I’m connecting a clone breakout board powering the pn532 from 3.3v (might be the case with other methods too!) using the following schematic

(credit DanyO for his Py532lib tutorial ).

I’m experiencing the following problems:

  • board disappearing when scanning the i2c bus
  • tag disappeared errors when reading a tag (libnfc 1.7.0)
  • error during tag reading
  • using debug level = 3 as suggested on the libnfc forums, the situation does not improve a lot

I’m not an electronics engineer, but it happened to me sometimes that devices are not correctly powered, so when I faced this problem, two piece of info came into my mind:

  1. I read more then once in the RPi forums that the 3.3v line might be dirty due to high integration in the board
  2. PN532 breakout board can also work with 5v power supply so it might become picky when powered via 3.3v

My proposed solution is to put a 10nF decoupling capacitor in parallel with the 3.3v power supply, as near as possible to the PN532 chip.

After I added the capacitor I was able to read a mifare classic 1024b three times in a row without errors (was not even able to start that before the mod).

I’m still experiencing some problems from time to time, so it might be the case to put another 100nF capacitor in parallel and, in extreme cases, add a 2.2uF-4.7uF electrolitic (add, not switch, from my googling around), but don’t go too high (disclaimer: if your Pi reboots or the 3.3v line blows, don’t blame me!) because that may introduce high current load at startup and some parasite effects that might nullify the decoupling benefits.

Here is the mod:

Hope this is of any help.


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