The Atmel AT43320 is a microcontroller-based USB hub with an embedded function. The USB hub has five downstream ports, one of which is permanently attached. Internally the AT43320 consists of a USB hub and function interface, a hub repeater, and a high-performance, efficient 8-bit RISC microcontroller. The hub and peripheral device controller function is implemented in the microcontroller's firmware. To the USB host, the embedded function appears as an attached port of the hub with its own device address and endpoints. The AT43320 can also be configured as a USB function without the hub. The USB hardware block consists of a USB transceiver, SIE, hub repeater, endpoint controllers, and an interface to the microcontroller. The USB hardware interfaces to the USB host at the transaction level. The CPU of the microcontroller is based on the Atmel AVR microcontroller and is capable of addressing up to 128K bytes of external program memory. The AVR architecture was developed to be programmed in C efficiently and with maximum performance.