Chapter 11: The High-Speed Signaling Environment

High-Speed RESET and Suspend

Both device RESET and device suspend are signaled by >3ms of bus idle. The high-speed device must distinguish between the two events. Another common trait of high-speed RESET and suspend is that the device must transition to fullspeed signaling. The common characteristics of RESET and suspend end at this point. The following sections define the differences between RESET and suspend signaling.

Signaling RESET

The hub signals RESET when software issues a PortReset command to the hub. This causes the hub to signal a SE0 for >10ms and <20ms. Since the high-speed idle state is indicated by SE0, the device must determine if the reason for a long period of idle is due to a RESET.

When high-speed devices are reset they must transition to full-speed signaling mode. The device disables its high-speed terminators (i.e., disables SE0 at its full-speed driver) and reconnects the 1.5kΩ resistor on D+. Since reset is being signaled by the hub, the device continues to detect SE0.

Signaling Suspend

Hubs may suspend all of their ports or may suspend selected ports by no longer transferring any packets to the suspended port(s). A suspended condition may last for very long periods to conserve power. Ports are suspended when software issues a PortSuspend command to a specific hub, or when the host controller suspends the entire network of USB devices, called global suspend. In either case, a device attached to a suspended port will no longer see any traffic, including all microSOF packets.

Devices are required to go to their suspended state after 3ms of bus idle time. During idle, high-speed devices must transition to full-speed signaling by disconnecting their high-speed terminators and reconnecting their 1.5kΩ resistor on D+. The hub in turn transitions to full-speed operation by disabling its highspeed terminators. These actions place the bus in a full-speed idle state (D+ pulled up and D- pulled down), which is consistent with full-speed suspend.

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USB System Architecture

Differentiating Between RESET and Suspend

A high-speed device must revert to full-speed mode after 3ms of high-speed idle but before 3.125ms of high-speed idle. Next, the device determines whether this prolonged interval of bus idle is caused by RESET or a suspend event. The device samples D+ and D- as follows to determine the cause of the extended idle:

•The device samples the state of D+ and D- after 100 s and before 875 s from its transition to full-speed mode.

•If D+ and D- are both driven low, then RESET is being signaled by the hub, and the chirp sequence is performed.

•If D+ is high (pulled up) and D- is low (pulled down), then full-speed idle is being signaled by the hub, and the device must enter suspend.

Note that following resume the device automatically returns to high-speed signaling mode.

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12 HS Transfers,

Transactions, &

Scheduling

The Previous 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. The previous chapter introduced the principles associated with USB high-speed signaling and the methods used to switch between fulland high-speed operation.

This Chapter

This chapter introduces the changes brought about by the USB 2.0 specification. The transfers defined in USB 1.0 are also used in 2.0 implementations and have the same primary characteristics in the high-speed environment; however, some changes have been made such as new packet sizes. Furthermore, 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.

The Next Chapter

Error detection and handling during high-speed transactions is very similar in concept to the lowand full-speed error detection methods. However, due to the faster clock rates, several of the timing parameters must be changed to support error detection implementations such as time-out values and babble detect. These issues are discussed in the next chapter.

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