Chat here is you are connecting ESP-xx type modules to existing AVR based Arduino

Moderator: igrr

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By JorgenVikingGod
#26477 Hmm strange - I can not find any failure.
Maybe something with the OLED or RFID library is faulty and forces this effects...
As I wrote above - each of them in single use is working - both together not :/

I hope to find the error - because I need both.

But thank you very much for your information - that on your ESP8266 both is working at the same time.
This is a good hint, that I should look at the libraries and fix it. Hardware issue would be terrible in this case.
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By martinayotte
#26483 If I were in this situation, I would do a new debugging sketch, by first make the display initialized, display some traces on both the LCD and Serial, and using serial input feedback doing step by step initialization of the SPI and further code in the MFRC522 lib. At a certain point, when you will seen that LCD stop reacting, you will probably narrow the guilty code.
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By danbicks
#37326 Hi had a quick play, was not to hard, have I2C display and RFC522 both working fine.

The issue you had was GPIO15 connected to RST of the RFID module. This caused the ESP not to boot or accept program mode from start up. To fix this I coupled RST with GPIO 0 instead. All working fine.

Hope this helps anyone else

PS: used my own sketch to test this instead of yours but should produce the same results :)