TAILIEUCHUNG - USB Complete fourth- P46

USB Complete fourth- P46: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 18 Differential 0 and Differential 1 When transferring data the two states on the bus are Differential 0 and Differential 1. A Differential 0 exists when D is a logic low and D- is a logic high. A Differential 1 exists when D is a logic high and D- is a logic low. Chapter 19 has details about the voltages. The Differential 0 1s don t translate directly into zero and one data states but instead indicate either a change in logic level no change in logic level or a bit stuff as explained later in this chapter. Single-ended 0 The Single-ended 0 SE0 state occurs when both D and D- are logic low. The bus uses the SE0 state when entering the EOP Disconnect and Reset states. Single-ended 1 The complement of SE0 is the Single-ended 1 SE1 . This state occurs when both D and D- are logic high. This is an invalid bus state and should never occur except as specified in the USB battery-charging specification. Data J and Data K In addition to the Differential 0 and Differential 1 states which are defined by voltages on the lines USB also defines two Data bus states J and K. These are defined by whether the bus state is Differential 0 or Differential 1 and the speed of the cable segment Bus State Data State Low Speed Full Speed Differential 0 Data J Data K Differential 1 Data K Data J Defining the J and K states in this way makes it possible to use one terminology to describe an event or logic state even though the voltages on low- and full-speed lines differ. For example a Start-of-Packet state exists when the bus changes from Idle to the K state. On a full-speed segment the state occurs when D- becomes more positive than D while on a low-speed segment the state occurs when D becomes more positive than D-. 426 Packets on the Bus Idle In the Idle state no drivers are active. On a full-speed segment D is more positive than D- while on a low-speed segment D- is more positive than D . Shortly after device attachment a hub determines whether a device is low or full speed by .

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