Web NFC (and writing to NFC tags)

Posted on Fri 25th, September 2020

• #1

Gordon

Hi, I'll try and turn this into a tutorial at some point, but it turns out you can now use 'Web NFC' on Android to read and write NFC tags including Puck.js/Pixl.js.

HTML looks like this:

<html>
<body>
<script>
/* Starting a scan at startup stops Android from
moving away from the Chrome window when a tag is found*/
const reader = new NDEFReader();
const writer = new NDEFWriter();
reader.scan();

function send(msg) {
  writer.write(msg).then(_=>console.log("Written ",msg));
}
</script>
<button onclick="send('red')">Red</button>
<button onclick="send('green')">Green</button>
<button onclick="send('blue')">Blue</button>
</body>
</html>

Puck.js/Pixl.js code is a bit more complex as right now it's emulating an NFC tag in JS:

var data = new Uint8Array(10+64);
var header = NRF.nfcStart();
var written = false;
data.set(header,0); // NFC device header
data.set([0,0,0xE1,0x10,(data.length-10)/8,0,0,3,0,0xFe], 0x0A); // NDEF tag header
// 0,0,e1
NRF.on('NFCrx', function(rx) {
  var idx = rx[1]*4;
  switch(rx[0]) {
    case 0x30: //command: READ
      NRF.nfcSend(new Uint8Array(data.buffer, idx, 16));
      break;
    case 0xa2: //command: WRITE
      written = true;
      if(idx > data.length) {
        NRF.nfcSend(0x0);
      } else {
        data.set(new Uint8Array(rx, 2, 4), idx);
        NRF.nfcSend(0xA);
      }
      break;
    default:   //just, re-enable rx
      NRF.nfcSend();
      break;
    }
});
NRF.on("NFCoff",function() {
  if (written) 
    onWritten(E.toString(new Uint8Array(data.buffer,26,data[21]-3)));
  written = false;
});

function onWritten(data) {
  console.log("NFC written", data);
  var colors = {
    red : 1,
    green : 2,
    blue : 4,
  };
  if (colors[data]) {
    digitalWrite([LED3,LED2,LED1], colors[data]); //  onwards
    setTimeout(function() {
      digitalWrite([LED3,LED2,LED1], 0);
    },1000);
  }
}

But basically this lets you send strings of data over NFC (no bluetooth connection needed). You just hold the phone near, tap 'red/green/blue' and then when you move the phone away Puck.js reads the text in the tag and decides what colour to make itself...

• #2

François

NDEFWriter has been merged into NDEFReader recently. See https://github.com/espruino/EspruinoDocs/pull/589
• #3

Gordon

Thanks! Updated code is here: http://www.espruino.com/Web+NFC

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Web NFC (and writing to NFC tags)

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