9 USB Power

Conservation

The Previous Chapter

Interrupt, bulk, and control transfers require that the successful delivery of data be verified by USB. CRC and other error checking is performed to verify data delivery, and if errors occur, retries of the failed transmission are performed. The previous chapter discussed the various sources of errors and the error detection mechanisms used by USB to identify them.

This Chapter

USB devices support power conservation by entering a suspend state. This chapter discusses the ways that devices are placed into the suspend state under software control. It also discusses how software re-awakens devices, and how a device such as a modem can initiate a wakeup remotely.

The Next Chapter

The next chapter provides a brief introduction to high-speed device operation and sets the stage for a detailed discussion of the high-speed environment.

Power Conservation — Suspend

Suspend is designed to reduce overall power consumption under software control. USB supports two types of suspend:

•Global suspend — all USB devices are placed into the suspend state

•Selective suspend — selected devices are placed into the suspend state

When a device enters its suspend state it must consume no more than 500µa of current. Devices enter suspend after 3ms of no bus activity. Normally devices are kept from entering the suspend state because they receive a SOF token at the beginning of every 1ms frame, even if no other USB traffic is occurring.

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USB System Architecture

Low-speed devices however, see only low-speed traffic, meaning that they do not see the SOF packet nor any full-speed transactions. Consequently, hubs must signal an idle to K state transition on all low-speed ports at intervals of less than 3.0ms to prevent low-speed devices from inadvertently entering the suspend state. To this end the specification minimally requires that the hub signal a low-speed EOP to all low-speed devices at the beginning of each frame.

Device Response to Suspend

When devices enter the suspend state, they must preserve their state and consume no more than an average of 500µa of current, or 2.5ma for high-power devices that are enable to generate remote wakeup. The specified current draw must not exceed the specified current limit when averaged over an interval of 1 second. A momentary current spike is permitted for configured devices during the averaging interval, but must not exceed the device’s power allocation (specified in the max power field of the configuration descriptor). This limit includes the current draw associated with the pull-up and pull-down resistors on the D- and D+ lines.

Some devices may need to awaken the system in response to an external event. For example, a modem function could be designed to awaken the system when it receives a “ring indicate” from an external line. It would then be necessary to notify system software that the modem requires attention.

Hub Response to Suspend

When a hub detects greater than 3ms of inactivity on its upstream port, it must also enter the suspend state. Since the hub has detected no bus activity for 3.0ms, by definition no activity will have been broadcast to any of the hub’s downstream ports for 3ms. All downstream ports, along with the hub itself, will detect the suspend state at approximately the same time.

Upon detecting suspend, hubs take the following actions:

•place their repeaters into the wait for start of packet (WFSOP) state

•float all output drivers

•maintain static values of all control and status bits

•preserve current state info for all downstream ports

All internal clocks are stopped and power consumption from the hub function is reduced to a minimum.

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