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Streaming data to the browser with web-bluetooth at 80Hz!

Demo of streaming data from the magnetometer from an Espruino Puck.JS / Bangle.JS at up to 80 Hz to the browser with Web-Bluetooth and plotting with plotly JS. In real time :)

Javascript all the way down!

Changelog / release notes:

v.0.3:

  • Update to work with Bangle.JS as well

v.0.2:

  • Implemented battery level service to show how to connect to two characteristics and receive notification

sample graph

Usage

  • Upload the PuckStreaming.js to your Puck, or BangleStreaming.js to your Bangle
  • Open index.html in a browser that supports web Bluetooth (tested with Chrome, and Edge "dev")
  • Press the DoIt button & select your Puck to connect to it
  • Streaming start as soon as you are connected
  • You can change the sample speed, or stop it with the dropdown

Code, features

Espruino

  • defined a custom BLE service & characteristic that uses notify to "push" the data form the magnetometer (and accelerometer in case of the Bangle)
  • the characteristic is writeable, so sample rate can be changed, or turned off
  • battery level service that auto-updates every 10 seconds
  • button click toggles the LED to quickly alter battery state. Only on Puck, but you can turn on the screen of the Bangle, and that changes battery level.

Web-Bluetooth

  • connects to the characteristics from the web page & subscribe to notifications
  • display magnetometer and battery level with Plotly.JS

Serialization & deserialization

The magnetometer outputs three integers, for example: { x: 1, y:2, z: 3 } so just used an Int32Array's buffer write the data: new Int32Array([d.x, d.y, d.z]).buffer.

On the browser side did the reverse: created an Int32Array from the received buffer: new Int32Array(raw.buffer). X is the 0th, y is the 1st, z is the 2nd element in the Int32Array.

Tricky parts?

There were two things that caused some hangup:

Plotly with the default SVG doesn't really like work with lots of points updating all the time. Solution: use scattergl

Trying to update the screen at 80Hz really dragged my browser, took almost half minute to set the sample rate back to 40Hz. 40 Hz was working fine, so I guess my screen update rate is 60Hz. Simply using window.requestAnimationFrame to send new data to Plotly only on available animation frames solved this.

Shaky cam video: https://www.youtube.com/watch?v=72ViaRna7ZM (not really visible, but after 13 seconds I move the Puck over my laptop's speaker, so magnetometer values skyrocket)