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Hardware: USB-HID Protocol Evolution (WIP)

Published on 2018-03-28.

Motivation

Existing Report Format for Keyboards

The USB descriptor for a standard keyboard looks roughly like this:

Usage Page (Desktop),                   ; Generic desktop controls (01h)
Usage (Keyboard),                       ; Keyboard (06h, application collection)
Collection (Application),
    Usage Page (Keyboard),              ; Keyboard/keypad (07h)
    Usage Minimum (KB Leftcontrol),     ; Keyboard left control (E0h, dynamic value)
    Usage Maximum (KB Right GUI),       ; Keyboard right GUI (E7h, dynamic value)
    Logical Minimum (0),
    Logical Maximum (1),
    Report Count (8),
    Report Size (1),
    Input (Variable),
    Report Count (1),
    Report Size (8),
    Input (Constant),
    Usage Page (LED),                   ; LEDs (08h)
    Usage Minimum (01h),
    Usage Maximum (05h),
    Report Count (5),
    Report Size (1),
    Output (Variable),
    Report Count (1),
    Report Size (3),
    Output (Constant),
    Usage Page (Keyboard),              ; Keyboard/keypad (07h)
    Usage Minimum (None),               ; No event (00h, selector)
    Usage Maximum (FFh),
    Logical Minimum (0),
    Logical Maximum (255),
    Report Count (6),
    Report Size (8),
    Input,
End Collection

It contains usage pages for reading the modifier keys (first mention of the keyboard usage page), reading the rest of the keyboard keys (second mention of the keyboard usage page), and writing status LEDs (LED usage page).

Given this descriptor, the payload sent from the keyboard to the computer looks like this:

+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | +---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────┬───────┘ Modifiers Reserved Scancode 1 Scancode 2 Scancode 3 Scancode 4 Scancode 5 Scancode 6

This means that each report can report the scancodes of at most 6 simultanouesly pressed keys, a limitation often described with the term 6KRO (six key roll-over).

Various approaches have been tried to work around this limitation in the past, but large-scale adoption has been limited.

Universal Report Format for Keyboards

+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | +---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ ╰──────┬──────╯ ╰──────┬──────╯ ╰──────────────────────┬──────────────────────╯ ╰──────────────────────┬──────────────────────╯ Modifiers left Modifiers right Universal code 1 Universal code 2 ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ ╰──────────────────────┬──────────────────────╯ ╰──────────────────────┬──────────────────────╯ ╰──────────────────────┬───────┈ Universal code 3 Universal code 4 Universal code 5 ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+┅ ┈──────────────╯ ╰──────────────────────┬──────────────────────╯ ╰──────────────────────┬──────────────────────╯ ╰──────────────┈ Universal code 6 Universal code 7 ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┅+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+ ┈───────┬──────────────────────╯ ╰──────────────────────┬──────────────────────╯ ╰──────────────────────┬──────────────────────╯ Universal code 8 Universal code 9 Universal code 10

Universal Code – Unicode codepoints

+---------------+---------------+---------------+ |0┊ ┊ ┊u┊u┊u┊u┊u|u┊u┊u┊u┊u┊u┊u┊u|u┊u┊u┊u┊u┊u┊u┊u| +---------------+---------------+---------------+ │ ╰────────────────────┬────────────────────╯ Unicode selector Unicode codepoint +------┅ |0┊0┊0┊ +------┅ ╰─┬─╯ Single codepoint +------┅ |0┊1┊0┊ +------┅ ╰─┬─╯ Multiple codepoints – start +------┅ |0┊1┊1┊ +------┅ ╰─┬─╯ Multiple codepoints – continue +------┅ |0┊0┊1┊ +------┅ ╰─┬─╯ Multiple codepoints – end

Universal Code – Legacy Usage Pages

+---------------+---------------+---------------+ |1┊s┊s┊s┊s┊s┊s┊s|p┊p┊p┊p┊p┊p┊p┊p|p┊p┊p┊p┊p┊p┊p┊p| +---------------+---------------+---------------+ │ ╰─────┬─────╯ ╰──────────────┬──────────────╯ │ Usage Page Usage ID Legacy Selector +---------------+---------------+---------------+ |1┊0┊0┊0┊0┊1┊1┊1|0┊0┊0┊0┊0┊0┊0┊0|0┊0┊0┊0┊0┊1┊0┊0| +---------------+---------------+---------------+ ╰─────┬─────╯ ╰──────────────┬──────────────╯ Keyboard Page 0x07 Usage ID 0x04 (letter "A")