NFC pins configured as GPIO will short together

[Eric Berseth]

The NFC pins of my nRF52840 MCU have been set to be used as GPIO pins through the NFCPINS register.  I can toggle the GPIO lines just but I find that after power on that the two NFC pins seem to be shorted internally together, for about 1 millisecond.  How can I prevent this from happening because the NFC/GPIO pins drive some critical circuitry that can't tolerate the apparent 1ms pulse.

[Simonr]

Hi

This sounds strange. Can you explain/show how you have configured the NFC pins as GPIOs exactly? What SDK and SDK version are you using for your project? For the nRF5 SDK the correct procedure is described in this case. Please note that if using the Development Kit you need to modify the DK with some extra resistors as explained in the DK user guide.

Also, please note that the NFC pins are not suited for high drive/freq. operations, and they are recommended to use at standard drive/low freq. in the pin assignments.

Best regards,

Simon

[JackLan]

Thanks Simon,

We used the following code to configure the NFC pins as generic IOs.

        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
   
        NRF_UICR->NFCPINS &= ~UICR_NFCPINS_PROTECT_Msk;
    
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
        NVIC_SystemReset();

We hold the mcu reset pin and then provide the power to the hardware. We could capture a pulse which is in the following picture. 

https://drive.google.com/file/d/1Sufa-ArswiFRmOJdXAPH-mhqfOxZ2j99/view?usp=sharing

 Actually, we can get the same pulse even if we did not hold the reset pin.

And the versions of SDK / Softdevice are  7.0.1 and S140.

[Eric Berseth]

Futhermore, NFC1 is in configured as an input and pulled up with 100K to 3.3V and NFC2 is output to a 100K pulldown.
We are using the 15.3.0 SDK, I believe.

[JackLan]

Thanks.