Marc Dougherty

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Bluetooth Part 1

25 November 2017

I’ve recently become interested in bluetooth, specifically web-bluetooth, which is newly enabled in most chrome/firefox/opera browsers. It allows a web app (javascript) to interact with bluetooth devices near the web browser. Combine this with eddystone beacons and the bluetooth device is advertising a URL that links a user directly to an app to interact with the device.

Hardware and software

To play with bluetooth, the first thing I needed was a bluetooth radio. Some simple beacon stuff can be done with the beacon toy app, I wanted to use a microcontroller, suitable for embedding into projects, as this was my eventual goal. There are plenty of good options on this front:

I went for the Adafruit nRF52, only because that was the first one that caught my eye. the micro:bit has several other onboard sensors, and is cheaper, so I may switch to that platform eventually.

To program the nRF52, i’m using the arduino toolchain, but since i’m a vim user, i’m editing primarily with vim, and only using the arduino IDE parts for the compilation. (I later switched to using the vim-arduino plugin, but i’ll talk about that setup separately).

There are also some mobile apps that are helpful here, notably nRF Connect, which is produced by Nordic Semiconductor, a major manufacturer of bluetooth chips. It has a variety of modes, and companion apps that i found indispensible for debugging and testing these examples.

First steps: Eddystone Beacon

My first goal was to just get an Eddystone beacon broadcasting, to direct an interested user to a web page. The code is nearly verbatim from the adafruit nRF52 example. it is important to note that the Eddystone protocol only allocates 17 bytes for the encoded URL. No error message is emitted when using a url that is too long. The best practice for avoiding problems here is to use a url shortener like goo.gl. This has the added benefit of letting you change the beacon’s destination without having to update the beacon device.

Next: Playbulb candle emulation

At this point, i’m more familiar with arduino/C++ than with javascript, so I opted to create a device that an existing webapp could interact with. I stumbled on the playbulb candle codelab, which interacts with a fairly simple custom bluetooth device.

Based on the javascript code, I discovered the protocol used to set the name, color and light effect on the playbulb candle devices. These are handled by individual Characteristics inside of a Bluetooth GATT service. Armed with this knowledge, i created a sketch that implemented enough of the service to interact with the app.

The webapp has some rough edges, notably that if web-bluetooth is not enabled, there is no error message displayed (though there is one printed to the javascript console if you open chrome’s developer tools). The same is true if you access the page over http, rather than https. (https is required for all web-bluetooth functionality, in accordance with the spec).

The big takeaway from this example is that i’ve now implemented entirely custom BLE services on the device, with read-only and read-write characteristics. These can serve as the basis for any custom services I build later.

Nordic Uart Service: not-exactly-standard, but close enough.

Chronologically, this project was the second one i built, not the third. but logically it makes more sense here.

In my research about bluetooth LE services, i kept seeing references to the Nordic Uart Service (NUS). This is a service common to many of the chips from Nordic Semiconductor, that emulates a standard bluetooth UART connection over BLE.

While the nRF52 i’m using is made by Nordic, it does not have built-in support for this service, so I decided to build a simple Echo service on the NUS protocol. A later addition can interpret commands delivered over this link, to perform actions. See the terminal echo sketch for the details here. I was now able to use nRF Connect mobile app to connect to the device over NUS, and send/receive text.

Wondering how to apply this to a web-bluetooth app, i came across a web bluetooth terminal app that had a similar behavior, but was built using only a single service and characteristic, where NUS uses two characteristics in the same service (one for Transmit, and one for Receive). With some refactoring of the main javascript file, I was eventually able to produce a terminal app that’s compatible with NUS, and could use it to connect and interact with my bluetooth echo device, which would dutifully reply with whatever it was sent. (as long as it was in chunks smaller than 20 bytes, which is the max write size for a ble characteristic). The original terminal had some buffering to work around this, but with my limited javascript experience, i decided to remove it, as it complicated the parsing of ble packets.

What’s next?

Well, now that i’ve mastered beacons, created custom services and characteristics, and exchanged simple text commands over BLE, its time to build something bigger! Maybe Zork over ble, or multi-player bluetooth hungry-hungry-hippos. I’m not sure exactly what’s next, but stay tuned to find out!