UART reading on Pico: Struggeling to understand/make it work

[Julu02]

Hello,
I'd like to read data from a radio control system (Radiomaster Pocket and FrSky ) using the FrSky ACCST transmission protocol. The receiver has a so called SBUS signal that consists of 25 bytes:

1 start byte: 0x0f
22 data bytes
1 flag byte
1 end byte: 0x00

The information per channel is coded with 11 bits, so more than one byte is needed to transmit the information, e. g. channel 1 using bits 0 to 10, channel 2 using bits 11 to 21...
For getting the channel information I have found some code examples but that comes next. As a first step I read the SBUS information using UART functionality:

import machine
import time

led_board = machine.Pin(25, machine.Pin.OUT)
sbus = machine.UART(1, baudrate=100000, bits=8, parity=None, stop=2, tx=machine.Pin(4), rx=machine.Pin(5), invert=machine.UART.INV_RX)
sbus_data=[]
while True:
    if sbus.any():
        sbus_data=sbus.read(25)
        led_board.value(1)
        time.sleep(0.1)
    
        if len(sbus_data) == 25:
            for i in range(0,25):
                print(sbus_data[i], " ", end="")
            print()

            led_board.value(0)
            time.sleep(0.2)
    else:
        print("n.ok")

System gives readings that fits to my expectation at the first glance

>>> print(type(sbus_data))
<class 'bytes'>`

>>> print(sbus_data)
b'\xe0\x03\x1f\xf8\xc0\x07>\xf0\x81o\xda\x00\x00\x0f\xac\x00_\x1c\xc1\x07>\xf0\x81\x0f|'

I'd expect that the transmitted information will change if I change positions of transmitter's gimbals, but I get either the same information per read or see changes that occur occasionally (not caused by any interaction on the transmitter).
Example 1 (no gimbal movements)

%Run -c $EDITOR_CONTENT
>>> %Run -c $EDITOR_CONTENT
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  15  219  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  219  3  159  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  219  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  219  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  219  3  159  20  193  7  62  240  129  15  124

Example 2 (with gimbal movement of channel 1)

>>> %Run -c $EDITOR_CONTENT
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  159  20  193  7  62  240  129  15  124  224  3  31  248  192  7  62  240  129  111  218  0  0
15  219  3  223  20  193  7  62  240  129  15  124  224  15  219  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  220  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  79  211  0  0  15  11  3  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  159  22  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  31  21  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  223  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  159  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  144  3  95  20  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  16  7  31  21  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  16  7  159  31  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  16  7  159  30  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  16  7  31  30  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  199  3  95  28  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  31  28  193  7  62  240  129  15  124
224  3  31  248  192  7  62  240  129  111  218  0  0  15  172  0  31  28  193  7  62  240  129  15  124

Of course, the data shown above is more or less nonsense because decoding of the eleven bits per channel isn't done yet, but according to the definition of SBUS the first byte should always be 0x0F as well as the last one being 0x00. If I move gimbal of channel 1 I'd expect that bytes 2 and 3 will change, but the others will stay.
Therefore I'm wondering whether UART reading is the right solution. Or are my expectations wrong?

Best regards
Juergen

System: MicroPython v1.21.0 on 2023-10-06; Raspberry Pi Pico with RP2040

[GitHubsSilverBullet]

Your best bet would be to read the incoming data properly (ie all eleven bits)
That can be accomplished using the PIO. IIRC there's an UART implementation which could act as a baseline.
One would need to modify that slightly from eight to eleven bit.

Does the SBUS protocol use a start/stop bit in addition to the eleven data bits?

EDIT:
OK, read a bit about the protocol. SBUS uses an inverted 8E2 protocol.
So should work with a regular UART:

from machine import UART
uart = UART(0, baudrate=100000, bits=8, parity=0, stop=2, invert=UART.INV_TX | UART.INV_RX )

[Julu02]

... should work with... UART...

That's what I'm hoping. Frankly, before I can go and use this PIO stuff I need to read a lot more.

[scruss]

Out of interest, are you basing your work on Sokrates80/sbus_driver_micropython: a Micropython driver for the SBUS protocol? There's an issue comment which appears to help it work on the RP2040: Inverted SBUS: Pico

[Julu02]

I've seen that post by Sokrates80, but there are still some to do's mentioned within so I thought it would be smarter to start my own story. But I'll have a closer look to his code although this will be another challenge for me as a beginner.