Chapter 9: USB Power Conservation

Since each device attached downstream also enters the suspend state, each device can each draw up to 500µa. This means that the hub itself may draw Configured bus-powered hubs may also draw up to 2.5ma with devices attached to it downstream ports. Unconfigured hubs like all other unconfigured devices must limit current draw to 500 a. Further, when in the suspend state, hubs must be able to supply the maximum current defined for their downstream ports to support remote wakeup. That is, a device may drive the bus during suspend to wake up the system as will be discussed later in this chapter.

Global Suspend

Global suspend places the entire USB network of devices into the suspend state from the top down. This provides for minimum power consumption from the USB. The host initiates global suspend typically in response to a prolonged period of bus inactivity.

Initiating Global Suspend

Global suspend is initiated when all downstream traffic from the root hub is terminated. This is done under software control by issuing a global suspend request to the root hub’s control endpoint. All USB devices (hubs and functions) automatically enter the suspend state when they encounter 3.0ms of inactivity.

Resume from Global Suspend

Devices awaken from their suspend state when they detect resume signaling on the bus. Resume is signaled by a non-idle state (K state), initiated in the following ways:

•by the root hub, causing the resume signaling to occur on all downstream cable segments.

•by a device on any downstream cable segment attached to an enabled port, causing the hub to reflect the resume signaling back to that port, to all other enabled ports downstream, and upstream to its root port.

•by device attachment.

•by device detachment.

•by reset, causing all devices to be reconfigured.

The following sections discuss the methods of handling suspend and resume.

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Resume Initiated by Host

Host software may initiate USB wakeup by issuing a resume request to the root hub. The root hub responds by signaling resume to all downstream ports that are enabled as illustrated in Figure 9-1 on page 198. The resume signaling must be maintained for 20ms to give each attached device sufficient time to recover from suspend and be ready to receive transactions. The root hub ends resume signaling by driving an EOP for two low-speed bit times.

Downstream hubs that receive the 20ms of resume signaling must also propagate the resume signaling to all of their enabled downstream ports as illustrated in Figure 9-1.

Figure 9-1: Host Initiated Resume

PCI Bus

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Remote Wakeup from Device

A device in the suspend state may respond to an external event by signaling a wakeup to the system via the USB. Resume signaling is initiated by the device by driving a K state (non-idle) onto the bus. The actions taken by the hub are:

•signal resume to upstream port

•signal resume to all enabled downstream ports

•reflect resume signaling back to the originating device

Figure 9-2 illustrates the sequence of events and describes the timing associated with device initiated resume. The following steps and timing apply:

1.A K state is signaled by the device to the hub port (t0).

2.The port detects the resume signaling.

3.Resume signaling is broadcast by hub 7 to its upstream port and to all enabled downstream ports within 900µs of receiving the resume (t1).

4.Hub 2 signals resume to all of its enabled ports within 900µs of receiving the resume from hub 7.

5.Hub 7 ceases driving resume in the upstream direction within 1-15ms of t0. but not earlier than 100µs and reverses connectivity. This is referred to as t2.

6.Hub 2 ceases driving resume in the upstream direction within 10ms of receiving resume from hub 7, but not earlier than 50µs.

7.When the root hub detects the resume signaling, it initiates 20ms of resume signaling downstream to all ports, including the originating port.

8.The root hub terminates resume signaling by driving two low-speed EOPs.

Note that during the interval between t0 and t2, the host may have started driving resume signaling from the upstream direction, while the downstream hub is still driving resume signaling. Since it is acceptable to drive both ends of a bus segment to the same state, no problem is encountered.

Remote Wakeup via Hub Port Event

Remote wakeup may be signaled by a hub if it is enabled to produce a remote wakeup. In this case, when a hub detects a status change event (a bit is set in either the hub port status or status change register), it signals wakeup to the system in the same fashion as described for remote wakeup. See “Port Status Fields” on page 457 and “Port Change Fields” on page 459 for definition of the events that can cause a hub to generate remote wakeup.

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Figure 9-2: Global Resume Signaling Due to Wakeup from Target Device

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