micropython/lora , modem.poll_recv() in a callback problem

[davefes]

I have used start_recv and poll_recv in some low-level API code to successfully get the messages send from a remote.

When I tried using an irq_my_rx() callback I determined that, at least poll_recv would get me the message. However, doing a decode on the data throws a unicode error.

I also tried:

py
sender_id, my_data = my_receiver.recv(wait=True)

in my_rx_ irq() gives Invalid length in payload header

Writing a Python script to decode the message I discovered that there is a start byte in my message. When I remove it the message decodes.

#my_data = bytearray(b'\xfa\x01}\x18Hello, ticks_ms=36083677\x12')
my_data = bytearray(b'\x01}\x18Hello, ticks_ms=36083677\x12')
print(type(my_data))
print(my_data)
my_data = my_data.decode('utf-8')
print(type(my_data))
print(my_data)

What is the correct procedure to recover the received data in a callback?

[sophiedegran]

Perharps you have to 'reset' the lora buffer before reading ?

[davefes]

Good suggestion but I don't see any methods to do that. I will look harder in the source code ... as Googling for ways to remove a encoded start byte (xfa) out of a byetarray has not been fruitful.

[davefes]

According to this snippet from sender.py

        # Prepare the simple payload with header and checksum
        # See README for a summary of the simple data message format
        payload = bytearray(len(sensor_data) + 5)
        struct.pack_into("<HBB", payload, 0, self.device_id, self.counter, len(sensor_data))
        payload[4:-1] = sensor_data
        payload[-1] = sum(b for b in payload) & 0xFF

I am probably seeing a lot more than the original sensor_data. Looks like post-processing is required after a poll_recv()