I’m trying to get multiple (up to 16) RC522 RFID readers to work with the Electron using a custom made PCB. Generally, it works great - I’ve had success with connecting 8 of them sharing the same SPI bus, but with individual SS (slave select) lines to each reader using a shift register and a customized RC522 library.
However, at random intervals the SPI bus will simply stop working on the particular Electron. It seems to be permanent and I have to use a new one (I’ve been through 3 Electrons so far). Everything else on the Electrons still work fine, even the other SPIs (SPI1 and SPI2). I’ve tested the circuit with a multimeter to find electrical problems, but I haven’t been able to identify any problems - there are no short circuits, and the pins are only exposed to 3.3V as far as I can tell. The behavior of the SPI pins after they are “broken” is that they simply output a flat 3.3V with no working SPI communication with the RFID readers, until I either change to one of the other SPI buses (using a breadboard) or install a new Electron on the PCB, and everything works great again for a while with the exact same firmware and hardware setup. I haven’t been able to pinpoint exactly when the problem arises, but generally it will work fine for a few days.
I’m using the standard SPI bus on pins A5, A4 and A3.
Any idea of what could cause (potential) damage to the SPI pins from this setup, assuming there is no excessive voltage on the pins? Or alternatively, anything that could interfere with the SPI pins (clock speed or similar) that can make them go into this state - and stay that way, even after reflashing the firmware?
Btw., these are the readers I’m using: Getting the RFID-RC522 to work! [SOLVED]