Solved by setting -DARDUINO_USB_MODE on 0 in platformio.ini: board_build.extra_flags = -DARDUINO_LILYGO_T_DISPLAY_S3 -DARDUINO_USB_MODE=0 ; <-- 1 is here by default -DARDUINO_USB_CDC_ON_BOOT=1 -DARDUINO_RUNNING_CORE=1 -DARDUINO_EVENT_RUNNING_CORE=1 Default values are from: https://github.com/platfor...

Introduction I have my head scratching by one thing regarding the HWCDC and serial communication for couple of days now. It seems like serial.open() in various frameworks (reproducible with QtSerialPort and PySerial) does some initialization that needs to finish before serial.write() goes through, ...