Different performance ST25R3911 Nucleo vs ST25R3911 Discovery

[mdesc]

Hello ST25 community

I'm working on a research project using ST25R3911B to design the reader. We developed our own antenna and tested power transfer with the ST25R3911 discovery kit. On the other side, I used a STM32L4 nucleo plugged to the nucleo ST25R3911 NFC05A1. At some point, I encountered reading problem.

My tag has an energy harvesting chain in parallel to the ST25DV64K (30mW to the charge). With the nucleo kit I was measuring 100% of inventory and read block correct at 3cm, and with the disco kit that distance was more 4 cm.

The both kits have exactly the same filter, everything has been measured. The antenna is the same. But the nucleo kit seems to be less performant than the disco kit.

I've tried to shield the STM32 on the nucleo but it hasnt really improved anything.

At the point, we will do everything with the disco kit which meet our requirements (which is not the best as it misses lots of outputs on the MCU - no spare SPI or IOs).

We don't understand why the nucleo kit isn't as good as the disco kit. We thought our system was bad but in fact with the disco kit it was all ok.

Any idea why nucleo STM32L4+nucleo NFC05A1 isn't as good as ST25R3911B disco ?

thanks a lot

[Travis Palmer]

​Hello mdesc,

first of all, thank you for reporting this.

We have not been aware of this. Since the DISCO and NUCLEO have different antennas we expect a different performance.

As you mentiond, to use the same antenna and matching, it should also give the same performance.

When talking about same matching, did you also populate the same capacitive voltage divider and are you using the same software + register settings?

For a successful reader operation two parameters are important.

First is the output power. It should be strong enough to power the tag in a desired distance.

The second one is the sensitivity of the reader. Typically with our ST25R391x ICs the sensitivity should not be a problem.

For a reader device, this means that it could either be power limited or sensitivity limited.

To find out, which one is the limiting factor, you can place a ISO calibration coil (or any pickup coil) connected via a high impedance probe to the oscilloscope close to the tag.

When moving the tag out of the reading area, the reader will stop reading the tag.

First case:

If the tag does not reply anymore to the reader command (REQA,…) then the device is power limited. (this is the typical case). Be aware that the tag can also stop replaying to later commands (RATS,…) due to the fact that more power is required by tag for computing operation.

Second case:

If the device is sensitivity limited, the reader would send his command, the tag replies, but the reader does not understand correctly and does not send any further commands.

This could also be seen using a protocol spy or digital (SPI) analyzer. You should then also check the exchanged commands.

The setup of the ST25R3911B-DISCO is a bit more ideal then the X-NUCLEO-NFC05A1. The DISCO has a low ohmic positive and negative power path (power and GND planes in the inner layer of the PCB). The NFC05A1 features some connectors, jumpers and narrow traces on the Nucleo64 MCU board. This additional resistance could either lower the supply voltage slightly or can also cause additonal noise in the receiver. This noise in the receiver could then reduce the sensitivity of the NFC05A1 system => the reader has less read range due to sensitivity limitation. Causing 25% reduction in read range seems to me a bit too high in this case.

I would therefore ask you to investigate if the device is power or sensitivity limited.

Could you please also share more details about the setup you are using? Antenna size, matching impedance power supply of the board(s)?

On upper and lower edge of the ST25R3911B we have two additional connectors P202 and P203 which feature one I2C and one UART interface.

This connections should also be useable as IOs.

Many Thanks,

BR Travis

[mdesc]

Hello Travis,

I can tell that it's a sensitivity problem, not a power problem. I've tested 2 nucleo kit and 2 disco kit. My reading distance while transferring 30 mW to my tag is different.

Longer with the 2 disco kit et shorter with 2 nucleo kit. As if it was a EMC or noise problem.

Power transfer is OK with all kits :) The ST25R3911 is really good for that.

All capa trim, capa dividor bridge, output filters and antenna are identical.

I think maybe it could be a conjuncture of different routing, connectors, noise etc..

Thanks

melanie