Saving Power in Displays for Electronics

VIP Expert

Apr 02, 2018 / 3 min read

Higher performance at lower power is the most critical requirement of SoC designs, specifically those targeted towards mobile and consumer electronics applications. VESA (Video Electronics Standards Association), the technical standards organization for computer display standards, came up with a new power saving feature called PSR (Panel Self Refresh) in eDP 1.3. It is also available as an optional feature in DisplayPort. PSR helps to extend battery life in mobile phones, notebooks, and tablets, and is quickly being adopted in high-end designs.

DisplayPort (DP) is a display interface used to connect a video source to display devices such as monitors and projectors. It provides the fastest refresh rate, high resolution with deep color modes, and increased bandwidth with support for all 3D video and audio formats. To know more about DisplayPort, read our previous blogs – connecting DisplayPort to multiple devices and industry’s first DisplayPort 1.4 with DSC 1.2 VIP and test suite.

PSR in DP enables power saving in the system when the image is static. When static content is displayed, the display pipeline and internal frame buffers consume power by sending display updates even when unnecessary. PSR incorporates a Remote Frame Buffer (RFB), allowing it to refresh the screen directly rather than continuously requesting image data from the source device. Ultimately, PSR is used to turn off the video processor and its circuitry when an image is static. For example, a frame with a resolution of 1920 x 1080p @120 Hz requires 8.9 billion bits to be transmitted per second. With this frame size, there is a total of 2.475 million pixels per frame, and therefore 297 million pixels are needed for 120 frames. This sums up to 891 million components (as there are 3 components per pixel) and 8.9 billion bits per second. With PSR technology in place, when a static frame is displayed, power required for transmission of 89 billion bits is saved every 10 seconds.

A sink device stores a static image locally in the RFB and displays it from the storage. Since the sink can display the image from its local buffer, the main link can be turned off to save power. The sink device will locally refresh the stored static images until new updates are available. Any source and sink devices connected to each other should support PSR. The source indicates to the sink device that the displayed image is static and sends a PSR active state indication to the sink along with a complete active frame. While in PSR active state, the source device can turn off its transmitter to save additional power. Let’s look at a few use cases where PSR is enabled and disabled :-

Case 1: CPU/Computer is in idle mode and the monitor’s display content is not changing (i.e. remains with home or an application screen). Since the display is static in nature, the CPU can turn on PSR static mode and stop transmission of the same frame during this time. Simultaneously, the monitor will store and retain a static frame in its local frame buffer.

DisplayPort PSR use case 1

Case 2: The monitor’s display content is changing dynamically in usage modes such as gaming, movies, and simulations etc. In this case of fast moving content, the PSR is disabled.


DisplayPort PSR use case 2

Case 3: Similar to Case 1, a static image is being displayed. In this case, the image is stored in the local RFB of the sink and therefore the source doesn’t need to send updates to the sink device. The source can turn off the transmitter.


DisplayPort PSR use case 3

PSR technology does not affect screen power consumption but it reduces the power consumed by the DisplayPort transmitter and receiver. PSR technology requires a frame buffer inside the display SoC design, adding to the cost of displays. eDP 1.4 defines a new version, PSR2, in which only a small portion of the frame is updated instead of the entire frame. PSR2 is a superset of PSR, providing self-refresh with selective updates. Stay tuned for upcoming blogs to learn more about PSR2 and other emerging display technologies.

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