Displays for the human/machine interface are becoming increasingly immersive in our daily lives. Displays can be embedded into fabric, built into transparent surfaces like eyeglasses or mirrors, molded into shapes that wrap around the wrist or along a curved wall, head mounted for virtual reality experiences or fabricated into car windshields to provide augmented reality features that replace dashboard applications.
All of these displays use complex interfaces within the device to exchange data between the application processor, data source and the display module. Thanks to advancing interface specifications, which standardize the interconnections to meet rigorous performance needs while reducing technology costs and power consumption, vendors can embed displays into devices that are both intuitive and enthralling. And today, people can’t get enough of displays, likely because their use is so simple and the value so obvious: they check their phones throughout the day and they are increasingly using smartphones as hubs for second-screen applications.
This article will briefly review advancements in display technologies and the integration challenges engineers should consider as they add new display capabilities to mobile and mobile-influenced products.
The Evolution of Displays, From Basic to Fantastic
Initially, displays on digital devices simply showed content on a screen with a one-way user interface. The displays used an application processor and/or a CPU or GPU to show digital content or visual imagery for a calendar, map, game, or camera, for example.
Later, the industry integrated a “touch fabric” into displays to improve the user experience by allowing customers to apply a stylus and/or finger for two-way interaction with their applications. The next advancement brought multiple touch points to a display to enhance productivity and gaming applications and provide pinch-to-zoom and other features. Today, touch technologies have begun adding “pressure sensitive touch” to add a third dimension, a depth-sensing capability, which customers can use to customize the touch experience or activate special features in an application.
Display quality can determine if a product succeeds or fails in the market. Vendors are supporting more pixels per inch (PPI) and, importantly, “better pixels” to deliver amazing, life-like image quality. As more devices offer ultra-high definition (4K) display capability and evolve to 8K resolution (initially in theater and home theater applications), there will be constant pressure to innovate toward higher quality yet lower power display and touch systems.
New Displays Create New Integration Challenges
When embedding high-performing displays into devices, the engineer must take considerable effort to ensure components are integrated and well controlled. This can be challenging in mobile applications that must deliver high performance without draining battery life or introducing electromagnetic interference (EMI).
Conserving Battery Life with Good Management Techniques and Efficient Interfaces
Battery life is a concern because displays are typically the most power-hungry components in mobile devices. Yet power demands are increasing as displays become brighter and provide higher resolution.
Engineers typically use two techniques to preserve battery life. One approach is to use strong management schemes to control display brightness and/or quality. A visible-light sensor, for example, detects if the customer is using the device in a dark or light environment and adjusts the display accordingly. Devices will also dim the display if the user doesn’t touch the device for a certain period of time. Some devices use an internal display frame buffer to self-refresh the display when the content is not changing from frame to frame. Products can also use content-adaptive lighting reduction to dim the backlight and save power without compromising the viewing experience.
Another essential technique is to interconnect the display with interface technologies that help conserve power. While interfaces use a fraction of the power compared to displays, an interface can’t add to the power demands. Designers can address this need by choosing interfaces that minimize power consumption while meeting the products’ most demanding display requirements.
Electromagnetic interference is a significant concern. Displays can flicker based on interference from internal sources such as high-speed clocks, long cable assemblies, and poor grounding approaches. Also, interference from the embedded wireless radio can affect display circuitry. In some cases, displays themselves can emit EMI that impacts other circuit components. For example, increasing “frame refresh rates” might generate interference that can impact components outside the display module. For this reason, interference mitigation schemes such as spread spectrum clocking in MIPI D-PHY, data whitening, embedded-clock interfaces such as MIPI C-PHY or the Video Electronics Standards Association (VESA) Embedded DisplayPort (eDP) are often used to ease EMI.
New Display Interfaces for High-Performance, Low Power and Low EMI
MIPI Alliance continues to advance its MIPI Display Serial Interface (DSI) for mobile and mobile-influenced applications. The newest version of the specification, MIPI DSI-2 v1.0, enables designers to support two different MIPI physical layers: MIPI D-PHY (v2.0) and MIPI C-PHY (v1.0 or 1.1). The options give designers the flexibility to support a variety of scenarios for integrating displays, depending on the display technology used and the designer’s needs to support different topologies and lane width, such as 4-lane MIPI D-PHY or 3-lane MIPI C-PHY display interfaces.
The VESA develops data compression and transport schemes that help reduce the required data rate for high-resolution mobile display applications, lower the power consumption and lower the costs for implementing these systems.
MIPI Alliance and VESA work collaboratively on improving display performance. MIPI DSI-2, for example, references the latest version the VESA Display Stream Compression Standard, DSC v1.1, as an optional feature. DSC can reduce the amount of data sent from the application processor and therefore reduce the workload and power consumption of the GPU and display interface.
MIPI DSI-2 v1.0 has recently been approved.
Anticipating the Future: Greater Reliance on Displays and Interface Technologies
The need for effective, compelling and efficient displays in mobile devices will only increase. By 2020, 5.4 billion people will have mobile phones, exceeding the number of people who have electricity. By then, smartphones will represent nearly half of global devices and 80% of mobile data traffic.
Smartphones will also have more responsibilities. Already, consumers stream content wirelessly from their devices to TVs, PC screens and even mirrors in their homes. As more devices offer wireless streaming, via Wi-Fi or WiGig, the connected displays will need to support compatible screen resolutions to ensure a seamless, high-quality user experience. Borrowing from the smartphone evolution, displays will continue to emerge in mobile-influenced designs such as wearables and Internet of Things devices such as smart meters of various types.
When integrating displays into devices and “things” to serve current and future needs, keep in mind that low power consumption and low EMI are just as essential as intrinsic display quality. By using industry-developed interface specifications designed to meet these many challenges, engineers can build successful, high-performing devices that continue to captivate the market.