Just going to go over communicating over bluetooth in Go. Over the summer, I had spent some time learning the basics of bluetooth and made a super simple central and peripheral example in Go. Summarizing those learnings here.

Definitely go over the above link to help understand the rest of this quick summary. I seem to frequently mess with bluetooth a bit and then don’t touch it for an extended period of time, so this is mostly a reference document for me to get a quick refresher on the basics until I spend significant time at the embedded level. Honestly, I spend way more time at the infrastructure and server level, so that comes more naturally to me.

Notes on GATT

GAP is covered first, but mostly just goes over the advertisement protocol for bluetooth. Here is a Go snippet that summarizes the basics of GAP:

// Define the peripheral device info.
adv := adapter.DefaultAdvertisement()
err = adv.Configure(bluetooth.AdvertisementOptions{
    LocalName: "loupi5",
    Interval:  bluetooth.Duration(20000),
})
if err != nil {
    log.Fatal(err)
}

The Generic ATTribute(GATT) Profile defines how BLE devices transfer data back and forth using concepts called Services and Characteristics. GATT is important to consider when a connection is established between two devices that have already gone through the advertising process governed by GAP.

Only one peripheral device can be connected to a central at a time using GATT, meaning that any advertising that had happened prior to GATT will not continue. This connection then allows for two-way communication between devices.

GATT Servers(Peripherals) and Clients(Centrals) negotiate a connection interval to transact data. This interval is just a recommendation, however, not a requirement. The GATT transactions contain nested objects called Profiles, Services and Characteristics.

Going over my simple example

The central peripheral code can be found in my ble-playground repository. It’s pretty simple and doesn’t do anything special, so I’ll skip an in-depth description and instead give this high-level diagram of what it does:

┌──────────────────┐            ┌──────────────────────┐
│                  │            │ Scan for device with │
│  PiZero loupi5   ┼─────────►  │ name loupi5. Once    │
│                  │            │ found: connect,      │
│                  ┼─────────►  │ discover services,   │
│    advertise..   │            │ and then disconnect  │
│                  ◄────────────┼                      │
│                  │            │   (Central)          │
│   (Peripheral)   │            └────────────────────── 
└──────────────────┘                                    

The PiZero in this case was on my local network and I had it advertise bluetooth using bluez and uploaded the Go binary with scp.

Where do I want to Go?

In a separate post, I would like to have a cylic TCP=>Bluetooth=>TCP relationship between my computer and the pi5. It helps understand how to allow the peripheral node to act as a sort of gateway to the central(where I need to understand this architecture and how this ble interest started).