Support Passive Mode

[gilphilbert]

In many projects it's desirable to support non-blocking code - instead of constantly checking to see if a card is present, put the PN532 into interrupt mode where it will tell the MCU when a card is detected. This allows the MCU to free up cycles and only read the card reader when a card has been detected.

Usage is simple, once the device is configured, simply call startPassiveTargetIDDetection(PN532_MIFARE_ISO14443A) and monitor the IRQ pin after setting it to input. It's also possible to use attachInterrupt() (check for a FALLING pin)

When a card is detected, the IRQ pin is pulled low by the PN532. At this point, read the card as usual. Once the read is complete, remember to call startPassiveTargetIDDetection() again.

[mrdc]

@gilphilbert

Hi!
Note: The PN532 only supports use of the IRQ pin in I2C mode
Any workaround for HSU or SPI connections? ESP32 has issues with PN532 when I2C is used...

BTW does readPassiveTargetID(PN532_MIFARE_ISO14443A); constantly checks for a card blocking the code?

[gilphilbert]

Good question, I found this referenced on a forum and went hunting through the datasheet to find the statement but couldn't. I've tried it with SPI and it works fine.

On a related note, I've actually found the PN532 doesn't share any bus particularly will on the ESP32, in the end I just created a dedicated i2c bus for the PN532 in my project.

[mrdc]

Good question, I found this referenced on a forum and went hunting through the datasheet to find the statement but couldn't. I've tried it with SPI and it works fine.

I've made some tests and it looks like the default readPassiveTargetID(PN532_MIFARE_ISO14443A,,) always brings ~1 sec delay when called in the loop, when no cards are present (blocks other tasks). When there is a card being read there is no delay in the loop execution.

[gilphilbert]

Ah, yes, I didn't answer that one.

This is why I added the interrupt feature! The default behavior is to make a blocking read request, which is fairly expensive from a cycles perspective. This also broke my I2S playback, which can't tolerate a delay of more than 14ms.

The interrupt feature puts the NFC into passive mode and waits for the IRQ pin to flag that a card is present, instead of checking every loop. Once the interrupt shows a card is present, you can make a read request. This removes the blocking nature!

[mrdc]

Once the interrupt shows a card is present, you can make a read request.
Thanks, good solution.

Note: The PN532 only supports use of the IRQ pin in I2C mode

I've tried it with SPI and it works fine.

So, it also works for SPI connection?

[gilphilbert]

It does work with SPI. I haven't tried HSU, but I don't see why it wouldn't work.

The delay is a timeout while waiting for a card - the default in the code is 1s. I thought it was a better solution to only call readPassiveTargetID(...) when there's a card actually present.

[mrdc]

the default in the code is 1s

Oh, I've missed it - it was in the header file.