USB Org resources
The full spec and downloadable tables etc can be found here (Device Class Definition for HID).
Get the 'HID descriptor tool' from the same page to help create descriptors – very handy!!!
Overview
A USB HID Report Descriptor is requested by a USB host from a USB device. It tells the host how to interpret the report data packets the device sends to it. It includes:
How many packets the device supports
Packet sizes
The purpose of each byte in each packet
The HID Report
An example USB HID Report Descriptor
0x05, 0x01, // USAGE_PAGE = Generic Desktop (USAGE_PAGEis much like a namespace, see "HID Usage" in the specs) 0x09, 0x02, // USAGE = Mouse (one of the available usages from the selectedUSAGE_PAGE)0xa1, 0x01, // COLLECTION = Application (Allows you to define different groups of related attributes etc)0x09, 0x01, // USAGE = Pointer (an available sub USAGE of the parent USAGE) 0xa1, 0x00, // COLLECTION = Physical (Allows you to define different groups of related attributes etc)0x05, 0x09, // USAGE_PAGE = Button (a change of USAGE_PAGE, like moving to a new namespace) 0x19, 0x01, // USAGE_MINIMUM = Button 1 (specifying the buttons) 0x29, 0x03, // USAGE_MAXIMUM = Button 3(specifying the buttons)0x15, 0x00, // LOGICAL_MINIMUM = 0 (the min value that can be reported) 0x25, 0x01, // LOGICAL_MAXIMUM = 1 (the max value that can be reported) 0x95, 0x03, // REPORT_COUNT = 3 (total number of reported data fields, in this case the number of buttons) 0x75, 0x01, // REPORT_SIZE = 1 (bits used per report field, size will be 3 fields x 1 bit = 3 bits)) 0x81, 0x02, // INPUT = Data,Var,Abs (add the above data variables to the report) 0x95, 0x01, // REPORT_COUNT = 1 0x75, 0x05, // REPORT_SIZE = 5 0x81, 0x03, // INPUT = Cnst,Var,Abs(add the above constant data to the report, in this case padding bits)0x05, 0x01, // USAGE_PAGE = Generic Desktop (returning USAGE_PAGE back again) 0x09, 0x30, // USAGE = X (specifying the mouse X axis) 0x09, 0x31, // USAGE = Y(specifying the mouse Y axis)0x15, 0x81, // LOGICAL_MINIMUM = -127 0x25, 0x7f, // LOGICAL_MAXIMUM = 127 0x75, 0x08, // REPORT_SIZE = 8 0x95, 0x02, // REPORT_COUNT = 2 0x81, 0x06, // INPUT = Data,Var,Rel 0xc0, // END_COLLECTION 0xc0, // END_COLLECTION
USAGE_PAGE
You first set a usage page. Above it is using 'Generic Desktop'
USAGE_PAGE page is much like a namespace.
See "HID Usage" in the specs.
USAGE
A usage from the available usages in the usage page setting
COLLECTION
Allows you to define different groups of related attributes etc
INPUT
Data,Var,Abs
Add the above data variables to the report
Cnst,Var,Abs
Add the above constant variables to the report (e.g. padding bits to byte align for the next data section)
Good Resources
http://eleccelerator.com/tutorial-about-usb-hid-report-descriptors/