Hot Plug Devices

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[oR]

In order to manage power within a computer system, three major categories require cooperative design.

• System and bus hardware

• Devices

• Operating system and device driver software

Windows 2000 and Windows 98 support the WDM driver model, which incorporates sophisticated power management for bus and device hardware. Since the remainder of this chapter discusses the software support for power management, it is appropriate to first review the hardware requirements for the feature.

Bus Considerations

First and foremost, before a device can be powered down, the bus into which it plugs must electrically provide for such a state. For example, many bus drivers require power to guarantee an on-bus or off-bus state. Without power to the electrical bus-driver chip itself, the driver and its effect on the rest of the bus is indeterminate. Clearly, the hardware bus specification must allow for powered down bus drivers in order to support robust power management.

In some cases, the bus design must meet physical constraints. Especially in mobile systems, devices are routinely inserted and ejected. If the device installation and removal is allowed on a "hot" system, the physical bus characteristics must be carefully designed. Besides the obvious requirement that each device be physically separate (i.e., one device cannot scrape another as it is inserted or removed) there is the less obvious concern for which bus signals will "break" first. Typically, the ground pins for such buses are physically longer than the power pins, ensuring that ground makes first and breaks last during insertion and removal.

Such trivia aside, all modern buses support power management and incorporate necessary electrical and physical specifications to meet the stated requirements of the bus. PCI, PCMCIA (PC Card), USB, and IEEE 1394 are all designed for the feature. All but PCI allow "hot plug" of devices.

Device Considerations

Besides meeting the requirements for a given bus, individual devices can allow for remarkably clever management of power. For example, a modem card might allow for power down of the UART and bus circuitry, while still powering a ring detection circuit that triggers an awaken sequence.

Indeed, when considering the possible segregation of power to a device's subsections, it becomes clear a device can have multiple states for describing its power condition. Between the extremes of "completely on" and "completely off," a device can implement decreasing power states corresponding to decreasing capability. The transition from state to state can be directed by system or user request, or simply after an extended idle time.

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