UART blocked communication

[bog739]

Hi! I am stuck with a problem related to UART on Pi Pico board with an RP2040 MCU. I use the module 0-UART (the default one) to connect to a GSM, everything works just fine when setting up the GSM, for example with text mode. The problem arises when it receives a SMS when I try reading some data the program just freezes. I used uasyncio module to create two tasks, one for constantly checking for data and other which will parse the message if GSM gets one. I used the GSM device on a separate board connected to my laptop and by using classic Python on windows it gets the job done seamlessly. If anyone can help me with an idea, I will be thankful!

[karfas]

The exact micropython version you use would be useful.
There were some recent changes to async UART read - see e.g. #9951 (reply in thread).

[bog739]

I use micropython 1.19.1 version

[peterhinch]

Given that you are using uasyncio are you using the StreamReader and StreamWriter classes? These are highly recommended. See this example.

[bog739]

I used them, but did not use the drain() method after a write. It's a must have?
I'd checked these links before started with uasyncio module:

1. https://github.com/peterhinch/micropython-async/blob/master/v3/docs/TUTORIAL.md#63-using-the-stream-mechanism
2. Uaysncio v3 Stream Writer Drain Bug #6621
3. https://github.com/peterhinch/micropython-async/blob/master/v3/as_demos/auart_hd.py

[peterhinch]

It is necessary to use .drain() as per your item 1.

Item 2 is an invalid way to use a stream writer as explained in my response.

Item 3 uses .awrite(). This is a legacy mechanism: if you look at the code it uses .drain() internally. I guess I should update that example.

I suggest you look at the code to see how the StreamWriter works. The .write() method simply buffers data. It's .drain() that does the actual output. It works this way for CPython compatibility. The mechanism has been subject to discussion and objections, including from me. But there it is.

[bog739]

It works now with .drain() after .write(). The StreamWriter and StreamReader do their job correctly now.