Bluetooth Notification

As mentioned earlier, I searched for ways to transfer data to, and(!) from the device. Bluetooth data exchange might be a standard feature of Android, Apple …

On a linuxbox there are ways to do this as well!

In the zephyr samples, I found a sample for Indication. I created a sample for Notification. (samples/bluetooth/Peripheral-notification)

What?

The BLE standard defines two ways to transfer data from the server to the client: notification and indication.

Notification don’t need acknowledgement, so it is faster. Hence, server does not know if the message reached the client….

How?

Notification has to be enabled.

Linuxbox (bluez) writes “enable” to the watch notification characteristic.

Once Nofication is enabled the watch sends continuously messages : the maximum data payload size defined by the specification in each message is 20 bytes.

Bluez

on linux you could use “bluetoothctl”

There is an example for heartrates.

#bluetoothctl

connect MAC-device

menu gatt
select-attribute 00002a37-0000-1000-8000-00805f9b34fb
notify on

if all goes well, the watch starts sending heartrates

in case it stops:

notify off
notify on

A word on attributes

The data that the server exposes is structured as attributes.

Attribute type (Universally Unique Identifier or UUID) This is a :

  • 16-bit number

  • or 128-bit number

The 16-bit number is converted to 128-bit by means of a 128-bit base UUID:00000000-0000-1000-8000-00805F9B34FB.

These 16-bit numbers are standardised. On each device the temperature measurement value is 0x2A1C.

Because I create a custom service to exchange data I use a custom attribute type, also sometimes referred to as vendor-specific UUID. BLE_CHARACTERISTIC_UUID= “12345678-1234-5678-1234-56789abcdef1”

Attribute Handle

This is a 16-bit value that the server assigns to each of its attributes — think of it as an address. This value is used by the client to reference a specific attribute and is guaranteed by the server to uniquely identify the attribute during the life of the connection between two devices. The range of handles is 0x0001-0xFFFF, where the value of 0x0000 is reserved.

Python

The scripts are provided. (scan+not.py) But here is a short explanation on how to enable notifications.

On linux one could use the Gatttool from the commandprompt.

Another option is Python!

In this manual the module Bluepy is used : https://github.com/IanHarvey/bluepy.

(custom service) BLE_SERVICE_UUID =”12345678-1234-5678-1234-56789abcdef0” BLE_CHARACTERISTIC_UUID= “12345678-1234-5678-1234-56789abcdef1”

ch = ble_service.getCharacteristics()[0] ch.valHandle (this gets you the handle to the notification characteristic

# Writing x01 is the protocol for all BLE notifications. # However …. you need to write it to the handle + 1

dev.writeCharacteristic(ch.valHandle+1, b”x01x00”, withResponse=True)