Patent Application
20080174606
Aspects of the present disclosure relate generally to the field of display systems and more particularly to reducing the power consumption of display systems in portable, battery-operated devices such as smartphones and PDAs.
Designers of devices such as laptops, smartphones, PDAs, multimedia players and other battery-operated portable devices frequently need to balance a desire to add power consuming features with a desire to have a long battery life. Device designers can improve batter life by using a larger battery, but this is often undesirable as it increases the overall size and weight of the device. A significant portion of a computing system's power consumption can be traced to the display subsystem, and as a result, increasing the size or resolution of a device's display can greatly lessen a device's battery life.
A display subsystem comprises a display controller as well a display screen. A significant source of the power consumption within the display subsystem can be attributed to the functionality of the display controller. The display controller accesses pixel stores in memory and processes the fetched pixels to render a frame to be displayed on a screen. Modern display systems often require the mixing and blending of multiple display streams to produce a single frame. The typical rate of refresh for LCD devices is on the order of 60 Hz, meaning the display subsystem must generate 60 frames per second.
Although the subsystem is generating 60 frames per second, the actual rate at which the content of the frames changes is typically much lower. Even when showing video or other rapidly changing images, the rate at which frame content changes rarely exceeds 30 Hz. In many usage situations such as email or web browsing, the refresh rate can be significantly lower than 30 Hz, meaning the display subsystem frequently generates one or more duplicates of the same frame.
In order to reduce power consumption, many devices can operate in a low power mode where the operating frequency of the device and the refresh rate of the display can be scaled down. In such a low power mode, however, the operation of the display is coupled to the operational state of the system's processor; such coupling can be undesirable. For example, when making a phone call, the processor needs to operate at a high frequency in order to process the incoming and outgoing audio data, but the screen might only be changing once per second to update a call length timer. Nevertheless, the system will still generate sixty frames per second, even though a significant majority of the frames will be duplicates of previously generated frames.
Given the increasing screen sizes and the high refresh rates of current LCD devices, it is desirable to reduce the power consumption associated with the activity of the display subsystem. Further, it is desirable to do so in a manner that is independent of the operational state of the system's processor.
Aspects of the present invention include a display subsystem that can operate in two different display modes—a composition mode and a low power refresh (LPR) mode. In the composition mode, a composition engine can take multiple images from different image streams and combine them into a single display frame formatted for the device's screen. In the LPR mode the display system can store a copy of the display frame produced by the composition engine, and in instances where the display content has not changed, output the stored display frame rather than having the composition engine go through the process of rendering a new frame that would be identical to the previous frame. By entering an LPR mode during periods when display content remains constant, the display system can reduce the amount of data fetched and reduce the amount of processing needed to render a frame, thus reducing bandwidth and power consumption.
A system embodying aspects of the present invention can further include a controller configured to switch between the different display modes upon the detection of a change to a system parameter. The particular mode the display subsystem operates in can be decoupled from the operational state of the processor. Further, the control system can be implemented in hardware, allowing it to avoid the delays associated with software processing, thus achieving a response time sufficiently fast to allow the controller to switch modes on a frame-by-frame basis.
a and 2b show display systems configured to operate in both a ion mode and a low power refresh mode.
Aspects of the present invention include a display subsystem that can operate in two different display modes—a composition mode and a low power refresh (LPR) mode. The system can contain a control system configured to switch between the different modes upon detecting a change to a system parameter. The particular mode the display subsystem operates in can be coupled from the operational state of the processor. Further, the control system can be implemented in hardware, allowing it to avoid the delays associated with software processing, thus achieving a response time sufficiently fast to allow the control system to switch modes on a frame-by-frame basis.
As with the system of
In order to save memory and bandwidth, the memory-sink mechanism 240 can optionally include a compression engine to reduce the amount of data needed to store a copy of the frame as well as the amount of data to be retrieved when accessing the frame. The compression engine can use various compression algorithms known in the art to achieve a desired compression ratio. The desired compression ratio can be determined by a system designer based on factors such as an amount of tolerable image degradation and the amount of memory available for storing images.
In order to reduce power consumption and bandwidth, the system can implement a compression scheme sufficient to reduce the amount of data enough to store the compressed image in on-chip memory 290 (as shown in
In systems utilizing a compression engine, the retrieval mechanism 250 can be configured to include a decompression engine for restoring the compressed image to a format suitable for the device's screen 230. Because the compressed image of the frame was already formatted to fit the screen 230 before it was saved to memory, the decompression engine should not have to significantly reconfigure the image before transmitting it to the screen 230, thus reducing the amount of power associated with processing image data. When the system operates in LPR mode, the composition engine 220 can be placed into a low power state by stopping the propagation of clock and power signals through techniques such as clock gating circuitry.
In order to switch between the composition mode and the LPR mode, the system can contain a controller 270 capable of selecting either a first or a second channel on a multiplexor (MUX) 280. Selecting the first channel might mean the composition engine 220 produces the frame supplied to the screen 230 (i.e. the system is in composition mode), while selecting the second channel might mean the retrieval mechanism 250 produces the frame being supplied to the screen 230 (i.e. the system is in LPR mode).
The controller 270 determines which mode to operate in based on a snoop mechanism 260 that monitors a system parameter such as the contents of the frame buffer 210. For example, the snoop mechanism 260 can monitor the frame buffer 210 to detect content changes. The snoop mechanism 260 can detect content changes by looking for changes to memory locations within an address range corresponding to the frame buffer 210.
If the system is operating in a composition mode, then the controller 270 might switch the system into LPR mode if the snoop mechanism 260 does not detect a content change in the frame buffer 210. If the snoop mechanism 260 does detect a change, then the controller 270 might keep the system in composition mode. When operating in LPR mode, the controller 270 can switch the system into composition mode if the snoop mechanism 260 detects a content change but otherwise can keep the system in LPR mode as long as no content changes are detected.
In LPR mode (block 330), the snoop mechanism can continue to monitor the frame buffer for content changes (block 340). If the snoop mechanism detects a content change (path 342), then the system will shift from LPR mode back into composition mode (path 342 and block 310). If the snoop mechanism does not detect a content change, then the system can remain in LPR mode (path 341 and block 330). The method can continually repeat itself alternating between the composition mode and the LPR mode while the display is functioning. The system can switch modes as often as between every frame.
Referring back to
Another feature of a system embodying aspects of the present invention can include using the compressed image stored by the memory-sink mechanism 240 but while still making minor alterations to it. For example, when playing a song on a portable music device the screen 230 might show a picture of an album cover as well as song information and a timer, all overlaid on a background. When the system operates in composition mode, the composition engine 220 has to retrieve the images from the frame buffer 210, resize them, overlay and blend them, and render a single frame in a format suitable for the device's screen 230.
Of the multiple component images that are used to create the displayed frame, it might be that only the image associated with the timer is regularly changing. Therefore, rather than resizing and relayering all the component images, the system can take the copy of the frame saved by the memory-sink mechanism 240, which has already been resized and formatted to the fit the device's screen 230, and only relayer the timer image. Alternatively, the system can be configured to have software overwrite portions of the stored image to make a minor change, such as updating the value of the timer.
The image stored by the memory-sink mechanism 240 can be stored either in a compressed form in the on-chip memory 290 (
The foregoing description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments without the use of inventive faculty. For example, some or all of the features of the different embodiments discussed above may be deleted from the embodiment. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope defined only by the claims below and equivalents thereof.
The present application claims the benefit of co-pending U.S. provisional application Ser. No. 60/886,214 filed on Jan. 23, 2007. The disclosure of the co-pending provisional application is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60886214 | Jan 2007 | US |
