NFC reader detection field?

[Jeff King]

Hello:

I've got a application where I want to detect if the NRF52 is near a NFC reader. I don't need two way communications, but it would be good if the reader could modulate the 13.56 mhz signal (and I can do this custom in hardware if need be).  Using just a oscilloscope and a raw NFC 13.56mhz antenna I can detect energy out to about 10 inches with just a stock reader or Android phone. 

Can the NFC hardware/software be set to detect a NFC field? 

[Kaja]

Hi again, 

yes I work in the technical support team at Nordic. 

So this is difficult to support on, since it means going out of spec, and you have to test, try and probably fail some. 
But I tried to find something that might help you on your way. 

Jeff King said:

Can I get the decoder to trigger/work below 1Vpp?  (if not I imagine I can add a amp ahead of things)

Can I decode one way data? 

 - As I said this is out of spec, the HW might still be able to support it, but you need to adjust the current value driven to the NFC load control. And play around and see if you get somewhere with this. 

CURRENTLOADCTRL

0x430

Current value driven to the NFC Load Control

What you need to take into account is the NFCT peripheral supports communication signal interface type A, and the reader needs to be configured after this. 

endnode said:

It has FIELDDETECT AND FIELDLOST events. You can forget for the moment what is written in the chip specification regarding field strength, simply write small piece of code which activates NFCT peripheral block and check when the events occur. The distance where you get FIELDDETECT or FIELDLOST (depending on direction you move) is the maximum which gets some chance for communication (but still not guaranteed, there might be too noise or other disturbance which will make actual data transfer failing). I would also expect some hysteresis so take the less favorable result.

 - Agree

endnode said:

As far as I understand ISO14443 there is nothing like one-way communication, you always need to "hear" also the other side because otherwise you cannot complete anti-collision on layer 3 and cannot operate "link" on layer 4.

 - So you have to set up so that it does not matter for the sender what it gets back. 

endnode said:

Nordic provide some NFC "tag" libraries which make the FW development relatively easy, however they are indeed designed for standard use case (NDEF Tag Type 2 and 4 communication over ISO14443 TypeA). If you cannot achieve your goal with them (I would anyway invest few days into that by recompiling the examples and test these with your antennas so you do understand how close or far are you from your thresholds) then you can still try to build your own NFC stack on top of HW registers but that is long way to go.

- I also agree with this, see if you can use the libraries to build a useful solution. 

Best regards,
Kaja