If your board is never used with ch55xduino before, you need to make the ch55x chip enter bootloader mode. You need to disconnect USB and unpower ch55x, connect the pull-up resistor on D+ line (generally a 10K resistor between D+ and 5V, controlled by a push-button or adjacent pads). Then you connect USB. and hit Upload. Also, a blank new chip will enter the bootloader automatically.
First time programming still needs a hardware connection to be made between P1.5 and GND, but after that holding the 1st key while connecting USB lands you in the bootloader!
