GitHub - jameslockman/Griffin-PowerMate-Driver

6 min read Original article ↗

Griffin PowerMate driver for modern MacOS

This small driver enables the Griffin PowerMate, a nifty little device from days gone by. What does the PowerMate do? It is a knob that you can twist or that you can press. That's it. It also has a blue LED in the base that can change intensity based on what you're doing.

When it was released, it was intended to assist video and audio production by adding a scrollable knob to your desktop. Of course, modern controllers exist that offer many more literal bells and whistles, but there is something... quaint... about this early device.

To install, open the DMG and drag PowerMate Agent to your Applications folder. Then, launch PowerMate Agent. Of course, it won't do anything without a PowerMate, so go dig around in your junk USB drawer and dust it off! You will see a new item in your top menu to control the behavior of the PowerMate. Screenshot_5_10_26__9_27 AM

The PowerMate acts as a scroll control, so if the active window or control has a scroll option, turning the dial will scroll the window or increase/decrease selected value. You can reverse the scroll direction if you don't like the default scroll direction.

The PowerMate also acts as a mouse button. A momentary push of the button acts as a mouse click. A long-press of the button acts as a right-click. You can also change the behavior so that a long-press acts as a double-click.

Pretty simple, eh?

Technical details.

A small macOS driver that opens the Griffin PowerMate (VID 0x077d, PID 0x0410) over USB HID, reads its 6-byte reports, and exposes button and rotation events so you can map them to actions (e.g. scroll, click, media keys).

The device reports on the bus but does nothing by default on macOS; this library seizes the device and delivers events to your app.

Report format (from device)

  • Byte 0: Button state — 0 = released, 1 = pressed
  • Byte 1: Rotation delta — signed; positive = clockwise, negative = counter‑clockwise (typically ±1 to ±7 per report). The device does not report speed directly; the driver derives rotation rate (deltas per second) from the time between reports.

Build and run

cd /path/to/USB
swift build
swift run PowerMateDemo

With the PowerMate plugged in, turn the knob or press the button; the demo prints events. Stop with Ctrl+C.

System-wide driver (PowerMate Agent)

PowerMateAgent turns the knob and button into keyboard/scroll events that any application receives (browser, editor, etc.):

  • Rotation → vertical scroll, or Up/Down arrow keys when a menu (or submenu) is focused.
  • Click (short press) → left mouse button (at cursor), or Return when a menu is focused (chooses the highlighted item).
  • Long press → right mouse button (at cursor).

Menu and submenu detection uses the Accessibility API: when the focused UI element is a menu (including submenus), rotation sends arrow keys and click sends Return. Grant Accessibility in System Settings → Privacy & Security so submenus work without “sticking” to scroll. A long-press still enters a fallback “menu mode” (arrow keys until click or 5‑second timeout) if Accessibility is not enabled.

The LED throbs while you turn and goes dim when idle; full on while the button is held.

On first run, macOS will prompt for Input Monitoring permission. Grant it in System Settings → Privacy & Security → Input Monitoring and add (or enable) the Terminal or the built executable, then run the agent again.

To run in the background: swift run PowerMateAgent & or run the built binary ./.build/debug/PowerMateAgent and add it to Login Items if you want it to start when you log in.

To create a signed, notarized app (or installer) so others can use it without security warnings, see DISTRIBUTION.md. You’ll need an Apple Developer account; users will still need to grant Input Monitoring (or Accessibility) once when they first use the dial.

Use in your app

1. Add the package

In your app’s Package.swift (or Xcode: File → Add Package Dependencies):

dependencies: [
    .package(path: "/path/to/USB"),  // or your clone URL
],
targets: [
    .target(name: "YourApp", dependencies: ["PowerMateDriver"]),
]

2. Start the driver and map events

import PowerMateDriver

let driver = PowerMateDriver()

// Optional: use closures for simple mapping
driver.onRotate = { delta, rate in
    // delta > 0 = clockwise, delta < 0 = counter-clockwise
    // rate = deltas per second (nil on first report); use for speed-dependent mapping
    // e.g. scroll: CGEventCreateScrollWheelEvent(..., delta * lineHeight)
}
driver.onButtonDown = { /* e.g. simulate click or toggle */ }
driver.onButtonUp = { }

// Or use the delegate for all events
driver.delegate = self  // implement PowerMateDriverDelegate

driver.start()
// Keep run loop running (e.g. main thread in an app)

3. Event types

  • PowerMateEvent.buttonDown / buttonUp — knob pressed / released
  • PowerMateEvent.buttonClick — short press and release (under longPressThreshold).
  • PowerMateEvent.buttonLongPress — press held at least longPressThreshold, then release.
  • PowerMateEvent.rotate(delta: Int, rate: Double?) — delta is the signed step count (e.g. +1, -2); rate is derived rotation speed in deltas per second (nil on first report).

Set longPressThreshold (default 0.4 seconds) to tune what counts as a long press. Use onClick and onLongPress (or the delegate) for separate handling.

Use driver.isConnected to see if the device is currently opened.

LED (blue light in base)

The base has a blue LED you can use for feedback. Control it only when the device is connected (isConnected == true).

  • setLEDBrightness(_ value: UInt8) — static brightness 0–255 (0 = off).
  • setLEDPulseAsleep(_ on: Bool) / setLEDPulseAwake(_ on: Bool) — turn the built‑in pulse when “asleep” or “awake” on or off.
  • setLEDPulseMode(table:op:arg:) — custom pulse: table 0–2, op 0 = slower, 1 = normal, 2 = faster, arg 1–255 for op 0/2.

LED commands use USB vendor control requests (same protocol as the Linux driver). They return true if the command was sent successfully. If the USB device is busy (e.g. another process has it open), LED calls may fail.

Mapping to system actions

  • Scrolling: In onRotate, create a scroll wheel CGEvent (e.g. CGEventCreateScrollWheelEvent) and post it, or feed the delta into your own scroll logic.
  • Click: In onButtonDown / onButtonUp, create and post a mouse click CGEvent, or call your own click handler.
  • Media / other: Map onRotate and onButtonDown to whatever you need (e.g. volume, key equivalents).

Posting events may require Input Monitoring (or Accessibility) in System Settings → Privacy & Security for your app.

If the device doesn’t respond

  1. Unplug and replug the PowerMate, then run your app again.
  2. Quit other software that might be using the PowerMate (e.g. old PowerMate apps).
  3. The driver uses kIOHIDOptionsTypeSeizeDevice so it takes exclusive access; only one process can use it at a time.

Requirements

  • macOS 13+
  • Swift 5.9+

References