Python library for Bluetooth Low Energy (BLE) Central on Linux
Project description
Python package for using BlueZ D-Bus API to create a device in the Central role
Goal
The goal of this library is to only have a small number of dependencies and to be easily installable (even in a Python virtual environment without –system-site-packages).
The commands should be installed and run by the user without sudo privileges.
Install
$ python3 -m venv venv_ble
$ . venv_ble/bin/activate
$ pip3 install BLE_GATTBasics of BLE
Hopefully you are here you are here with some basic knowledge of Bluetooth and you understand that Bluetooth Classic and Bluetooth BLE are different.
There is an introduction to BLE at:
https://www.bluetooth.com/bluetooth-resources/intro-to-bluetooth-low-energy/
This library is only going to work with BLE. It will also only be a Central device connecting to a Peripheral device.
The two key things that you will need to know about the peripheral device you want to connect to is its address and the UUID of the GATT characteristic you want to interact with.
Addreess
This will be in the form of a mac address. This is 6 pairs of numbers separate by colons. e.g. 11:22:33:44:55:66 This should be unique to each device.
UUID
A UUID is a 128-bit value that are written in groups 0f 8-4-4-4-12. For example 00002A00-0000-1000-8000-00805F9B34FB.
Each characteristic will have a UUID that represents what it is. The number above is for the Device Name Characteristic.
Writing those long numbers is cumbersome so Bluetooth official characteristics can be shortened to 16-bits. This means you will often see the above Device Name Characteristic written as 0x2A00 although on the system it will still be the 128-bit value. The official Bluetooth base UUID is: 0000xxxx-0000-1000-8000-00805F9B34FB and the 16-bit value replaces the x’s.
One-time provisioning of peripheral device
A BLE peripheral device will be advertising some summary information, such as what services it offers, and our Central device needs to have read this advertisement before it can connect. Some devices will also need to exchange security information before they can connect and exchange information securely.
This security information exchange is called pairing or bonding. As this is a one-time provisioning step this library does not do the discovery or pairing step. Those can be done with the Linux bluetoothctl tool.
To launch the tool:
$ bluetoothctl
To start and stop the discovery of nearby advertising peripherals:
[bluetooth]# scan on [bluetooth]# scan off
Discovered devices will scroll up the screen. Once you stop discovery, to get a list of devices use:
[bluetooth]# devices
If you need to pair with the peripheral then the commands are:
[bluetooth]# agent KeyboardDisplay [bluetooth]# pair 11:22:33:44:55:66
If you don’t need to pair, then doing a connect will save the device in the Bluetooth information on the machine:
[bluetooth]# connect 11:22:33:44:55:66 [Name Of Device]# disconnect
Getting Started
Now you have the peripheral device address, UUID’s of interest, and have done the provisioning of the device we are ready to do some coding.
Create a device object
Import the library to you code and tell it about the address of the peripheral device to connect to.
import BLE_GATT
my_device = BLE_GATT.Central('11:22:33:44:55:66')Connect and disconnect
Establish (or remove) a Bluetooth connection between the Linux computer your code runs on and the peripheral device.
my_device.connect()
my_device.disconnect()Exchange Information
The API uses the structure of the command name is the activity you want to perform on the characteristic. The first parameter is the UUID of the characteristic you want to perform that on. To save us keep writing the long UUID, it is a good idea to create a constant/variable with the UUID value
Bluetooth data is always an array of unsigned bytes. We can represent that in Python as a list of values between 0 and 255. Numbers that are bigger than 255 will take multiple bytes. We can connect these together in Python with int.from_bytes or struct.unpack.
To create the values to write we can use int.to_bytes or stuct.pack
Expect Bluetooth data to be in little endian format.
my_custom_uuid = '12345678-1234-1234-1234-123456789ABC'
value = my_device.char_read(my_custom_uuid)
my_device.char_write(my_custom_uuid, [255, 255, 0, 123])Asynchronous Data
As well as reading and writing data, it is also possible get notifications from a Bluetooth peripheral when the value of a characteristic has changed. This is very efficient on Bluetooth traffic and also the battery of the peripheral as it can turn the radio off when there isn’t new data. For programming the client it means we don’t know when there is going to be data to handle. This requires us to code using asynchronous technique.
We do this be using the on_value_change command for the GATT characteristic UUID of interest. We give it a function that will get called when the value changes. We also need to use wait_for_notifications to stop the code exiting. This runs an event loop listening for updates.
def my_callback(value):
print(value)
on_value_change(my_custom_uuid, my_callback)
wait_for_notifications())If you want to stop getting notifications from a GATT characteristic UUID then there is:
remove_notify(my_custom_uuid)There is also a command that will remove all notifications, exit the event loop, and disconnect from the peripheral device.
cleanup()Bytes and Values
With Bluetooth values will always be in bytes which isn’t very readable to humans so most of the time we will want to covert them to an integer or floating point number.
Let’s use an example from GATT Specification Supplement at https://www.bluetooth.com/specifications/specs/
We will use Electric Current Specification (0x2AF0) which has three fields within the characteristic each two bytes (octets) in size. Those three fields are minimum, typical and maximum electric current. With current being defined as:
Field |
Current |
Data Type |
uint16 (Represents a 16-bit unsigned integer) |
Size (in octets) |
2 |
Description |
|
The represented value information helps us convert an integer to a floating point number:
M = multiplier, positive or negative integer (between -10 and +10)
d = decimal exponent, positive or negative integer
b = binary exponent, positive or negative integer
From bytes
If we had three values of 12.34, 23.45, 34.56 they would arrive as [210, 4, 41, 9, 128, 13]. Let’s see how we could covert them using firstly struct.
>>> value = [210, 4, 41, 9, 128, 13]
>>> import struct
>>> struct.unpack('<HHH', bytes(value))
(1234, 2345, 3456)
>>> [value * (10 ** -2) for value in struct.unpack('<HHH', bytes(value))]
[12.34, 23.45, 34.56]And doing the same using int.from_bytes
>>> int.from_bytes(value[0:2], byteorder='little', signed=False)
1234
>>> int.from_bytes(value[0:2], byteorder='little', signed=False) * (10 ** -2)
12.34
>>> int.from_bytes(value[2:4], byteorder='little', signed=False) * (10 ** -2)
23.45
>>> int.from_bytes(value[4:6], byteorder='little', signed=False) * (10 ** -2)
34.56To Bytes
If we were sending this data then it needs to go from being floating point numbers to a list of bytes.
Using struct
>>> c_min = 12.34
>>> c_typ = 23.45
>>> c_max = 34.56
>>> expo = 10 ** 2
>>> list(struct.pack('<HHH', int(c_min * expo),
int(c_typ * expo),
int(c_max * expo)))
[210, 4, 41, 9, 128, 13]Using int.to_bytes
>>> list(b''.join((int(c_min * expo).to_bytes(2, byteorder='little', signed=False),
... int(c_typ * expo).to_bytes(2, byteorder='little', signed=False),
.... int(c_max * expo).to_bytes(2, byteorder='little', signed=False))))
[210, 4, 41, 9, 128, 13]Advanced Information
The BlueZ D-Bus API’s used in making this library is documented at:
You can get help on accessing those full APIs with the following commands:
import BLE_GATT
my_device = BLE_GATT.Central('11:22:33:44:55:66')
my_custom_uuid = '12345678-1234-1234-1234-123456789ABC'
help(my_device.adapter)
help(my_device.device)
help(my_device.chrcs[my_custom_uuid.casefold()])
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