Receiving >20 bytes at once from Bluefruit Connect app not working

[dhalbert]

@ Thornhill in discord says a UART message of >20 bytes from the Bluefruit Connect app to, say, ble_uart_echo_test, is dropped. Sending to/from two CircuitPython boards does work. Also, does work sending to Arduino.

Confirmed by @caternuson.

[caternuson]

Yep. Just adding results of my simple and brief test. Phone is Android. BLE Connect v3.3.2. Running this example:
https://github.com/adafruit/Adafruit_CircuitPython_BLE/blob/master/examples/ble_uart_echo_test.py

Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit ItsyBitsy nRF52840 Express with nRF52840
>>> import ble_uart
b't'
b'e'
b's'
b't'
b'\n'
b't'
b'e'
b's'
b't'
b'2'
b'\n'

[tekktrik]

Aha! I'm working on adafruit/Adafruit_CircuitPython_BluefruitConnect#16 and this seems to be blocking that, given the size of images. My testing confirms what @caternuson has described. I can continue without this I suppose, but just won't be able to receiving images from the app.

[tekktrik]

@dhalbert and @caternuson I can also send longer UART between boards. Should this be transferred to the app's repo?

[dhalbert]

Maybe this is due to the app. I was afraid it is more general. There was this bug: adafruit/circuitpython#3975, but it's now fixed.

[caternuson]

Seems potentially something with the app. What's some other non-CP non-app thing we could try sending a >20 bytes message with? Just to prove a CP board will happily receive a >20 bytes message from some non-CP source.

[tekktrik]

We could try to do it via Arduino I suppose. I also just got a Bluetooth sniffer, I can try to do it the other way and see if I can actually detect the problem from the app?

[dhalbert]

This is due to the default max_length for variable-length Characteristics being 20. It would be fixed by #37.

I looked at a couple of implementations of NUS (Nordic UART Service). The max length of the characteristics is not fixed by the NUS definition. Instead, it's chosen at build time, based on the maximum length on the MTU size - 3.