NFC Field while Device is Off

[bdub94]

Hello, if an NFC field is applied to the nRF52832 (with NFC coil) while the device is off, is there a chance of damaging the chip?

[Naeem Maroof]

Hi,

NFC uses two pins to connect the antenna. These pins are shared with GPIOs, and 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.

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 GPIO function will be disabled on those pins as well.

When NFCPINS.PROTECT=Disabled, the device will not be protected against strong NFC field damages caught by a connected NFC antenna, and the NFCT peripheral will not operate as expected, as it will never leave the DISABLE state.

See more here: NFCT Pin Protection     Battery Protection

[bdub94]

What kind of protection hardware is in place?

Can you confirm that the chip is protected when the device is completely off (e.g., battery disconnected)?

Is there a maximum voltage and current limit specification for when the device is completely off?

[Naeem Maroof]

If the antenna is exposed to a strong NFC field, current may flow in the opposite direction on the supply due to parasitic diodes and ESD structures. If the battery used does not tolerate return current, a series diode must be placed between the battery and the device in order to protect the battery.

Max values of Imax (input current on NFC pins) is 80mA, and max Vswing (Diff Input voltage swing on NFC pins) is VDD as per the electrical specification.

[bdub94]

Can you clarify how this applies when the device is off? I understand the the ESD structures may cause the NFC voltage to backflow to VDD, even when the device is off. Is this only situation I should consider?

Would I need to use external protection circuity (e.g., Zener / TVS diode) if the device off and in the presence of a strong NFC field?

[Naeem Maroof]

Hi Brian,

I have asked internally regarding the specific question of NFC protection while the device is off. I will let you know accordingly.

[bdub94]

Any update to this?

[Naeem Maroof]

Hello Brian,

Apologies for the delayed communication.

I have received below response regarding this:

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The reset value of 0x20C (NFCPINS) is 0xFFFFFFFF, so NFCPINS.PROTECT = NFC, hence the best clamps for strong NFC fields are enabled.

In simulated case, when all supplies are zero with large magnetic field applied to the NFC pin, it is observed that the NFC protection cell still works by limiting the swing on the NFC pin. This was done for other SOCs, but the protection cell for nRF52 should be similar to those chips, and hence should work.

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We also suggest customer to do the testing and see how large fields could be tolerated as compared to their real use-case.

Regards,

Naeem