This has been resolved, but resolving this issue took quite a bit effort:
I was wondering why my NFC was not working on my custom board. First I thought it would be an antenna matching issue and we made improvements. But it didn't help although the matching looked ok. Then I thought that it must be something with the sw. Unfortanetely I had only nRF52dk in my hands, and (simplified verstion) of the sw worked fine on that board.
In the end the problem turned out the be in the TFM. The KConfig has
config TFM_NFCT_PINS_AS_GPIOS
bool
default y if !DT_HAS_NORDIC_NRF_NFCT_ENABLED
depends on SOC_NRF5340_CPUAPP
help
Two pins are usually reserved for NFC in SoCs that implement the
NFCT peripheral. This option switches them to normal GPIO mode.
HW enabling happens once in the device lifetime, during the first
system startup. Disabling this option will not switch back these
pins to NFCT mode. Doing this requires UICR erase prior to
flashing device using the image which has this option disabled.
NFC pins in nRF5340: P0.02 and P0.03
TF-M configures these pins as GPIOS when the NFCT peripheral is not
enabled in the device-tree of the target.
So, you will need tfm.overlay where you enable the NFC (and make sure that the overlay file is been used in the build). Enabling the NFC in the main application is not enough.
This is quite annoying, for example because UICR documentation says that the NFC pins are functioning as NFC by default.
I don't know how, but this shuold be documented better. And not only in TFM documentation, but in general inside NFC documentation.
