johntyree / knobby

An experiment in reverse engineering the Griffen Powermate USB knob

Knobby

An experiment in reverse engineering the Griffin PowerMate USB jog wheel/button on Linux.

First, install the udev rule to make sure your PowerMate appears at /dev/powermate. Adjust the RUN line to something suitable for your machine or remove it altogether if you want no notification of success. If all is done correctly, a symlink should appear when the device is plugged in. If it doesn't seem to be working, try running # udevadm control --reload-rules and replugging the device.

A small but fully functional example is in knobby/handlers/volume_control.py. Test it out with:

python -m knobby.handlers.volume_control

The example assumes that /dev/powermate is readable.

Events

Events come in from the device node as a byte stream with each event having the form

struct {
    timeval {long sec, long usec}
    unsigned int id
    int data
}

where id is unique for each kind of event and data describes the event in some way.

The events have one of three identified ids and always come in groups. All events in a group occur at the same time. The final event in a group is always end and signals that no more events will arrive with the current time stamp.

Turn:

• id: 0x00070002
• data: Magnitude of turn. Sign indicates direction with positive clockwise.

Button:

• id: 0x01000001
• data: 1 if pressed. 0 if released.

End:

• id: 0x0
• data: 0x0

API

from knobby.api import EventHandler, main

counter = 0

def my_callback(event):
    print(event)
    counter += 1
    # Stop processing after 4 events
    return counter == 4

my_handler = EventHandler(callback=my_callback)

if __name__ == '__main__':
    main(handler=my_handler)