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Port Receives Connect or Disconnect

If the selectively suspended port receives either a single-ended zero (SE0) to idle transition (device connect) or an idle to SE0 transition (device disconnect), resume signaling is not sent back to the selectively suspended port; rather, the port’s output buffers are placed in the high impedance state. Status bits will be set by the hub to indicate the connect or disconnect event and that the port is no longer in the suspend state.

Selective Suspend Followed by Global Suspend

Consider the situation in which several devices have been selectively suspended prior to a global suspend occurring. Figure 9-5 illustrates such an example. Port 1 on the root hub, port 1 on device 2, and port 2 on device 7 are selectively suspended. All other devices are in their suspend states due to the global suspend. In this example, when a remote wakeup is signaled by device 9, the hub (device 7) forwards resume signaling to its other port and to its root port. Device 2 recognizes the resume signaling and forwards resume signaling to all ports except port 1, since it has been selectively suspended. The root hub detects resume signaling on its port 2 and reflects resume back to device 2, but not to port 1, since it is selectively suspended, nor to port 3, since it is disabled.

Once the system completes global resume processing, the host may then selectively awaken the devices that were selectively suspended prior to the global suspend.

Resume via Reset

When a suspended hub detects a reset on its upstream port (>2.5µs of SE0), it must initiate its wakeup sequence. The hub must be awake and have completed reset no later than 10ms from the completion of reset signaling. After reset has been completed, the hub controller must be in the following state:

•Default address is zero

•Control bits set to default values

•Hub repeater in the WFSOP state

•All downstream ports in Powered Off state (power-switched hub)

•All downstream ports in Disconnected state (no power switching)

A bus containing power-switched ports cannot guarantee that host initiated reset will propagate all the way downstream. This is because an upstream hub

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that supports power switching goes to the Powered Off state after reset. Consequently, power downstream might be removed from bus powered hubs and devices before reset processing has completed. However, a powered off device will effectively be reset if power is removed long enough. Note that once reset has been performed, all devices must be reconfigured.

Figure 9-5: Resume with Selective and Global Suspend

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Hub Frame Timer After Wakeup

While in the suspend state hubs shut their frame timers off to reduce power consumption. When a resume occurs, the frame timer must be restarted and given time to synchronize to the host by receiving SOF packets. Normally, the hub checks the state of the bus near the end of frame to determine if a device is babbling or hung in a non-idle state. However, immediately after leaving suspend, the hub has no concept of host frame timing and cannot detect these forms of bus errors until it synchronizes to the host’s frame timing. All hubs must be synchronized to the host frame timing after detecting two SOF packets.

To prevent babbling devices or LOA from hanging the bus, a hub must not propagate bus activity in the upstream direction until it has received a SOF packet. Upstream traffic is blocked by a hub when it enters the wait for start of packet from upstream (WFSOPFU) state. Hubs leave the suspend state and enter the WFSOPFU, thereby blocking upstream bus traffic. Hubs transition from WFSOPFU to WFEOPFU when they receive the first SOF packet after being awakened. The repeater then transitions back to the WFSOPFU if the frame timer is not locked to SOF packets. Once the frame timer is locked, the SOP from upstream causes a transition from WFEOPFU to WFSOP. (See Figure 9-6 on page 209.)

Note that downstream traffic is propagated downstream even when the hub is in the WFSOPFU state. If devices downstream are addressed, they will likely respond by sending packets back to the host. Since the packet transmission is blocked by the hub, a time-out will be detected by the host, due to no response from the device. Consequently, the specification recommends that the host not address any device residing on a bus segment that has been just resumed, thus avoiding needless bus time-outs.

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Figure 9-6: Repeater State Machine With Suspend and Resume Transitions

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