After turning on the analog section of the mfrc522, a lot of spurious
interrupts could be observed. Debugging revealed that the mfrc522 did not
give consistent information about an interrupt source.
A closer look into the chip documentation (chapter 9.3.1.4) revealed that
the interrupt pin is open drain by default, but can be set to CMOS.
Switched to CMOS, deactivated internal and external pull down resistors.
No more spurious interrupts.
Stefan Kratochwil
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