Why not choose a microcontroller with built-in USB hardware? There are several of these available (see ). You can choose the License: Open Source or commercial.Only about 1150 to 1400 bytes code size.High level functionality is written in C and is well commented.Can be clocked with 12 MHz, 15 MHz, 16 MHz 18 MHz or 20 MHz crystal or from a 12.8 MHz or 16.5 MHz internal RC oscillator.No UART, timer, input capture unit or other special hardware is required (except one edge triggered interrupt).Runs on any AVR microcontroller with at least 2 kB of Flash memory, 128 bytes RAM and a clock rate of at least 12 MHz.Comes with freely usable USB identifiers (Vendor-ID and Product-ID pairs).Transfer sizes up to 254 bytes by default, more as configuration option.(Note that the USB specification forbids bulk endpoints for low speed devices, but V-USB supports them to some degree.) Supports multiple endpoints: one control endpoint, two interrupt/bulk-in endpoints and up to 7 interrupt/bulk-out endpoints.Example projects demonstrate device and host driver implementations on Linux, Mac OS X and Windows.Fully USB 1.1 compliant low-speed device, except handling of communication errors and electrical specifications.

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