Chapter 6: LS/FS Transfer Types & Scheduling

Establishing Synchronous Connections

Many types of devices that connect to systems via USB are by nature synchronous devices. A partial list of devices that deal with synchronous data include:

•Telephones

•High-Speed Modems

•Microphones/Headsets

•High-End Digital Speakers

•CD Audio Player

•DVD Players

•Video Conferencing Camera

•Digital Satellite Receivers

The data at the source and sink is by nature synchronous, and the link between the source and sink must deliver the data synchronously. However, USB does not support synchronous streams. Instead, USB provides isochronous transfers as a way to establish a synchronous connection between a source and sink (such as a CD audio player and speakers). The following discussion of isochronous transactions is based on full-speed devices and 1 ms frames. However, the same principles can be applied to and are used by high-speed devices and 125 s microframes.

The Problem with Isochronous Transfers

Figure 6-3 on page 126 illustrates an example of an isochronous application consisting of a CD-ROM and speaker. An isochronous application must maintain synchronous communications between the CD-ROM and the speaker. The actual communications path includes device drivers and application software.

The sample clock at the source determines the amount and rate of data to be transmitted. The sink must receive this synchronous data and produce the required output. Figure 6-4 on page 127 illustrates an example isochronous transaction from the CD player. The synchronous data that is sourced during frame N is delivered across the USB bus in frame N+1, etc. This represents a synchronous to isochronous conversion of the data. Note that in this example, the isochronous transaction carries the synchronous data, but the clock is not encoded within the data stream.

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USB System Architecture

Figure 6-3: Isochronous Application Using USB CD-ROM and Speakers

Isochronous Pipe

Source

USB Device Driver

USB Device Driver

Sink

Speaker

Isochronous Pipe

Figure 6-5 on page 127 illustrates the isochronous transaction delivered to the speaker. It receives the data during clock N but does not know the source clock rate; thus, the isochronous to synchronous conversion cannot be made.

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Chapter 6: LS/FS Transfer Types & Scheduling

Figure 6-4: Example of Source Device Delivering Isochronous Data to the Bus

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Figure 6-5: Example of Sink Device Receiving Isochronous Data from the Bus

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