BLE Communications

In other tutorials like the Wireless temperature Display we use Bluetooth Advertising to communicate. This is great for many-to-one type applications, but often you just want one-to-one communications.

In this case you probably want one Espruino device to create a connection to the other device, which is what we're covering here. Depending on the 'Connection Interval' chosen with NRF.setConnectionInterval this can be very power efficient as well (the default in Espruino 2v00 and above is to use a power efficient interval, but to switch to the fastest speed possible as soon as data starts being transmitted/received).

NOTE: Advertising is very power efficient for the sender, but the receiver has to be listening for packets a lot of the time which is extremely inefficient. If the receiver needs to run off a battery you might want to consider using a connection as well.

In this example we're going to use an MDBT42Q breakout board attached to a Servo Motor. There are a few main ways you might want to communicate:

• Sending commands as JavaScript code
• Sending commands to a custom Bluetooth LE Characteristic
• Streaming data to a custom Bluetooth LE Characteristic

Check out the headings for more information. The video below covers sending commands, and the one further down the page covers streaming.

Sending Commands (as JavaScript)

Receiver (Bluetooth Peripheral Device)

All the code that's needed here deals with moving the servo. To control this, we're just going to send the command flag() to execute the function.

var s = require("servo").connect(D14);
s.move(1,3000); // move to position 0 over 3 seconds

var timeout;
function flag() {
  if (timeout) clearTimeout();
  s.move(0.2,2000);
  timeout = setTimeout(function() {
    timeout = undefined;
    s.move(1,2000);
  },2000);
}

setWatch(flag, BTN, {repeat:true});

Transmitter (Bluetooth Central Device)

For the transmitter, we'll just use the Bluetooth LE UART module to allow us to send the command as if we were typing it in the REPL. Note that we need a newline to execute it - like we'd have to press Enter in the left hand side of the IDE.

function flag() {
  NRF.requestDevice({ filters: [{ namePrefix: 'MDBT42Q' }]
                    }).then(function(device) {
    require("ble_simple_uart").write(
          device,
          "flag()\n",
          function() {
            print('Done!');
          });
  });
}

setWatch(flag, BTN, {repeat:true});

NOTE: By default Espruino is expecting a REPL, so it echoes back the characters it was sent (plus the result of executing the command). You might not want this normally as it will slow down communications - check out the notes at the top of the page in Interfacing for the characters you can send to stop the echoing.

Sending Commands (Custom Service)

Receiver (Bluetooth Peripheral Device)

To create a more 'normal' Bluetooth device you'll want to create your own service, and also disable the REPL that we used previously with {uart:false} in setServices.

You should always generate your own UUID - just search online for a UUID generator (or on Linux type date | md5sum into the terminal). Then keep that same UUID and replace the 5th to 8th characters with 0001, 0002, etc for all the different services and characteristics you need - this is efficient in BLE as the 128 bit UUID only has to be sent once.

// Same code as before
var s = require("servo").connect(D14);
s.move(1,3000); // move to position 0 over 3 seconds

var timeout;
function flag() {
  if (timeout) clearTimeout();
  s.move(0.2,2000);
  timeout = setTimeout(function() {
    timeout = undefined;
    s.move(1,2000);
  },2000);
}

setWatch(flag, BTN, {repeat:true});

// New code to define services
NRF.setServices({
  "3e440001-f5bb-357d-719d-179272e4d4d9": {
    "3e440002-f5bb-357d-719d-179272e4d4d9": {
      value : [0],
      maxLen : 1,
      writable : true,
      onWrite : function(evt) {
        // When the characteristic is written, raise flag
        flag();
      }
    }
  }
}, { uart : false });
// Change the name that's advertised
NRF.setAdvertising({}, {name:"Flag"});

Transmitter (Bluetooth Central Device)

The code for the Puck is a little more complicated than needed because it traps errors and deals with multiple calls at once.

However the basic requestDevice,connect,getPrimaryService,getCharacteristic,writeValue code can be used in Web Bluetooth too.

var busy = false;

function flag() {
  if (busy) {
    digitalPulse(LED1,1,[10,200,10,200,10]);
    return;
  }
  busy = true;
  var gatt;
  NRF.requestDevice({ filters: [{ name: 'Flag' }] })
  .then(function(device) {
    console.log("Found");
    digitalPulse(LED2,1,10);
    return device.gatt.connect();
  }).then(function(g) {
    console.log("Connected");
    digitalPulse(LED3,1,10);
    gatt = g;
    return gatt.getPrimaryService(
        "3e440001-f5bb-357d-719d-179272e4d4d9");
  }).then(function(service) {
    return service.getCharacteristic(
        "3e440002-f5bb-357d-719d-179272e4d4d9");
  }).then(function(characteristic) {
    return characteristic.writeValue(1);
  }).then(function() {
    digitalPulse(LED2,1,[10,200,10,200,10]);
    gatt.disconnect();
    console.log("Done!");
    busy=false;
  }).catch(function(e) {
    digitalPulse(LED1,1,10);
    console.log("ERROR",e);
    busy=false;
  });
}

setWatch(flag, BTN, {repeat:true});

NRF.setServices({}, { uart : false });
NRF.setAdvertising({}, {showName:false, connectable:false, discoverable:false});

NOTE: The video shows how you can improve on the connection speed by remembering characteristics.

Streaming Data

In this case, we don't want to send a command, but instead to send a stream of data to Espruino.

Receiver (Bluetooth Peripheral Device)

The receiver sets up services as in the last example, but this time it actually uses the data that was sent on the onWrite event.

NRF.setServices({
  "3e440001-f5bb-357d-719d-179272e4d4d9": {
    "3e440002-f5bb-357d-719d-179272e4d4d9": {
      value : [0],
      maxLen : 1,
      writable : true,
      onWrite : function(evt) {
        // Data comes in as a byte, make it 0..1 range
        var n = evt.data[0] / 255;
        // Send data directly to servo as PWM
        analogWrite(D14, (1.2 + n*0.8)/20, {freq:50});
      }
    }
  }
}, { uart : false });

// On disconnect, stop the servo
NRF.on('disconnect', function() {
  digitalWrite(D14,0);
});
// Change name to 'Flag'
NRF.setAdvertising({}, {name:"Flag"});

Transmitter (Bluetooth Central Device)

The transmitter is much the same as before, however this time we don't disconnect, and just repeatedly call writeValue. Note the code to detect if a transmit is in progress or if the connection is lost.

var gatt,characteristic;

NRF.requestDevice({ filters: [{ name: 'Flag' }] }).then(function(device) {
  console.log("Found");
  return device.gatt.connect();
}).then(function(g) {
  console.log("Connected");
  gatt = g;
  return gatt.getPrimaryService(
    "3e440001-f5bb-357d-719d-179272e4d4d9");
}).then(function(service) {
  return service.getCharacteristic(
    "3e440002-f5bb-357d-719d-179272e4d4d9");
}).then(function (c) {
  console.log("Got Characteristic");
  characteristic = c;

  startWriting();
});


function startWriting() {
  var busy = false;
  var i = setInterval(function() {
    if (!gatt.connected) {
      clearInterval(i);
      return;
    }
    if (busy) return;
    busy = true;
    var n = analogRead(D5)*255;
    characteristic.writeValue([n]).then(function() {
      busy = false;
    });
  }, 50);
}