NFC T4T response timing SDK 15.3

[Jefferson]

I am using nfc_t4t_lib and I need to send a response 100ms after I receive the NFC_T4T_EVENT_DATA_IND event.  How do I extend the response timeout by sending S(WTX)?  There is a similar post but I don't see the same conditions:

https://devzone.nordicsemi.com/f/nordic-q-a/29168/nfc_t4t_response_pdu_send-returns-nrf_error_invalid_state

If I put a 6ms delay after NFC_T4T_EVENT_DATA_IND, nfc_t4t_response_pdu_send() returns success but the reader failed to receive the data.  Is S(WTX) being sent automatically by the library or do I need to manually send it?  Also, how do I know when I can send the response after a S(WTX) request?  I don't see any event "RX: S(WTX) response".

[Kenneth]

I think it will be a bit difficult to debug this. I assume you have checked that the reader here will expect a response within a certain time window, and you are fulfilling this requirement (e.g. 6ms may not work if it expect something else).

My suggestion here is that you add in addition an unpowered nRF52-DK as a simple sniffer here, by connect an NFC antenna to the unpowered nRF52-DK, and connect an oscilloscope probe as close as possible to the NFC pins on this unpowered kit. You should possible be able to sniff some activity between the reader and tag this way, by measure "something" during the NFC read and response (a change in voltage or similar that indicate the difference in timing), that may indicate that the timing you are trying to accomplish here actually is occurring as expected. Maybe if you have already a working setup you may compare the timing (the voltage will likely be different, but that doesn't matter).

Best regards,
Kenneth

[Jefferson]

I am transferring some data over BLE while the reader is waiting for the response.  Does BLE traffic interfere with NFC operations?  I see the NFC_T4T_EVENT_FIELD_OFF event sometimes when I am transferring BLE data even though I am not moving the device away from the reader.  I am using the PCA10040 board.

[Kenneth]

Hi,

I am not aware of anything no, the carrier frequency is significantly different between NFC and BLE, so I don't expect any problem. What NFC antenna&size have you used? Have you tried a larger antenna for comparison? What is the supply voltage here?

Best regards,
Kenneth

[Jefferson]

I am using the NFC antenna that came with the PCA10040 and using USB power.  Can BLE activity in the soft device block the NFC interrupt for more than 5ms?  If I put the 6ms delay in the NFC_T4T_EVENT_DATA_IND event, it will block the NFC library from sending the S(WTX) and cause the timeout.

[Kenneth]

I have tried to ask around a bit, but I can not find any reason for what you are experiencing. The softdevice will only block the application for the amount of time specified in the corresponding softdevice specification (typically in the order of tens of us). I assume you have checked that if you close down the BLE link and/or use advertisement only you can confirm it is really related to BLE and not the delay itself.