BlueZ BTGATT-client.py WriteValue

Question

I had a successful attempts with BlueZ-5.50 > test > example-gatt-client.py with ESP32 set as HeartRate Sensor Server.I am able to read from ESP32 which i modified to suit the BlueZ example.

However i have a doubt. What if i need to send some byte or string to ESP32 from the existing example.I wrote in such manner (a simple function to see whether it throws an error):

def write_val():

value = 0x50

#Write to a UUID
cg_ctrl_pt_chrc.WriteValue(value,reply_handler=body_sensor_val_cb,
                                error_handler=generic_error_cb,
                                dbus_interface=GATT_CHRC_IFACE)    

I am still new & learning Dbus & Python.

OS - Raspbian Stretch

Answer 1

The examples in the BlueZ test directory use the dbus-python library which is not the best of python libraries according to https://wiki.python.org/moin/DbusExamples.

A library from that page is pydbus https://pypi.org/project/pydbus/ which seems to work well with the BlueZ DBus API for a GATT client.

I don't have an ESP32 or Heart Rate Sensor, so I will do an example with a BBC micro:bit and hopefully it will be easy to modify to your needs.

The BlueZ DBus API documentation is available at:

https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/adapter-api.txt

https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/device-api.txt

https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/gatt-api.txt

Some useful things to know to get you started:

1. The Dbus service for bluez is called org.bluez
2. The Bluetooth adapter on a Raspberry Pi normally has the DBus object path of /org/bluez/hci0
3. The DBus Object path to a device is the adapter path plus the mac address prepended by 'dev_' and the semi-colons replaced with underscores. i.e. 'DE:82:35:E7:43:BE' would be found at /org/bluez/hci0/dev_DE_82_35_E7_43_BE

This script assumes that the device has already been paired with Raspberry Pi. As the pairing is a one-off provisioning step, it makes things simplier to do it manually. I usually use bluetoothctl on the command line.

import pydbus
from gi.repository import GLib

# Setup of device specific values
dev_id = 'DE:82:35:E7:43:BE'
btn_a_uuid = 'e95dda90-251d-470a-a062-fa1922dfa9a8'
temp_reading_uuid = 'e95d9250-251d-470a-a062-fa1922dfa9a8'
temp_period_uuid = 'e95d1b25-251d-470a-a062-fa1922dfa9a8'

# DBus object paths
bluez_service = 'org.bluez'
adapter_path = '/org/bluez/hci0'
device_path = f"{adapter_path}/dev_{dev_id.replace(':', '_')}"

bus = pydbus.SystemBus()
adapter = bus.get(bluez_service, adapter_path) 
device = bus.get(bluez_service, device_path)

# Assume device has been paired already so can use connect
device.Connect()
# Get commands and properties available
print(dir(adapter))
print(dir(device))

To read and write you need to find the path to the characteristic which is a little more work. Typically you know the UUID of the characteristic you are interested in so to get the value from button A on a BBC micro:bit it would be:

mngr = bus.get(bluez_service, '/')

def get_characteristic_path(dev_path, uuid):
    mng_objs = mngr.GetManagedObjects()
    for path in mng_objs:
        chr_uuid = mng_objs[path].get('org.bluez.GattCharacteristic1', {}).get('UUID')
        if path.startswith(dev_path) and chr_uuid == uuid:
           return path

char_path = get_characteristic_path(device._path, btn_a_uuid)
btn = bus.get(bluez_service, char_path)
print(btn.ReadValue({}))
# [0]

Writing to a characteristic is similar. Here is an example of reading and writing to the Temperature Period on a BBC micro:bit

tmp_period_path = get_characteristic_path(device._path, temp_period_uuid)
tmp_period = bus.get(bluez_service, tmp_period_path)
print(tmp_period.ReadValue({}))
# Result is:
# [232, 3]
# To get it as an integer:
print(int.from_bytes(tmp_period.ReadValue({}), byteorder='little'))
# 1000

# To write a new value of 1500
new_value = int(1500).to_bytes(2, byteorder='little')
tmp_period.WriteValue(new_value, {})
print(tmp_period.ReadValue({}))
# [220, 5]
device.Disconnect()

If you want to run this in an eventloop with notifications from the remote device then remove the disconnect above and add the following code:

temp_reading_path = get_characteristic_path(device._path, temp_reading_uuid)
temp = bus.get(bluez_service, temp_reading_path)

# Enable eventloop for notifications
def temp_handler(iface, prop_changed, prop_removed):
    """Notify event handler for temperature"""
    if 'Value' in prop_changed:
        print(f"Temp value: {as_int(prop_changed['Value'])} \u00B0C")

mainloop = GLib.MainLoop()
temp.onPropertiesChanged = temp_handler
temp.StartNotify()
try:
    mainloop.run()
except KeyboardInterrupt:
    mainloop.quit()
    temp.StopNotify()
    device.Disconnect()

Hope that is helpful.

Answer 2

I am posting another answer to see if this fits better given that you have posted the link which has the code you are using at the other end of the link:

https://platformio.org/lib/show/1841/ESP32%20BLE%20Arduino

Looking at the BLE_write.ino, my understanding is that if you write text to "beb5483e-36e1-4688-b7f5-ea07361b26a8" the ESP32 will display the text on its console.

import pydbus
from time import sleep

# Setup of device specific values
dev_id = 'DE:82:35:E7:43:BE'
# BBC micro:bit text display characteristic
# txt_uuid = 'E95D93EE-251D-470A-A062-FA1922DFA9A8'
# ESP32 BLE_write.ino text characteristic 
txt_uuid = 'beb5483e-36e1-4688-b7f5-ea07361b26a8'

txt_to_send = 'This is a test'

# DBus object paths
bluez_service = 'org.bluez'
adapter_path = '/org/bluez/hci0'
device_path = f"{adapter_path}/dev_{dev_id.replace(':', '_')}"
# Get adapter and device objects
bus = pydbus.SystemBus()
adapter = bus.get(bluez_service, adapter_path) 
device = bus.get(bluez_service, device_path)

# Assume device has been paired already so can use connect
device.Connect()

# Wait for the remote device to resolve its services
while not device.ServicesResolved:
    sleep(0.5)

mngr = bus.get(bluez_service, '/')

def get_characteristic_path(dev_path, uuid):
    """Get DBus object path for Characteristic UUID"""
    mng_objs = mngr.GetManagedObjects()
    for path in mng_objs:
        chr_uuid = mng_objs[path].get('org.bluez.GattCharacteristic1', {}).get('UUID')
        if path.startswith(dev_path) and chr_uuid == uuid.casefold():
           return path

txt_path = get_characteristic_path(device._path, txt_uuid)
txt_obj = bus.get(bluez_service, txt_path)
# Turn text into 
new_value = [ord(c) for c in txt_to_send]
# Write a new value ensure it is not too long for Bluetooth 
txt_obj.WriteValue(new_value[:19], {})

# Disconnect
device.Disconnect()