HID Usage Tables Document 1.1 - C. Scott Ananian

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96 Universal Serial Bus HID Usage TablesA.8 A Device with a DisplayThe following example is of a 2x16-character display device. The device uses one Feature, one Input, andtwo Output reports.A Feature report is declared for identifying fixed features of the display and display status. All of theFeature report’s fields are constants.The Character Spacing usage is not declared, so it can be assumed that the respective inter-character spacingis forced by the pixel layout of the display, and any downloaded font characters do not have to include it.In this example, the Character Height and Width are fixed. The fields are declared in the Report descriptorand the actual values are reported when the Feature report is read. For example, the Character Height andWidth fields will always return 7 and 5, respectively.Uploading of the font is not supported by this display so there is no Font Report Input report.UsagePage(Alphanumeric Display),Usage(Alphanumeric Display),Logical Minimum(0),Collection(Application),The first report defined is a Feature report with seven fields. The Rows, Columns, Character Height andWidth fields are Static Values (SV) and their report size is set to 5 to demonstrate how the bit packing takesplace in a report. Standard Character Set, Data Read Back and Vertical Scroll are Static Flags (SF).Usage(Display Attributes Report),Collection(Logical),Usage(Rows), ; Constant = 2Usage(Columns), ; Constant = 16Usage(Character Width), ; Constant = 5Usage(Character Height), ; Constant = 7ReportID(1),Logical Maximum(31),ReportSize(5), ReportCount(4),Feature(Constant, Variable, Absolute), ; Four 5-bit fieldsReportSize(1), ReportCount(3),Logical Maximum(1),Usage(ASCII Character Set), ; Constant = 1Usage(Data Read Back), ; Constant = 1Usage(Vertical Scroll), ; Constant = 1Feature(Constant, Variable, Absolute), ; Three 1-bit fieldsReportCount(1),Feature(Constant, Variable, Absolute),End Collection(),; 1-bit padVersion 1.1 April 8, 1999
Universal Serial Bus HID Usage Tables 97The second report defined is an Input report that is generated on the interrupt endpoint each time the statusof the display changes. Each of the possible states that can be identified by the display are identified in theDisplay Status collection. This report can also be read over the control pipe to determine the current status.ReportSize(8), ReportCount(1),Logical Maximum(2),Usage(Display Status),Collection(Logical),Usage(Stat Not Ready),Usage(Stat Ready),Usage(Err Not a loadable character),Input(Data, Array, Absolute, No Null),End Collection(),; 8-bit status fieldA second Feature report is defined for getting or setting the current cursor position.Usage(Cursor Position Report),Collection(Logical),ReportID(2),ReportSize(4), ReportCount(1),Logical Maximum(15),Usage(Column),Feature(Data, Variable, Absolute, No Preferred State), ;ColumnLogical Maximum(1),Usage(Row),Feature(Data, Variable, Absolute, No Preferred State), ;RowEnd Collection(),There are a number of ways that data can be transferred between the host and the display: one byte at a time,multiple bytes, or the whole screen using a 32-byte buffered-byte transfer. The choice may depend onwhether the device is implemented as a low-speed or a high-speed device. In this example, a third Featurereport is defined for writing up to four sequential characters from the display in a single report. Note that theData Read Back usage is not declared in the Report descriptor, which implies that the display character datais write-only.Usage(Character Report),Collection(Logical),ReportID(3),ReportSize(8), ReportCount(4),Logical Maximum(126),Usage(Display Data),Feature(Data, Variable, Absolute, Buffered Bytes),End Collection(),;4-byte data bufferVersion 1.1 April 8, 1999
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HID Usage Tables Document 1.1 - C. Scott Ananian

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