11 The High-Speed

Signaling

Environment

The Previous Chapter

The previous chapter provided a brief introduction to high-speed device operation and was intended to set the stage for a detailed discussion of the highspeed environment starting with this chapter and others that follow.

This Chapter

High-speed capable devices must also be able to communicate in the full-speed signaling environment. High-speed devices add many extensions to the fullspeed environment to permit reliable signaling at a 480Mb/s rate. This chapter introduces the principles associated with USB high-speed signaling and the methods used to switch between fulland high-speed operation.

The Next Chapter

The next chapter introduces the changes brought about by the USB 2.0 specification. The transfers defined in USB 1.0 have the same primary characteristics in the high-speed environment; however, some changes have been made such as new packet sizes. Also, new features have been added to the high-speed environment such as high-bandwidth transfers and the ping protocol. These and other changes are reviewed in this chapter.

Overview

High-speed devices must be able to operate at both full-speed and high-speed. This ability ensures that any high-speed device attached to a 1.x port (i.e., root ports in 1.x system or 1.x hub ports) will work properly. This requirement is

217

USB System Architecture

important for compatibility with the large number of existing 1.x hubs and systems. Similarly, high-speed hubs must support low-, full-, and high-speed devices attached to their downstream ports to ensure compatibility with all USB devices. Figure 11-1 on page 218 summarizes the required port speed capabilities of each high-speed hub and device transceiver.

High-speed hubs must be able to detect whether they are connected to a fullspeed or high-speed port and operate correctly at that speed. When the highspeed hub is connected to a high-speed port, it must be able to detect whether a high-speed/full-speed/low-speed device is attached and perform transactions to devices at their speed.

Figure 11-1: High-Speed Capable Ports Must Support a Variety of Speeds

+RVW &RQWUROOHU

URRW KXE SRUWV PXVW RSHUDWH DW /6 )6 +

$OO +6 GHYLFHV PXVW RSHUDWH

DW )6 +6

+6 +XE)6 'HY /6 'HY

218