rpi2040 gives correct output on logic analizer for 8bit but not for 9bit communication

Port, board and/or hardware

rpi2040

MicroPython version

MicroPython v1.23.0 on 2024-06-02; Raspberry Pi Pico with RP2040

Reproduction

import machine
import utime

# Configure UART1 on GPIO8 (TX) and GPIO9 (RX)
uart = machine.UART(1, tx=machine.Pin(8), rx=machine.Pin(9), baudrate=25000, bits=9, parity=0, stop=1)

# Function to send the known pattern
def send_pattern():
    #test bytes
    uart.write(bytes([0x55]))  # 0x55 (binary 01010101)
    uart.write(bytes([0xAA]))  # 0xAA (binary 10101010)
    uart.write("AAA")
    #what I want to actually send
    uart.write(bytes.fromhex('0102'))
    uart.write(bytes.fromhex('0104'))
    uart.write(bytes.fromhex('00F9'))
    #uart.write(b'\xAA')
    #uart.write(b'\xFF')

# Send the known pattern every 50ms
while True:
    send_pattern()
    utime.sleep_ms(50)

Expected behaviour

expected to see those values in the logic analyzer

Observed behaviour

the logic analyzer sees this:

name	type	start_time	duration	data	error
Async Serial	data	0.0087	0.00042	0x0075
Async Serial	data	0.00914	0.00042	0x0029	framing
Async Serial	data	0.00966	0.00042	0x0161	framing
Async Serial	data	0.0103	0.00042	0x0061
Async Serial	data	0.01074	0.00042	0x002C	framing
Async Serial	data	0.01126	0.00042	0x0064	framing
Async Serial	data	0.0119	0.00042	0x01F9
Async Serial	data	0.05908	0.00042	0x0175	framing
Async Serial	data	0.05958	0.00042	0x010A
Async Serial	data	0.06002	0.00042	0x0161	framing

Additional Information

If I change the program and analyzer to use 8bits instead, things look better. I may have some issues there but the 0x55 and 0xAA are picked up for example.

uart = machine.UART(1, tx=machine.Pin(8), rx=machine.Pin(9), baudrate=25000, bits=8, parity=0, stop=1)

name	type	start_time	duration	data	error
Async Serial	data	0.01646	0.00038	0x55	framing
Async Serial	data	0.0169	0.00038	0xAA	framing
Async Serial	data	0.01734	0.00038	0x41	framing
Async Serial	data	0.01778	0.00038	0x41	framing
Async Serial	data	0.01822	0.00038	0x41	framing
Async Serial	data	0.01866	0.00038	0x01
Async Serial	data	0.0191	0.00038	0x02
Async Serial	data	0.01954	0.00038	0x01
Async Serial	data	0.01998	0.00038	0x04

Code of Conduct

Yes, I agree

Start by reproducing the UART1 RP2040 example with 9-bit and 8-bit settings, using GPIO8 and GPIO9 at 25000 baud, and compare the logic-analyzer output. Check whether the reported framing errors and decoded values match the configured communication; done means 9-bit transmissions are decoded as the expected values without unexplained framing errors.