How to send data to characteristic over BLE

  • I could use some help. I can connect to the service and characteristic, been able to receive data from another device I'm using. What I don't know is how to send over data to the device that should receive data. Is there a command for that? I'm using Windows 7 and Atom.

    I've been looking at the documentation for the LoPy and it should be characteristic.write that sends the data.

    # Handler for data Received (Working on Send)
    def characteristic_callback(char):
        #v = int.from_bytes(char.value(), "little")
        #print("Data: {}".format(chr(v)))
        i = 25
        characteristic.callback(trigger=Bluetooth.CHAR_NOTIFY_EVENT, handler=None, arg=None)```

  • @codemaniac64
    Umm I'm not sure, do you have any debug visibility on your PSoC code?

    Seems like it's rejecting something. Can you see if the BLE connection is still up when you try the .write(b'x0f')?

    It really helps to have a BLE sniffer like this

  • 0_1521018341352_Request fail.PNG

    So I've tried both using bytes([4]) and payload = struct.pack("<H", 2048) and I get this error everytime. What am I doing wrong? Is it because I'm not doing anything in my characteristic_callback?

    Also I did change char to characteristic just as Seb said.

  • @codemaniac64

    bytes([]) really expects an array of one byte integer to convert, if you try to covert something larger than one byte you'll get an error.

    To do it properly you should use struct.pack functions() where you can specify the byte order, for example to get the byte representation of 2048 as a little endian 16 bit short you''d use:

    import struct
    payload = struct.pack("<H", 2048)

    For Strings you can use payload_str = mystring.encode('ascii') (or Unicode, etc)

    Finally you can simple concatenate bytes with + e.g. payload + payload_str

  • @jmarcelino
    Thank you so much! This has helped a lot.
    Is it also possible that I store it in a variable and print it out, that way I can enter numbers and letters I want to send over. bytes([value])

  • @codemaniac64
    Yes that should do it.

    As explained on the documentation "For now it only accepts bytes object representing the value to be written." so you need to covert your integer to a bytes type with (for example to write one byte with the number 4):


  • 0_1520947932312_Skärm.PNG
    Should it be like this? Also what does the error I get mean?

  • @codemaniac64

    If the PSoC 4 is your GATT Server and you’ve already connected and found the characteristic it should just be a matter of doing characteristic.write(value)

    But do this outside of the callback function. Just use the characteristic you found via service.characteristics()

  • @jmarcelino
    My LoPy would be my GATT Client that I will send to my PSoC 4 BLE that will be my GATT Server.

    Another thing, in your trigger you have Bluetooth.CHAR_READ_EVENT, but the documentation says that it can must be Bluetooth.CHAR_NOTIFY_EVENT.

    So if I want to write something wouldn't it be: char.callback(trigger=Bluetooth.CHAR_WRITE_EVENT, handler=char_cb_handler)

  • @seb

    I already have the connection established to the desired characteristic, so that step is not needed. I also already know that the characteristic has a writable bit set because I've already programmed it that way and done tests.

    I have tried the characteristic.write(value) aswell but it doesn't work. The only step I want to know is how to send something to the GATT server.

  • Hi @codemaniac64

    Can you clarify what is your GATT Server and Client in your setup?

    If the LoPy is your GATT Server you'd trigger on a CHAR_READ_EVENT in order to send data to the remote device

    def char_cb_handler(chr):
        global some_number
        return some_number
    char.callback(trigger=Bluetooth.CHAR_READ_EVENT, handler=char_cb_handler)

  • @codemaniac64

    So firstly you need to find the desired characteristic from the device using code like:

    from network import Bluetooth
    import time
    bluetooth = Bluetooth()
    while True:
        adv = bluetooth.get_adv()
        if adv and binascii.hexlify(adv.mac) == 'XXXXXXXXXX':
                conn = bluetooth.connect(adv.mac)
    wanted_char = None
    services =
    for service in services:
        print("Service: {}".format(service.uuid()))
        if service.uuid() == b'XXXXXXXXXXXXX':
            characteristics = service.characteristics()
            for characteristic in characteristics:
                print("Characteristic: {}".format(characteristic.uuid()))
                if characteristic.uuid() == b'XXXXXXXXXXXX':
                    wanted_char = characteristic

    You will then need to check the to see if it has the writable bit set, if it does you can then do wanted_char.write(b'x0f')

  • @seb
    It's not a LoPy, I'm using a PSoC 4 BLE

  • @codemaniac64

    Are you sending to another LoPy or a third party device?

  • @seb

    I want to send something over BLE from LoPy. That is all. A simple 'Hello' or a value that I can enter in putty.

  • @codemaniac64

    What exactly are you trying to achieve?

  • @seb
    Do I need to change every char to characteristic?
    Nothing changed when I changed the char in the characteristic_callback

  • @codemaniac64

    In the code you have posted the callback parameter is called char not characteristic so please adjust your code accordingly.

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