Resulting HID report has to fit into 8 byte payload of low speed interrupt frame, which works out as 6 bytes for keys, 1 byte for LED states (which is arguably

Resulting HID report has to fit into 8 byte payload of low speed interrupt frame, which works out as 6 bytes for keys, 1 byte for LED states (which is arguably redundant) and one byte for modifier keys. On the other hand I can't understand why they had designed HID in this way, when essentially all previous PC keyboard interfaces worked by sending state changes (key down/key up) and not full state. It's not like that generating down/up events would add significant complexity to keyboard, there has to be whole lot of stuff for the USB itself and also it's already there for PS/2.