Why are the dedicated NFC pins on "low frequency I/O only" pins?

[david.drinn]

The dedicated NFC pins on the nRF52840 are P0.09 and P0.10. However, these are noted as "Standard drive, low frequency I/O only" pins, with "low frequency I/O" defined as "signals with a frequency up to 10 kHz". 

I don't understand. NFCT has a 13.56 MHz AM receiver and load modulator, and has a 106 kbps data rate, neither of which are anywhere near 10 kHz.

How do you square this?

[RK]

That’s just telling you if you use them as normal GPIO pins instead of NFC they are limited to lower frequency than other GPIOs. That’s quite understandable as, even with the NFC disabled there’s going to be some unusual loading on the pins and you’d expect that to be worse at high frequencies as the disable circuit probably has frequency dependent impedance. 

When used as NFC pins they will respond to NFC frequencies of course. They are just limited in the alternate GPIO configuration. 

[haakonsh]

Absolutely correct! :)

[david.drinn]

I guess when configured for NFC, the pins don't go "thru" the GPIO module, since they're no longer used as digital I/O pins, whereas the SPIM module, for example, still requires GPIO to be configured correctly for the MOSI, MISO, and SCK pins, and so the low frequency I/O recommendation would apply for those while it wouldn't for NFC.

Is that a correct way to put it?