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I am currently planning a battery-powered project, where one of the tasks involves reading Desfire EV1 tags. Since the cheap RC522 based NFC shields are reported not to work with the Desfire EV1 tags, I wanted to make an attempt with a PN532 based shield. Due to offering the fastest shipping option, I ordered an Aptotec PN532 shield, although there are many other, and likely cheaper options out there as well. The Aptotec shield seems identical to the Seeed NFC Shield V2.0.

Communicating with the Desfire EV1 tags works as expected, but I am not understanding the shield's power consumption. If I understand the PN532 datasheet correctly, the chip itself should only draw 45µA when in standby mode, some 30mA when active, but not transmitting and additionally up to 150mA more during actual transmission.

After sending an 'InRelease all targets' command to the PN532, the shield however still draws about 50mA, although the PN532 should be in stand by mode. Using the RFConfiguration command to turn the RF field off completely still does not decrease the shield's power consumption. Activating the RF field e.g. by issuing an InListPassiveTarget command, the current draw increases to about 80mA, which matches with my assumption that the PN532 now draws 30mA more than when idle. There seems however to be something else on the shield causing a 'base line' 50mA current draw, which is not caused by powering the PN532 chip.

Does someone know where this power is lost, if it is possible to software-based turn off whatever is causing the power draw or if other PN532 shields show the same behaviour? Since my project must run on a battery, the 50mA unexplained current draw is not acceptable.

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  • Please provide an exact reference of the shield you used, so that we may take a look at its specs (if any). Note that power consumption of an IC may be different for a shield or more complete circuit that integrates this IC. I have had this problem in the past with NRF24L01P modules, which consumed far more than the IC itself. – jfpoilpret Oct 30 '16 at 18:42
  • @jfpoilpret Sure. I've linked to Aptotec's web page, but there is not much documentation available from them. The board seems identical to the 'Seeed NFC Shield V2.0', where some more documentation is available, but no technical specifications explaining the power consumption. – jarnbjo Oct 30 '16 at 18:51
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After some trial and error, I think I can answer this one myself.

The circuit schematic from the Seeed module is actually broken and the Apotec model is built almost identically. The PN532 chip has an SVDD output pin to power an optional security access module for cryptography support. In the Seeed design, this pin is connected to the 3.3V input voltage from the voltage regulator, which is simply wrong and the pin sinks an essential amount of current.

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  • Good catch about the SVDD being an output is connected to 3.3V! I too am facing the same problem. Have your tried to cut the trace on the PCB coming out of SVDD (pin 37), and connected this pin to ground (or left floating?). Do let me know if you have tried this. Thanks Tilak. – user31092 Feb 18 '17 at 17:01
  • Accept your own answer to close this question. – user31481 Nov 16 '17 at 7:33
  • @user31092 Sorry that I missed your comment. No, I haven't tried to fix the module. If you cut the PCB trace to the SVDD pin, you have to let the output pin float and should not connect it to ground. – jarnbjo Nov 16 '17 at 14:34

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