Pololu A-Star 32U4 - Manual (Page 22)

6. The A-Star 32U4 USB interface
The A-Star 32U4 boards are based on a single AVR ATmega32U4 microcontroller that runs the user program and
also handles the USB connection to the computer. The AVR has a full-speed USB transceiver built into it and can be
programmed to present almost any type of USB device interface to the computer.
USB is an asymmetric system that consists of a single “host” connected to multiple “devices”. The host is typically a
personal computer. The ATmega32U4 can only act as a USB device, so an A-Star cannot be connected to other USB
devices; it can only be connected to a host such as your computer.
Programming an A-Star using the Arduino IDE as described in Section 5.2 will automatically configure it as a
composite device with a single virtual serial port and a human interface device (HID) with a keyboard and mouse.
Since we preload an example Arduino sketch on each A-Star before it ships, this is the default USB configuration of
the A-Star.
On a Windows computer, you can see the virtual serial port by going to your computer’s Device Manager and
expanding the “Ports (COM & LPT)” list. You should see a COM port labeled “Pololu A-Star 32U4”. In parentheses
after the name, you will see the name of the port (e.g. “COM3” or “COM4”). Windows will assign a different COM
port number to the A-Star depending on what USB port you plug it into and whether it is in bootloader mode or not. If
you need to change the COM port number assigned to the A-Star, you can do so using the Device Manager. Double-
click on the COM port to open its properties dialog, and click the “Advanced…” button in the “Port Settings” tab.
From this dialog you can change the COM port assigned to the A-Star.
Windows 8 device manager showing the A-Star’s virtual COM port.
On a Windows computer, you can see the rest of the A-Star’s USB interface by going to the Device Manager, selecting
View > Devices by connection, and then expanding entries until you find the “Pololu A-Star 32U4” COM port. Near
it, you should see the parent composite device and the human interface device.
Pololu A-Star 32U4 User’s Guide © 2001–2014 Pololu Corporation
6. The A-Star 32U4 USB interface Page 22 of 30