Display brightness control

Windows 8 has added new support and standardized a solution for keyboards (external or embedded on laptops) to control a laptop’s or tablet’s screen brightness through HID. The HID committee recently approved HID Review Request 41 (http://www.usb.org/developers/hidpage#Approved_Usage_Table_Review_Requests)

Architecture and overview

Windows 8 provides support for screen brightness increase/decrease as part of the consumer controls top level collection. Windows 8 supports the HID usages listed in the following table:

Usage IDUsage NameUsage Type
0x006FBrightness IncrementRe-trigger Control (RTC)
0x0070Brightness DecrementRe-trigger Control (RTC)

 

Note  These HID usages operate only on mobile systems (battery powered) and require Windows 8.

Sample report descriptor

The following section provides sample report descriptors that PC Manufacturers must leverage. Please note that if the Top Level Collection is part of a report descriptor that already has another Top Level Collection, a Report ID MUST be included (not shown in samples below).

Usage Page (Consumer)
Usage (Consumer Control)
Collection (Application)
   Logical Minimum (0x00)
   Logical Maximum (0x3FF)
   Usage Minimum (0x00)
   Usage Maximum (0x3FF)
   Report Size (16)
   Report Count (1)
   Input (Data, Array, Absolute)
End Collection

Important Notes

  • When a user presses a key, an input report is generated to identify the key. When the key is released an input report with usage value=0 is issued.
  • Only one usage is active and sent at a time. Consumer controls do not allow multiple buttons to be pressed simultaneously. When a new usage is sent, it is assumed that the usage for the previous key is released.
  • Brightness up/down are retriggering keys and their rate of repeat is handled by Windows. Hardware should not keep resending the usage when these keys are kept depressed by user. Hardware should only send an input report when button is pressed down and another when the key is released.

Troubleshooting common errors

Tip #1: Brightness increment/decrement HID usages only operate only on Mobile systems (battery powered) and require Windows 8.

Tip #2: If the system is attached to an external monitor, the brightness increment/decrement will not function as legacy monitor transports do not support the ability to channel HID messages to them / from them.

 

 

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