TAILIEUCHUNG - USB Complete fourth- P34

USB Complete fourth- P34:This book focuses on Windows programming for PCs, but other computers and operating systems also have USB support, including Linux and Apple Computer’s Macintosh. Some real-time kernels also support USB. | Chapter 12 Converting Units The Physical Minimum Physical Maximum Unit Exponent and Unit items define how to convert reported values into more meaningful units. Physical Minimum and Physical Maximum. The Physical Minimum and Physical Maximum define the limits for a value when expressed in the units defined by the Units tag. In the earlier example of values of zero through 250 in units of 2 mA the Physical Minimum is zero and the Physical Maximum is 500. The receiving device uses the logical and physical limit values to obtain the value in the desired units. In the example reporting the data in units of 2 mA means that the value can transfer in a single byte with the receiver of the data using the Physical Minimum and Maximum values to translate to mA. The price is a loss in resolution compared to reporting 1 bit per mA. If the report descriptor doesn t specify these items they default to the Logical Minimum and Logical Maximum. Unit Exponent. The Unit Exponent specifies what power of 10 to apply to the value obtained after using the logical and physical limits to convert the value into the desired units. The exponent can range from -8 to 7. A value of zero causes the value to be multiplied by 100 which is the same as applying no exponent. These are the codes Exponent 0 1 2 3 4 5 6 7 -8 -7 -6 -5 -4 -3 -2 -1 Code 00h 01h 02h 03h 04h 05h 06h 07h 08h 09h 0Ah 0Bh 0Ch 0Dh 0Eh 0Fh For example if the value obtained is 1234 and the Unit Exponent is 0Eh the final value is . Unit. The Unit tag specifies what units to apply to the report data after the value is converted using the Physical and Unit Exponent items. The HID specification defines codes for the basic units of length mass time temperature current and luminous intensity. Most other units can be derived from these. Specifying a Unit value can be more complicated than you might expect. Table 12-4 shows values to work from. A value can be as long as four bytes with each nibble having a defined function. Nibble 0 .

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