Thanks Felix! I will study your new example.

Thanks all friends! I finally figure it out.
I think it is still related to AXP2101. When I keep the USB-C connected and use PORT C to communicate, it works. When plug off the USB-C, PORT C UART port just never send and receive data. I think it is because the USB-C supply power to the PORT C, not the inside battery. I will try to figure out how to let the battery to give power to PORT C.

It looks like the bad thing is from AXP2101 power chip, all the PORTs were set to 5V? Please, any idea of how to lower voltage to 3.3V.
I will also find a multimeter to give a test.

@felmue I tested my target device the PIN is 3.3V, so, I need to downgrade M5 from 5V to 3.3V? Many thanks!

@felmue Thanks Felix! The device i'm working on is an Arduino 328P board, there were a UART port on the board (as far as the info i have, there printed "UART port", not I2C or whatever). It's a quite old device, it's difficult find link and more introduction. And it is the control board of an open source laser engraver machine.
I will try your suggestion of the port voltage. And I also ordered a M5 UART GPS module, I will try that module to get more understanding of serials port programing first.

@felmue Understand! Thanks! I'm using a 3rd part device, not M5's product.

@felmue I studied your example. Can I know what device you used to connect to S3 through the UART port(port C of the S3 core)? On my side it still doesn't work, can't read anything neither write out. I noticed in the introduction of S3, there is an official note:"Q: After the sensor is connected, what is the case that the sensor does not work or does not exist?
A: The CoreS3 uses the AXP2101 power management chip, and the Port port and the M5-Bus power supply need to be configured for normal output voltage. The configured API is in the AXP2101.cpp library file, and can be configured as required. eg: M5.Axp.powerModeSet(POWER_MODE_USB_OUT_BUS_OUT) in the setup function."

Would that be the reason? How can I do that in mpy? In your mpy and C project, you never use this powerModeSet function.

Many thanks again!

@robski Thanks alot robski! Thanks for your clue about uiflow 1， so I tried to code in mpy, “from machine import UART”. At least, it no longer crashes, but still can't read and write data, I will keep on trying and let everyone know.

@felmue Thanks alot! I will study your example!

I did nothing, only init a UART module. When i select UART 0, it throws out an error "ValueError: UART buffer size is fixed". When I select UART 1 or 2, it throws out an error "invalid pin"....
There were too insufficient documents!