputs without pull-ups except for the GPIO pins with special considerations that are documented in the datasheet. That is probably not what you are looking for here, but that is the other way to reset the pin output state.

Bryan Grant1 said:

Will repeated clamping events eventually damage the pin in any way?

Yes, that is possible for certain.

First off, you would need to ensure the ESD event is within specifications:

Note that there are timing specifications regarding this too - these are ESD events and thus extremely short and prolonged exposure will damage the device. The specifics of the timing can be found by looking up the HBM and CDM specifications for ESD which are industry wide standards.

If you remain within ESD specs however, repeated clamping can still damage the pin as over time it is being stressed for a collective longer total time duration for the life time of the device. Eventually these stressed would lead to damage to the pin.

The risk of this can be reduced by using proper external ESD protection which segue ways perfectly into your last question:

Bryan Grant1 said:

Do you have any recommendations to improve the hardware design? Such as a TVS diode right on the output of the pin to prevent the ESD clamp from triggering in the first place?

We do, and that part is well documented in our System Design Guidelines document linked here: https://www.ti.com/lit/pdf/spma059

Section 3.10.3 specifically covers ESD / EMC protection and our recommend is both a TVS diode and a current limiting resistor. The exact values for these would be system dependent, but we have some part number suggestions within the Design Guidelines and you can always reach out to our Interface team for newer part suggestions too: https://e2e.ti.com/support/interface-group/interface/f/interface-forum

I hope this was sufficient detail to help with your understanding of how these ESD clamps operate and that the resources provided will be useful for you to improve the robustness of the ESD protection for your application. Let me know if anything was unclear.

Best Regards,

Ralph Jacobi

Thanks Ralph. This is all very helpful.