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  • Case ID: 290256
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NFC pins configured as GPIO will short together

user116797

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.

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  • 0

    Thank you for the update. Did you try configuring the NFC pins as GPIOs as explained in the case I linked to? It should be the correct procedure to do so for SDK v15.3.0.

    If that doesn't help either, can you provide some more information on the following. Do you see this on one specific custom board or multiple ones? Are you able to recreate it on a Development Kit? Can you check what revision of the nRF52840 SoC you are using. What does the print on top of the nRF52840 SoC say?

    Best regards,

    Simon

  • 0 in reply to user75734
    user75734 said:

    Thank you for the update. Did you try configuring the NFC pins as GPIOs as explained in the case I linked to? It should be the correct procedure to do so for SDK v15.3.0.

    Hey  Simon, we have configured the nfc pins as gpios and they can be controlled by the software.
    The issue is we saw a pulse before the firmware was loaded by the chip.
    How can we know that? The steps we done were:
    1、Pulling down nordic chip‘s reset pin
    2、Provide the power to the hardware
    3、Captured the pulse

    We saw thIs pulse on multiple custom boards.

    On the 209 page of nRF52840_PS_v1.1.pdf, there are some descriptions of the protection circuit for the NFC pins.
    6.14.3 Pin configuration
    NFCT uses two pins to connect the antenna and these pins are shared with GPIOs.
    The PROTECT field in the NFCPINS register in UICR defines the usage of these pins and their protection level against excessive voltages. The content of the NFCPINS register is reloaded at every reset. See Pin assignments on page 575 for the pins used by the NFCT peripheral.
    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.
    When NFCPINS.PROTECT=Disabled, the device will not be protected against strong NFC field damages caught by a connected NFCT antenna, and the NFCT peripheral will not operate as expected, as it will never leave the DISABLE state.
    The pins dedicated to the NFCT antenna function will have some limitation when the pins are configured for normal GPIO operation. The pin capacitance will be higher on those (refer to CPAD_NFC in the Electrical Specification of GPIO — General purpose input/output on page 148), and some increased leakage current between the two pins is to be expected if they are used in GPIO mode, and are driven to different logical values. To save power, the two pins should always be set to the same logical value whenever entering one of the device power saving modes. For details, refer to INFC_LEAK in the Electrical Specification of GPIO — General purpose input/output on page 148.

    So we removed the pull up with 100K to 3.3V on the nfc pin1. And did the same testing steps as above, this time we could not see that pulse any more.
    Is this issue could be caused by the protection circuit in the chip shorts the NFC_PIN1 and NFC_PIN2 together due to the fact that the hardware provides 3.3V on the NFC_PIN1 before the protected field has been loaded?
  • 0 in reply to user116799

    Hi

    Yes, this does indeed seem to be what has caused the pulses you've been seeing, as the protection circuit is there to make sure the device won't be damaged by high voltage fields. This should not be necessary to implement when using the pins as GPIOs though.

    Best regards,

    Simon

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