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Также существуют состояния System PowerState (Windows OS) S1..S5, and Device Power State:
Если специфицировать в дескрипторе, что устройство SelfPowered, то операционная система может отключить питание от устройства (VBUS=0), Как результат устройство может потерять управление. Чтобы предотвратить это желательно оставить в дескрипторе минимальное стандартное потребление тока 100 mA, тогда система будет удерживать питание VBUS=5V.
Драйвер, также должен обеспечивать устройство информацией, что сыстема перешла в режим пониженого потребления, в свою очередь устройство должно прекратить активную работу с USB.
Более детально об этом можете посмотерть в DDK, личше в Windows 2003 Server DDK.
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Power States
A power state indicates the level of power consumption — and thus the extent of computing activity — by the system or by a single device. The Power Manager sets the power state of the system as a whole. Device drivers set the power state of their individual devices.
The ACPI specification defines two sets of discrete power states – system power states and device power states. Each power state has a unique name.
System power states are named Sx, where x is a state number between 0 and 5. Device power states are named Dx, where x is a state number between 0 and 3. The state number is inversely related to power consumption: higher numbered states use less power. States S0 and D0 are the highest-powered, most functional, fully on states. States S5 and D3 are the lowest-powered states and have the longest wake-up latency.
These clearly defined power states allow many devices from various manufacturers to work together consistently and predictably. For example, when the Power Manager sets the system in state S3, it can rely upon drivers that support power management not only to put their devices in the corresponding device state but also to return to the working state in a predictable fashion.
For detailed information about power states, see System Power States and Device Power States.
System Power States
System power states describe the power consumption of the system as a whole. The operating system supports six system power states, referred to as S0 (fully on and operational) through S5 (power off). Each state is characterized by the following:
Power consumption: how much power does the computer use?
Software resumption: from what point does the operating system restart?
Hardware latency: how long does it take to return the computer to the working state?
System hardware context (such as the content of volatile processor registers, memory caches, and RAM): how much system hardware context is retained? Must the operating system reboot to return to the working state?
State S0 is the working state. States S1, S2, S3, and S4 are sleeping states, in which the computer appears off because of reduced power consumption but retains enough context to return to the working state without restarting the operating system. State S5 is the shutdown or off state.
A system is waking when it is in transition from the shutdown state (S5) or any sleeping state (S1-S4) to the working state (S0), and it is going to sleep when it is in transition from the working state to any sleep state or the shutdown state. The following figure shows the possible system power state transitions.
Device Power States
A device power state describes the power state of a particular device. Device power states are named Dx, where x is a state number between 0 and 3. The state number is inversely related to power consumption: higher numbered states use less power.
Device power states are defined in terms of the following criteria:
Power consumption: How much power does the device use?
Device context: How much of its operational context does the device retain in this state?
Device driver behavior: What must the drivers for the device do to restore the device to the fully operational state?
Restore time: How long does it take to restore the device to the fully operational state?
Wake-up capability: Can the device request wake-up from this state? In general, if a device can request wake-up from a given power state (D2, for example), it can also request wake-up from any higher-powered state (D1).
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