README

This test program implements an USB CDC ACM serial gadget. It works on the
Raspberry Pi (3)A(+), Zero (2) (W) and 4B only. These models allow to connect
the on-board DW USB controller directly to an USB host for USB gadget (aka
device) operation. The Raspberry Pi 4B uses the USB-C power connector for that
purpose. You need a suitable USB cable. The Raspberry Pi is normally bus-powered
via the USB host in any case for this test. A separate power supply need not be
connected.

If you want to self-power your Raspberry Pi, you have to ensure that the supply
voltages at the USB connector and the power connector are exactly the same.
Otherwise there is a risk of damage. Be careful!

The program continuously sends log messages to the connected host computer, and
it displays messages, which were received from the host, on the screen and on an
eventually connected serial terminal (via GPIO14/15).

A configuration is required for this test. You have to define the macro
USB_GADGET_VENDOR_ID with your USB Vendor ID to be used for the USB device. See
the file include/circle/sysconfig.h for details!