Without changing beforehand REPL to UART, it works here with:

import os, machine
os.dupterm(machine.UART(0, 4800), 0)

This dupterm enables REPL on UART.

Hi Robert,
That also doesn't work , there even is no serial activity now anymore ...
I will try it with an original image.
Am I correct to put this in boot.py on the same folder where I put the uf2 image ?
This I am not sure of.

Also no result with an original uf2 for the sparkfun_thingplus
Only a repl on ttyACM0

The bootloader file system is NOT identical to the MicroPython file system. The only file that can be loaded to the bootloader files system is the UF2 firmware file, and doing that it will be copied to flash.
The MicroPython file system is NOT accessible to the bootloader. You can access it with tools like mpremote.
If I enter the two commands above at REPL, I get a REPL interface at pins 0 and 1, at a baudrate of 4800 baud. But using it is no fun. The output get's corrupted as soon as it should scroll.

I'm sorry guys , but baudrate stays at 115200 :-(

Hi Robert ,
I'm using the sparkfun_thingplus board , so I used that for compiling , but as said without any luck.
I did compile it in a way you said.
There must be a way , not ???

There will be a way. I will go back to the supermarket method and see how one can get rid of the output corruption. It looks like a buffer overflow.

So the initial approach still works for me, and I can avoid the corruption by setting the txbuf option of machine.UART() to a larger value. setting timeout and timeout_char should do the same, but they do not. I have to investigate that further. Until then, if you know how many bytes are sent by the board in bursts, you can set the buffer accordingly. The largest accepted value is 32766.
The setting then is still:

import os, machine
os.dupterm(machine.UART(0, 4800, txbuf=16384), 0)  // 16 k output buffer

If you do not get serial output, then there may be a problem with the connection, signal levels or signal polarity.

• did you connect all of TX, RX and GND?
• does your terminal work with digital levels 0 and 3.3V?
• Is the interface by change of V24/RS232 type? Then the signals have to be inverted and the signal levels should at least be 0-5V. V24/RS232 usually as signal levels +/-5-12V, but some devices accept 0 and 5V.

Ok, this works:

1. mpconfigport.h - #define MICROPY_HW_USB_CDC (0)
2. mpconfigport.h - #define MICROPY_HW_ENABLE_UART_REPL (1)
3. board/MY-PICO/mpconfigboard.h:

// Board and hardware specific configuration
#define MICROPY_HW_BOARD_NAME                   "Raspberry Pi Pico"
#define MICROPY_HW_FLASH_STORAGE_BYTES          (1408 * 1024)

// Override Pico SDK UART default BAUD_RATE
#define PICO_DEFAULT_UART_BAUD_RATE             (4800)

4. Edit uart.c

$ diff uart.c-4800 uart.c
53d52
<     uart_init(uart_default, PICO_DEFAULT_UART_BAUD_RATE); // set to 4800

5. make BOARD=MY-PICO

You get MY-PICO by:

$ cd boards
$ cp -r RPI_PICO MY-PICO

MY-PICO is not defined in the Pico-sdk, but mpconfigboard.cmake will set it to "pico" and "rp2040".

Now we will get REPL at /dev/ttyUSB0 with 4800 baud.

$ tio -b 4800 /dev/ttyUSB0 
[23:57:23.830] tio v2.7
[23:57:23.830] Press ctrl-t q to quit
[23:57:23.832] Connected

>>> 
MPY: soft reboot
MicroPython v1.27.0-preview.317.g9999553aae.kaki5 on 2025-12-06; Raspberry Pi Pico with RP2040
Type "help()" for more information.
>>>