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Manufacturers are always looking for innovative ways to save battery life on laptops. With the embedded DisplayPort 1.3 interface (eDP 1.3), VESA, the Video Standards Association behind DisplayPort, came up with a new idea, the Panel Self Refresh (PSR). Let’s see how it works.
The video processor needs to keep sending video signals to the video monitor at a constant rate. This rate, known as refresh rate or vertical frequency, is at least 60 Hz, meaning that the video processor has to send to the video display the contents of the screen at least 60 times per second, even when there is no change in the image.
The idea behind the Panel Self Refresh is to shut down the video processor and associated circuitry when the image is static – for example, when you are reading a PDF file. The display would have a video memory (a.k.a. frame buffer) to store the contents of the last screen update, the computer would shut down all video circuitry, and the display would keep repeating the contents of the last update until there is an update to the screen contents (e.g., you scrolled the document you were reading).
When the Panel Self Refresh is used, the display engine inside the CPU can be turned off, making the laptop save around 50% power. The DisplayPort interface, which is used in the communication between the computer and the LCD panel, can also be turned off, providing an additional 25% power savings. There is also some power savings on the RAM memory, because on systems with integrated graphics engines, as is usually the case with laptops, part of the RAM memory is used as a video memory (a.k.a. frame buffer). While the Panel Self Refresh is used, the RAM doesn’t need to be accessed by the video engine, providing an additional 10% power savings. When all the numbers are added, the Panel Self Refresh technology can result in the laptop saving up to 85% power while it is being used.
Please understand that this doesn’t mean that battery life will last 85% longer. This saving only occurs when the computer is idle and the display isn’t refreshed. When the display needs to be refreshed (e.g., you are watching a video), this technology isn’t used, and your laptop will have the same power consumption as before. This should translate in an increase of 45 minutes to one hour of battery life.
Also, because now the display has to have a piece of RAM to store the contents of the image to be displayed, the panel itself actually consumes a little bit more power when this technology is in use, but this increase in power consumption is offset by the overall power savings of the system. When the technology is not in use, the display turns off the RAM chip to save energy. By the way, manufacturers won’t use a RAM chip for this function; this functionality will be embedded inside the timing controller (TCON) chip, which is in charge of decoding the DisplayPort signal and sending the decoded signal to the rest of the circuitry inside the display.
Let’s see in a little bit more detail how the Panel Self Refresh works.[nextpage title=”Inside the Panel Self Refresh Technology”]
The Panel Self Refresh technology works by using a new DisplayPort command, “capture static frame.” When this command is issued by the video engine, the display will capture the last image (frame) sent, storing it in its RAM memory (frame buffer). After this command is issued, the DisplayPort main link is turned off, and the Panel Self Refresh mode is enabled.
To exit the Panel Self Refresh mode, a wake-up command is issued through the DisplayPort auxiliary (AUX) link. After a brief synchronization period (“link training”), which lasts a few milliseconds, the display will be ready to receive new images (frames), and the system will be working as it did before entering the Panel Self Refresh mode.
In Figures 1 and 2, you can see a system supporting Panel Self Refresh technology. In Figure 1, the system is working normally, while in Figure 2, the system entered the Panel Self Refresh mode. The blocks in black are turned off for power saving.