Multiple Antennae or multiple ST25R100/200

[Daniel Studer]

Hello,

We want to implement the new ST25100 or ST25R200 in our project. There we need to have 8 RFID antennae to read and write to 8 NFC Tags as fast as possible. The NFC Tags will be placed on a drawer, so all 8 Tags are in the range of the antennae at the same time.

We already did a prototype 1 years ago with ST25R95 (CR95HF). There we had one stm microcontroller and 8 ST25R95 connected by the same SPI. The used RFAL firmare has only one instance of ST25R95 and we had to startup and initialize every chip one by one to read out the tags. This setup needs time to start every chip and reading the tag. At problems with initializing, resetting this chip was very bad, but the new chips have a reset line.

The 8 RFID antennae and ST25R95 have a distance of 80mm next to each other, so we have a large distance from the first to last chip and antennae. 

The antenna is not an integrated PCB antenna. Every antenna is connected by a 200mm cable to the PCB.

My question now is what will be the better way to implement this with the new ST25R100/200? Is it a better approach to multiplex 8 antennae with one ST25R100 (AN6121) or to have 8 ST25R100/200 connected parallel (SPI) to one stm microcontroller? We have the pins to use SPI (8 CSN lines) so we don't use I2C. 

I don't know, if multiplexing antennae at this distances is possible or even a good idea.

Also considering the firmware, what would be the faster way to read all 8 tags, by multiplexing antennae or with separated chips for each tag?

Does anyone have any suggestions?

Thanks a lot.

[Travis Palmer · Accepted solution]

Hello Daniel,

The R200 is well suited for such an application. If you consider moving closer with the antennas, I would recommend to switch to the R3916B or R300. Both feature RSSI. It can be used to decide which antenna has better coupling (closer distance) when the tag is found by two antennas.  

In General, both approaches are feasible.

Using the multiplexer described in AN6121, each antenna is increasing the parasitic capacitance, which makes it harder to tune the antennas. 

Using 8 ST25R devices is the standard approach which for sure works. Here also one ST25R could drive two single ended antennas. 

I would propose to design a PCB implementing all three possibilities via assembly / solder jumper option.

It would allow you to benchmark all three options:

1.) Multiplexer as per AN6121

2.) 8 ST25R devices driving the antenna direct

3.) 4 ST25R devices driving 8 single ended antennas (AN5592)

BR Travis.