VUSB

V-USB, commonly written as V-USB or VUSB, is a software-only USB protocol stack for AVR microcontrollers that do not include native USB hardware. It enables inexpensive ATtiny and AVR-based devices to connect to USB hosts by bit-banging the USB signals on general-purpose I/O pins.

The implementation targets USB 1.1 low-speed operation (1.5 Mbit/s) and relies on precise timing rather than dedicated USB peripherals. D+ and D- lines are driven by software, with a 1.5 kΩ pull-up applied on D- to advertise a low-speed device. The stack provides the USB device framework, including control transfers and interrupt endpoints, and is written primarily in C for AVR-GCC. It is distributed as open-source code for a variety of AVR models, such as ATtiny45/85 and ATmega series.

Use cases are predominantly hobbyist and educational. V-USB is well-suited for simple USB devices such as keyboards, mice, and custom HID peripherals. It is not designed for full-speed or high-bandwidth applications, and performance depends on the microcontroller’s speed and memory. Complex USB classes or bulk transfers may be impractical on devices with limited resources.

V-USB originated as an open-source project in the mid-2000s and has since influenced how hobbyists prototype USB devices without dedicated hardware. The project is licensed under the GNU General Public License, and its source code and documentation are widely available online.

Overall, V-USB remains a notable reference implementation that demonstrates how USB can be accessed from software on simple MCUs, enabling numerous low-cost USB peripherals in the maker community.