While mobile device display performance continues to increase, system chip processes geometries continue to shrink, resulting in a greater proportion of system power consumed by the display and its high-speed interface. The new Embedded Display Port (eDP) v1.4 standard offers several new features that maximize system power efficiency, further consolidate the display interface, and address a wide range of system profiles to satisfy the growing demand for power optimization in the embedded display system.
The importance of lowering display-related system power
Today, mobile devices are a major driving force in the electronics industry. Every year new mobile devices are introduced with increased processing capability, better displays, a smaller and lighter form factor, and extended battery life. Taking into account typical CPU idle time, an average display consumes about 75 percent of system power. While system chip power reduction is accomplished through shrinking semiconductor process geometries, display power reduction comes through improvements in backlight and LCD technologies, as well as new pixel structures.
However, the recent trend toward brighter, higher-resolution displays is driving up display power. The second-generation iPad had a 1024x768 display and a 25 watt-hour battery, while the latest iPod has a 2048x1536 display (a 400 percent pixel increase) and a 42.5 watt-hour battery (a 70 percent power increase), both delivering a 10-hour battery life. The higher-resolution display requires additional pixel-driving circuitry and a higher data rate display interface, as well as fasterGraphics Processing Unit (GPU) rendering and display image processing circuitry.
This display power challenge has led to many new architectural developments at the platform level. Reducing display power means longer battery life and less battery capacity requirement and therefore smaller, lighter, and less expensive systems. Rather than being treated as a simple rendering device, display deployment has become more integrated into the overall system design. The new eDP v1.4 brings many of these concepts together, as explained in the following discussion.
Panel Self Refresh (PSR)
Introduced in eDP v1.3, the PSR function provides a means to lower display-related system power by allowing portions of the GPU and display interface to enter a low-power state during a static display image condition (see Figure 1). When the system enters PSR mode, the remote frame buffer built into the LCD timing controller (Tcon) performs the routine display refresh task, offloading the GPU and display interface. The local frame buffer can then be updated later by the GPU with a new static image (such as when a flashing cursor is turned on or off), or the system can exit from PSR mode to display constantly changing images (such as when playing a video). Because most displays are refreshed 60 times per second, PSR allows the GPU circuits and display interface to remain in a low-power state for most of the system operating time, resulting in significant power savings.
PSR with selective frame update
The latest version of eDP enhances the PSR mechanisms by enabling updates to selected regions of the video frame in the Tcon frame buffer. With eDP v1.3, the entire frame must be updated every time any portion of the image changes. With eDP v1.4, only the new portion of the frame requires updating (see Figure 2). This lets the GPU display interface remain active for a shorter duration, further reducing system power.
