What Is a USB Selective Suspend?
USB stands for Universal Serial Bus, an external serial bus standard for connecting peripheral devices to a computer. The USB specification defines support for low power devices -- devices, such as mice, keyboards and joysticks, that draw less than 100 milliamps of electric current from the USB power line -- and one of the low power mode features is called USB Selective Suspend.
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Operation
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USB Selective Suspend allows any USB device driver -- a program that determines how a computer communicates with a USB peripheral device -- to turn off, or suspend, the device it controls when the device is idle. When the device ceases to be idle, the device driver wakes it from Selective Suspend so that it can resume normal operation. Selective Suspend is extremely useful because it allows one or more USB ports on a system to be suspended -- hence reducing overall power consumption -- without affecting the operation of any of the other USB ports.
Idle Determination
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The USB Selective Suspend process starts with the USB client device driver detecting that its USB device is idle. What constitutes “idle” varies from one device to another; a USB mouse is considered idle if it hasn’t moved for a certain length of time, a USB camera is considered idle if the lens cover is closed and so on. The ability to detect when a USB device is idle is critical to extending the battery life of mobile platforms, such as laptop computers, and can make the difference between one product and another.
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Low Power Mode
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Once the USB client driver has detected that its USB device is idle, the driver must prepare the device for low power mode. It does so by submitting a data structure, known as an Input/Output Request Packet, to the root hub, or USB controller, which controls the whole USB bus connected to it. The process of suspending a USB device involves the USB controller sending it one or more commands, which force the device into low power mode.
Current Restrictions
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The USB Specification imposes a restriction of 2.5 milliamps on USB devices that support Selective Suspend with another low power feature, known as Remote Wakeup, enabled. Remote Wakeup essentially allows an USB device to wake itself from Selective Suspend rather than being woken by the system. For USB devices without Remote Wakeup, the USB Specification imposes of restriction of 500 microamps. These current restrictions are often the principal reason that USB devices on mobile platforms are self-powered.
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References
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