I received a set of wired Teensy 3.5 AudioShield and RFID RC-522. The audio shield is mounted on the Teensy but I cannot connect the MFRC522 Using MFR222 library. Pins are set to 0 and 31:

#define RST_PIN         0          // Configurable, see typical pin layout above
#define SS_PIN          31         // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance


Firmware Version: 0x0 = (unknown) 
WARNING: Communication failure, is the MFRC522 properly connected? 
Scan PICC to see UID, SAK, type, and data blocks..

Tried any other combination I can think of. Checked all soldered points. I did not find any guide on how to connect teensy 3.5 with RC522. only older versions. Do I need to tell the library where to find the other pins (MISO, MOSI...) Any help will be appreciated. Thanks Yehuda

 AudioShield is mounted on teensy and RFID RC-522 wired (soldered) enter image description here

  • the wiring diagram does not show a ground connection to the teensy
    – jsotola
    Mar 10, 2022 at 17:31
  • The teensy is mounted to audio, wired to RFID, and connected via USB to PC. Do I need to add another wire?
    – judthedude
    Mar 11, 2022 at 11:20

1 Answer 1


The answer is (as far as I understand) that when both audio shield and rfid are mounted, they have protocol collision (same protocol for both). So if you want them to work. You need to wire rfid to different pins.

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