nRF5340 NFC/GPIO pins during reset

[Samuel Carey]

During reset and initialization are the NFC pins configured as GPIO or as NFC?

If they are in NFC mode can you clarify if they have any special behavior on boot (pulls, impedance to ground, etc)?

I have found a note that the nRF52832 NFC Pins default to NFC but it is unclear if this is true of the 5340.

[Amanda Hsieh]

Hi Samuel, 

When UICR.NFCPINS is at its documented reset value (0xFFFFFFFF), PROTECT is NFC, the NFC protection path is enabled, and the pads are not usable as normal GPIOs. When NFCPINS.PROTECT=NFC, a protection circuit will be enabled on the dedicated pins, preventing the chip from being damaged in the presence of a strong NFC field. The protection circuit will short the two pins together if voltage difference exceeds approximately 2V. The GPIO function on those pins will also be disabled. See PS nRF5340 NFC / Pin configuration 

Regards,
Amanda H.

[Samuel Carey]

You say "approximately 2V". How approximately? Is 1.8V close enough that it may sometimes short?

[Samuel Carey]

Similarly, are the NFC pins effectively floating inputs? If I have an external input with a weak pull down, will the NFC defeat the pull-down and initialize high?

[Amanda Hsieh]

When UICR.NFCPINS.PROTECT=NFC, the PS describes a protection path that ties NFC1 and NFC2 together if their differential voltage exceeds ~2 V, and states GPIO is disabled on those pads (see NFCT — Pin configuration); the PS gives no tolerance on that ~2 V, so 1.8 V differential cannot be assumed safe. Vsense (~1.2 Vp differential, high-Z in sense mode,see NFCT electrical spec) is carrier/field detect, not the same as that clamp. With PROTECT=NFC, do not treat the pads as GPIO inputs for strapping; external pulls are not a supported, specified way to set a digital level (and they still electrically load the NFC pad). For GPIO or reliable digital behavior, use PROTECT=Disabled (and accept loss of NFC antenna + protection per PS) or another GPIO. See NFCPINS and NFCT — Pin configuration.

[Samuel Carey]

Thank you for the reply. So to clarify: if I set the PROTECT to disabled and then power cycle the 5340, will the pins reboot as exclusively GPIO or will there be a short time on boot that they will use the NFC inputs?

[Amanda Hsieh]

The NFCPINS register content is reloaded at every reset (see the nRF5340 PS NFC).  This means that if you have set NFCPINS.PROTECT = Disabled (stored in UICR), after a power cycle, the register will be reloaded from UICR with your Disabled setting. The pins should come up configured as GPIO. 

[Samuel Carey]

I'm glad the register settings store and the pin will be GPIO once the register has been loaded.

So to make very clear: During boot, before the register is loaded, do the NFC pins behave like all other GPIO in that they are inputs with the input buffer disconnected until configured otherwise?

[Amanda Hsieh]

No, the NFC pins do not behave like ordinary GPIOs at boot. nRF5340 NFC pin configuration description states that the NFCPINS register in UICR is reloaded at every reset, meaning the NFC protection/mode is re-applied at each reset.