USB System Architecture

Overview

High-speed transfer types are the same as those used by fulland low-speed devices and include:

•isochronous

•interrupt

•bulk

•control

Although these transfers are the same as those defined by USB 1.x, in some cases the high-speed transactions have new characteristics that are designed to improve performance. Also, note that high-speed packets differ from their fullspeed cousins in the following ways:

1.The synchronization sequence at the beginning of each packet is 4 bytes in length rather than 1 byte.

2.EOP is 1 byte rather than 2 bits.

Consequently, packet overhead associated with high-speed transaction are higher.

This chapter describes the characteristics of the high-speed transfers and transactions, and describes the mechanisms used for scheduling and executing them. Since isochronous and interrupt transfers have important characteristics in common, they will be discussed in the section entitled “Periodic Transfers,” while bulk and control transfers are discussed in the section entitled “Non-Peri- odic Transfers.”

High-Speed Transaction Scheduling

All high-speed devices share bus bandwidth when the devices are attached to ports of the same controller. High-speed transactions also use the same token, data, and handshake protocol that is used with fulland low-speed transactions. Software calculates the time required to perform transactions and schedules transactions to be completed during each microframe.

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