AVR Programmer Built From a USB Keyboard

Recently, my keyboard became louder and louder while typing, so I decided to buy a new one. But what to do with the old one? Ever since I saw the AVR programmer that uses a USB hub, I thought of abusing a USB keyboard for that.

Theory

Most keyboards have at least three indicator LEDs (Num-, Caps- and Scroll-Lock), which can be controlled from the host using a HID Set_Report request, and thus can be used as general purpose outputs. The inputs are a bit more tricky, since the keyboard uses a scan matrix divided in rows and columns. Most keyboards also do some debouncing and detect rows that are ‘stuck’, which means that we need to simulate the keypress of a single key. If a key has been pressed, the keyboard triggers an interrupt transfer with 8 bytes of data, containing the current state of all keys. The first byte reflects the state of the modifier keys (shift, ctrl, alt, etc.) which I’ll be using as inputs. Additionally, the keys can be polled using a Get_Report request to the control endpoint, but the interrupt transfers need to be handled either way, since my keyboard just locked up after the first keypress when I didn’t handle them first.