HID Sensor Usage Tables - USB.org

104 HID Sensor Usages
o Performing a HID Get Input Report request to retrieve a single “table row” consisting of
the GUID_OR_PROPERTYKEY struct;
o After each HID Get Input Report request, the sensor firmware automatically increments
the HID_USAGE_SENSOR_DATA_ENUMERATOR_TABLE_ROW_INDEX.
Once this operation is complete, the platform driver will now know:
The top level collection number (from 2 to n) of every sensor represented by the device;
The Sensor Category and Sensor Type of each sensor;
The Sensor Events emitted by each sensor;
All of the Data Fields reported by each sensor; and for each of those Data Fields, its Report ID,
packing position, data type, size, unit of measure, and so on;
All of the Properties supported by each sensor; and for each of those Properties, its Report ID,
packing position, data type, size, unit of measure, and so on.
When the platform driver wishes to interpret any Data Field or Property, it has all the information it needs
without requiring any of the identifying information to be “inline” in the Reports themselves. This frees up
the entire packet (other than the single byte for the Report ID) for holding the data values.
Here is an example of a HID Report Descriptor illustrating this concept:
// Complete HID report descriptor
const unsigned char enum_sensor_report_descriptor[] = {
HID_USAGE_PAGE_SENSOR,
HID_USAGE_SENSOR_TYPE_COLLECTION
HID_COLLECTION(Application),
};
// Global items
HID_REPORT_SIZE(8),
HID_LOGICAL_MIN_8(0),
HID_LOGICAL_MAX_8(255),
// Report ID 0x08: an Input Report with 37 bytes of Data Fields
HID_REPORT_ID(0x08),
HID_USAGE_SENSOR_DATA_GENERIC_DATAFIELD,
HID_REPORT_COUNT(37),
HID_INPUT(Const_Arr_Abs),
// Report ID 0x09: a Feature Report with 63 bytes of Read-Only Properties
HID_REPORT_ID(0x09),
HID_USAGE_SENSOR_DATA_GENERIC_PROPERTY,
HID_REPORT_COUNT(63),
HID_FEATURE(Const_Arr_Abs),
// Report ID 0x0a: a Feature Report with 42 bytes of Read/Write Properties
HID_REPORT_ID(0x0a),
HID_USAGE_SENSOR_DATA_GENERIC_PROPERTY,
HID_REPORT_COUNT(42),
HID_FEATURE(Data_Arr_Abs),
HID_END_COLLECTION
Note from the above that the contents of the Input Reports and Feature Reports are completely opaque.
The Generic Enumerator table rows must be consulted to make any sense out of the data values in the
Reports. The advantage of this approach is that it allows complete “run time” flexibility in the semantics of
the data being transferred.
4.3 Illustrative Sensor Report Descriptors
4.3.1 Biometric: Human Presence
// Complete HID report descriptor
//Human Presence
const unsigned char pres_report_descriptor[] = {
HID_USAGE_PAGE_SENSOR,
HID_USAGE_SENSOR_TYPE_BIOMETRIC_PRESENCE,
HID_COLLECTION(Physical),