My project includes 4 RFID-RC522 that have to be read at the same time by one single Raspberry Pi, I disable the pin CE0, CE01 that are usually used for slave select and I use instead regular GPIO pins set as outputs. I put them in low state to read/write and them put them back in high state after the data transfer. CLK and RST are shared among the 4 readers.
I first built this setup using breadboard and short jumper wires and it was working fine. Now I have built a more durable setup with longer wires because the RFIDs have to be placed at a certain distance to each other. The total chain, from master MOSI, through the slave RFID readers, to master MISO, is 3-4m long. But it is not working anymore and I cannot detect any tags...
I have already gone through several forums and these are some ideas I have found:
- lower SPI bus speed: which baud rate would be appropriate?
- use shielded wires.
- use separate power supplies: I am currently using the same 3.3V rail for the 4 readers, maybe this might help to use separate 3.3V supplies.
- use the smallest gauge wire for the data lines: I am currently using 24AWG, maybe this is not thin enough.
If you have any comments regarding the above ideas or any additional advice, I would be glad to hear them!