Xiao ESP32C6 hardware UART swapped TX (GPIO16) <=> RX (GPIO17) pins?

[siditm]

Hi, I have had some problems utilizing Xaio's RX and TX pins on the ESP32C6 board (MicroPython v1.26.0-preview on 2025-06-26).
Based no Serial Communication > Hardware Setup section:

Connect the TX pin of the external device to the RX pin (D7/17) of the XIAO ESP32C6.
Connect the RX pin of the external device to the TX pin (D6/16) of the XIAO ESP32C6.

the connection to the external device should be crossed as usual for UART, but in fact this will prevent USB serial communication with the board.

The weird part is it looks like pins are swapped just like in Xiao ESP32C3 per Serial - UART section:

However, if you want to use UART0 as the serial, you need to connect pin D6 as the TX pin and pin D7 as RX pin with a USB-Serial adapter.

Look at the image where they are connecting Device's TX to Xiao's TX and Device's RX to Xiao's RX.

In my case I'm connecting to the HLK-LD2450 radar module; if I define UART1 on other GPIO pins, it works as expected while cross-connected TX -> RX, RX <- TX.
The radar's TX connected to GPIO1, RX to GPIO0:

import esp
esp.debug(0)

# TX-RX, RX-TX
uart = UART(1, 256000, tx=0, rx=1)

Radar's TX connected to GPIO01, RX to GPIO00:

import esp
esp.debug(0)

# swapped pin assignment to keep cross connections TX-RX, RX-TX contradictory to Seed's documentation
uart = UART(1, 256000, tx=17, rx=16) 

but not when using TX (GPIO16) & RX (GPIO17) for UART1 as per Seeed's documentation.

import esp  
esp.debug(0)

# cross connecting TX-RX, RX-TX crashes USB serial; works if connected TX-TX, RX-RX why?
uart = UART(1, 256000, tx=16, rx=17) 

What am I missing?

[bikeNomad]

According to the ESP32-C6 datasheet, GPIO16 is indeed UART0 TXD, and GPIO17 is UART0 RXD.
Have you tried another build, perhaps the M5STACK_NANOC6 (which doesn't have MICROPY_HW_ENABLE_UART_REPL defined like the ESP32_GENERIC_C6)?

[siditm]

Thanks bikeNomad. Following your suggestion, I've tried the Seeed's official Xiao ESP32C6 build (it's v1.24 preview) and M5STACK_NANOC6 (v1.24 and v1.25), and all worked with:

uart = UART(1, 256000, tx=16, rx=17)  #cross connected C6_TX - DEV_RX, C6_RX - DEV_TX

My understanding from this is that defining MICROPY_HW_ENABLE_UART_REPL swaps pin logic, making ESP32_GENERIC_C6 behave as described in the Xiao ESP32 C3 Serial - UART section. Is that correct?

[bikeNomad]

No, what MICROPY_HW_ENABLE_UART_REPL does is to use those UART GPIOs (16 and 17) for the internal UART used for communications with the user. Which doesn't make any sense, since the main REPL is provided via the internal USB peripheral and its CDC device handler.