ST25R3916 transferring power but not communicating

[LltWc]

I'm working on getting the ST25R3916 to communicate with an NTAG215 on the target application, and had it wired up with the antenna today. When I send a measure amplitude command, I get 89 = 1.159 Vpp without a tag in the field (the same value I got from the NFC08A1 dev kit).

I have it sending an ALL request every 2 seconds, and when a tag is at the center of the field aligned with the antenna, I measure 49 = 0.637 Vpp when a tag is in the field, but no receive-related interrupt is triggered, just the same TXE and NRE interrupts as is typical during the 2s cycle.

Is 0.637 Vpp sufficient to communicate with an NTAG215? Is there anything else from the ST25R3916's perspective I can measure to debug?

[Ulysses HERNIOSUS]

Hi LltWc,

measuring RFI levels will only indicate if the receiver of the reader could potentially decode a tag answer. The 0.6 Vpp is rather low, typically we recommend something in the 2.4-2.7V area. I don't have really experience with such low levels but I would not expect immediately it to be the root cause for non-operation.

I would start by using a field probe (could be ground loop of an scope probe attached to the tip to create a small loop. If you put this close to the tag you should observe both the reader modulation and the tag answer. Please verify first that the tag is powered and answering. The answer to the ALL request should start 86us after the ALL request. 

If unsure you can also share scope shots of this.

BR, Ulysses

[LltWc]

Thanks - I'm working on setting up a spectrum analyzer now and will post results when I can.

What do you mean by "potentially decode a tag answer"? This is a tag that I know works; what does the measurement represent if not the power harvested by the tag?

I'm surprised that voltage won't suffice since I've measured the X-NUCLEO-NFC08A1 Vpp at 1.6 V without a tag consuming power. I did just do an amplitude measurement while tag reading with the dev board and got 1.02 V, which could definitely contribute. Why would the tag be consuming so much more power from the target device compared to the dev board?

[Ulysses HERNIOSUS · Accepted solution]

Hi LltWc,

if you check the schematics then the signal on RFI is just a feedback of the signal taken from the reader antenna. It relates to the outputted power at the antenna and will also be divided by a capacitive voltage divider to be in a ~2.5V range.

From that voltage you cannot deduce what a tag is seeing or even if it gets powered sufficiently. So, please investigate using a sniffer coil. This will also tell about tag responses, guard times, etc.

The other approach as you mentioned is a spectrum analyzer for checking the matching. As a software guy I will not be able to tell you if a matching is good or bad or how much field strength you can expect from it.

Regards, Ulysses

[LltWc]

Understood, thanks. I tried a different, external antenna and got the expected response, so now the work is in antenna tuning so that the target device can initiate properly.