Universal Serial Bus Specification Revision 1.1

Note: when the driver is turned on, the upstream port will be in the 'J' state and the downstream ports should be driven to the same state.

Low-speed data follows the PID and is propagated to both low and full-speed devices. Hubs continue to propagate downstream signaling to all enabled ports until a downstream EOP is detected, at which time all output drivers are turned off.

Full-speed devices will not misinterpret low-speed traffic because no low-speed data pattern can generate a valid full-speed PID.

When a low-speed device transmits, it does not preface its data packet with a PRE PID. Hubs will propagate upstream-directed packets of any speed using full-speed signaling polarity and edge rates.

For both upstream and downstream low-speed data, the hub is responsible for inverting the polarity of the data before transmitting to/from a low-speed port.

Although a low-speed device will send a low-speed EOP to properly terminate a packet, a hub may truncate a low-speed packet at the EOF1 point with a full-speed EOP. Thus, hubs must always be able to tear down connectivity in response to a full-speed EOP regardless of the data rate of the packet.

Because of the slow transitions on low-speed ports, when the D+ and D- signal lines are switching between the 'J' and 'K', they may both be below 2.0V for a period of time that is longer than a full-speed bit time. A hub must ensure that these slow transitions do not result in termination of connectivity and must not result in an SE0 being sent upstream.

11.8.4.1 Low-speed Keep-alive

All hub ports to which low-speed devices are connected must generate a low-speed keep-alive strobe, generated at the beginning of the frame, which consists of a valid low-speed EOP (described in Section 7.1.13.2). The strobe must be generated at least once in each frame in which an SOF is received from the host. This strobe is used to prevent low-speed devices from suspending if there is no other low-speed traffic on the bus. The hub can generate the keep-alive on any valid full-speed token packet. The following rules for generation of a low-speed keep-alive must be adhered to:

A keep-alive must minimally be derived from each SOF. It is recommended that a keep-alive be generated on any valid full-speed token.

The keep-alive must start by the eighth bit after the PID of the full-speed token.

11.9 Suspend and Resume

Hubs must support suspend and resume both as a USB device and in terms of propagating suspend and resume signaling. Hubs support both global and selective suspend and resume. Global and selective suspend are defined in Section 7.1.7.4. Global suspend/resume refers to the entire bus being suspended or resumed without affecting any hub’s downstream port states; selective suspend/resume refers to a downstream port of a hub being suspended or resumed without affecting the hub state. Global suspend/resume is implemented through the root port(s) at the host. Selective suspend/resume is implemented via requests to a hub. Device-initiated resume is called remote-wakeup (see Section 7.1.7.5).

Figure 11-10 shows the timing relationships for an example remote-wakeup sequence. This example illustrates a device initiating resume signaling through a suspended hub (‘B’) to an awake hub (‘A’). Hub ‘A’ in this example times and completes the resume sequence and is the "Controlling Hub". The timings and events are defined in Section 7.1.7.5.

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