Serial Debugging

Creating and using a virtual COM port over USB is really easy (provided the USB port is not used for anything else).

First step is to enable USB in device mode under Connectivity in STM32CubeMX:

Next, set the USB Device Middleware to Communication Device Class:

Generating code and the device should now show up as a virtual COM port.

Final step is to redirect output from "printf" to go to this virtual COM port rather than one of the UARTS.

int _write(int file, char *ptr, int len) {
    CDC_Transmit_FS((uint8_t *)ptr, len);
    return len;
}

You can now use printf to print debugging statements - for example:

uint32_t then = 0, now = 0;

while (1) {

	now = HAL_GetTick();
	if (now % 1000 == 0 && now != then) {

		printf("Tick (now = %lu)\n", now / 1000);

		then = now;

	}
}

The above "while" loop result in the following output on the virtual serial port:

lth@ncpws04:~$ microcom -f -s 921600 -p /dev/ttyACM0 
connected to /dev/ttyACM0
Escape character: Ctrl-\
Type the escape character to get to the prompt.
Tick (now = 1)
Tick (now = 2)
Tick (now = 3)
Tick (now = 4)
Tick (now = 5)
Tick (now = 6)
...

Notice the virtual com port can automatically adjust to a number of different speed settings. Above setting of 921600 appears to be around the max.