6 LS/FS Transfer

Types & Scheduling

The Previous Chapter

USB employs NRZI encoding and differential signaling to transfer information across USB cables. The previous chapter discussed the lowand full-speed signaling environment, including the differential signaling and NRZI encoding techniques used by the USB. The signaling environment must also support a wide range of other signal-related functions such as: detecting device attachment and removal, suspending and resuming operation, resetting a device, and others, all of which were discussed in the previous chapter.

This Chapter

USB supports four transfer types: interrupt, bulk, isochronous, and control. These transfer types and the process used to initiate and perform them are described in this chapter.

The Next Chapter

Every transfer broadcast over the USB consists of a combination of packets. These packets are combined to define individual transactions that are performed as part of a larger transfer. Each transaction type is defined, along with the individual packets that compose it.

Overview

Each endpoint within a given USB device has particular characteristics that dictate how it must be accessed. The transfer characteristics relate to the requirements of the application. The following four transfer types have been defined by the USB specification, each of which reflects the nature of transfers that may be required by a USB device endpoint:

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