Camera power management for connected standby platforms

The cameras in modern mobile platforms allow users to capture still and motion video of their surroundings, and to use video and audio to communicate with other users over the Internet. The general goal of power management for a camera device can be described simply—the camera subsystem must be powered off, consuming zero watts, unless the camera is in active use.

When the camera is actively being used to stream video to an application, the camera sensor and related components should be powered on. Windows enables the camera hardware to be turned off during connected standby by suspending any foreground Windows Store apps that might be streaming data from the camera device. Windows makes no provisions for the use of camera devices while the system is in connected standby, so a camera device can be used only when the display is turned on.

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Camera subsystem

In terms of power management, cameras are one of the most complex device subsystems in a mobile platform. This complexity is a result of the tight coordination required between the digital and image signal processing units, which are integrated into the System on a Chip (SoC), and the camera sensor, auto-focus, and flash components, which are external to the SoC chip.

Camera hardware topology overview

In terms of Windows driver support, the camera subsystem contains off-SoC components such as the camera sensor, an optional auto-focus unit, and flash—and possibly other associated hardware. The camera hardware also includes the on-SoC image processing units.

Camera power management modes

The off-SoC components of the camera subsystem must support two power management modes. The camera components must support an active mode in which the camera device is actively streaming content to an application. In addition, the camera components must support a power-removed mode in which the camera device is turned off, power is removed, and the camera device consumes zero watts.

Software power management mechanisms for cameras

Both the on-SoC image processing units and the off-SoC camera components are expected to consume no power (zero watts) when the system is in connected standby and the display is turned off. The primary software mechanism for power management is reference counting of the camera capture pin.

Supported hardware power configuration

Windows supports a single hardware power management configuration for camera devices in connected standby platforms. In short, each camera sensor must be connected to the SoC via a MIPI-CSI link, and can optionally be connected to an I2C bus and to one or more GPIO pins. The camera sensor device, its optional flash, and any other off-SoC camera components must be placed on a power rail that can be switched on and off by ACPI firmware.

Camera power management checklist

System integrators, camera sensor vendors, and SoC vendors should use the checklist in this article to ensure that their system power management design is compatible with Windows 8 and Windows 8.1.




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