use NFC and PCD(such as adafruit_pn532) at the same time and the same antenna

[fx5700]

Hi,

My system has both PCD card reader (such as adafruit_pn532) and nrf52840 internal NFC. How to make them work together and use the same antenna?

1.when i connected the NFC pin (NFC IN1 and NFC IN2) to the reader antenna Rx path, the field can not trigger a NRFX_NFCT_EVT_FIELD_DETECTED even， because the Rx path field strength have been limited to under 2Vpp

2. but  when i connected the NFC pin directed to the antenna coil and matching well， it can generate  a NRFX_NFCT_EVT_FIELD_DETECTED even , and the nrf52840 NFC function working well, but my RFID system (such as adafruit_pn532) can not word, because when the reader(PN532) open antenna field , the field strength voltage turn up to 20V ,this will cause the Nrf52840 NFC pin voltage to high,and can not work any more.

3. i have try to use a different antenna of them, but when the two antenna at the same window, this make non sense。

How to deal with this problem？

thanks！

[ketiljo]

It's not possible for the tag and the reader to share the antenna. As you say, the nRF52840 will limit the voltage swing, as it should and when connected as in #2, the voltage becomes too high and will fry the nRF52840 NFC pins. 

You have to use two antennas and in two different physical locations. Or you can try to use an analog switch to select _either_ the reader, _or_ the tag to connect to the antenna.