RFID reader and SD card shield SPI problem

Hi, I recently encountered a problem that's been bugging me for days. I want to make a project using a RFID reader and log data on an SD card with an Arduino Nano, but apparently no matter what I do, I can't seem to make them work together on SPI. They both work individually, and the SD shield works in parallel with the RFID reader, but the RFID doesn't work in parallel with the SD shield.

I'm absolutely sure that the wiring is correct, slave select on both modules are wired to different digital pins, and both are connected through bidirectional level shifters.

I noticed that the RFID reader works fine when I disconnect the MISO pin from the SD shield. I tried putting a diode between SD MISO and digital pin 12, I tried adding pullup resistors on both SS pins, I tried adding resistors on MISO lines, I tried pulling HIGH the SS i don't use, and low the one I want active. Nothing seemes to work and I'm out of ideas. The last thing I will try is conencting the modules on separate VCC, but I don't have faith that it will make any difference.

I hope someone has some advice for my problem.

Thanks in advance

It might help to provide a link to that SD card shield. I guess it's not a part from Arduino but from a cheap Chinese shop. They often don't put the MISO line into high impedance state if CS is pulled HIGH. Your description sound exactly like you have such a board. You could isolate it using a 74HC125: https://github.com/sumotoy/RA8875/wiki/Fix-compatibility-with-other-SPI-devices.


I had a similar problem, the workaround for this issue is to set the pin Mode and Value of the Chip Select of the SD Card to OUTPUT and HIGH.