USB System Architecture

Since selective suspend in this case has not been initiated by the host, the host must check status on a regular basis (poll the hub) to determine if resume has occurred. (The sections entitled “Get Port Status Request” on page 456 defines the mechanism used to check port status.)

If wakeup at the port is due to device attachment or detachment, the same actions are taken by the hub, except that resume signaling is not sent back to the initiating port. Instead, the port’s output buffers are placed into the high impedance state, and port status bits are changed to reflect the device attachment or detachment.

Selective Suspend When Hub is Suspended

After an individual hub port has been suspended, host software could subsequently suspend the entire hub (via either global or selective suspend). Three conditions may occur that require the hub to take action:

•device currently attached to suspended port signals resume

•device is connected to selectively suspended port

•device is disconnected from selectively suspended port

Device Signals Resume

If the device attached to the suspended port signals resume (wakeup), the suspended hub takes the following action as illustrated in Figure 9-4:

1.Resume is signaled to all downstream ports that are enabled and back to the suspended port. (The hub must signal resume within 100µs of detecting resume from the device, and must continue resume signaling for a minimum of 20ms.) Note that selectively suspended ports are left alone and remain in their selectively suspended state.

2.Resume is signaled upstream to its root port, also within 100µs of receiving resume from the device. However, resume must be released and connectivity reversed within 10ms.

3.The upstream port will signal resume back to the hub for a minimum of 20ms. This resume signaling is reflected to all enabled downstream ports, including the device that initiated wakeup.

4.The root port drives a low-speed EOP at the end of resume signaling and the hub enters the wait for start of frame (WFSOF) state.

5.Status is set by the hub to indicate that the selectively suspended port is now awake. The bit must be set no earlier that 3ms to give time for the hub’s frame timer to synchronize with the host.

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Chapter 9: USB Power Conservation

Note that the upstream port may either be suspended due to global suspend, or may have been selectively suspended. In the event of global suspend, resume signaling will be reflected upstream to the root hub. If the upstream port is selectively suspended, as illustrated in Figure 9-4, then resume signaling will be reflected back to the downstream hub, but not propagated to other ports. The hub labeled device 2 has not been suspended, but its port number 4 has been selectively suspended. When resume is detected at port 4, the hub returns resume signaling to device 7 and takes no further action.

Figure 9-4: Device Initiated Selective Resume to Suspended Hub

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