How to send data to characteristic over BLE

[CodeManiac64]

@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.

[seb]

@codemaniac64

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

[CodeManiac64]

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

[seb]

@codemaniac64

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

from network import Bluetooth
import time

bluetooth = Bluetooth()
bluetooth.start_scan(10)

while True:
    adv = bluetooth.get_adv()
    if adv and binascii.hexlify(adv.mac) == 'XXXXXXXXXX':
        try:
            print('1')
            conn = bluetooth.connect(adv.mac)
        except:
            bluetooth.start_scan(5)
            continue
        break

wanted_char = None
services = conn.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 wanted_char.properties() to see if it has the writable bit set, if it does you can then do wanted_char.write(b'x0f')

[jmarcelino]

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]

@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.

[CodeManiac64]

@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)

[jmarcelino]

@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()

[CodeManiac64]

Should it be like this? Also what does the error I get mean?

[jmarcelino]

@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):

bytes([4])

[CodeManiac64]

@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])

[jmarcelino]

@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)
characteristic.write(payload)

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

[CodeManiac64]

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.

[jmarcelino]

@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 https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF-Sniffer