Published in March / April 2009 issue of Chip Design Magazine
Can MIPI and MDDI Co-Exist?
Since MIPI and MDDI standards both target interfaces to cameras and displays on mobile devices, are two separate standards really needed?
The accelerating use of smartphones and the emergence of an exciting class of mobile Internet devices (MIDs) and Netbooks are creating an explosion of data transfer across wireless networks. Such full-featured devices give the consumer a multimedia viewing and listening experience, higher-resolution photography, and a richer set of applications like Web browsing and Global Positioning System (GPS) navigation. Cell-phone manufacturers and the chip providers that supply them need to decide which interface bus to use to support the required low-power, high-speed data transfer between the components that make up these new devices. The Video Electronics Standards Association’s (VESA’s) Mobile Display Digital Interface (MDDI) and the Mobile Industry Processor Interface Alliance’s (MIPI’s) display and camera interfaces provide overlapping standards to meet these requirements. Each standard is a reflection of its origin and the industry participants driving it. This article will explore the different aspects of MDDI and MIPI to help designers determine whether they need to support both of them in their next chip or system design.
Figure 1: Here is an example of a serial versus parallel display interface. Fewer serial wires simplify routing.
Parallel interfaces are less attractive for mobile devices now because of pin count, electromagnetic-interference (EMI) radiation, signal-integrity concerns at higher bus speeds, and higher power-consumption profiles. Large parallel-bus harnesses aren’t suited for connecting across hinged products like slider, swivel, or clamshell-type devices (see Figure 1). In addition, parallel buses add complexity to routing on the small printed-circuit-board (PCB) footprints in mobile devices.
The need for serial interfaces began with the need to address the shortcomings of legacy parallel-bus connections. To solve these issues, companies jumped in to provide proprietary interfaces. For example, Texas Instruments introduced the FlatLink3G serializers and deserializers to provide a high-speed interface between liquid-crystal-displays (LCDs) and mobile application processors, such as its OMAP platform. These sub-low-voltage-differential-signaling (sub-LVDS) serializers work for red/green/blue (RGB) color data. They support screen resolutions from QVGA through XGA including VGA. QUALCOMM entered the fray as well with MDDI. A key aspect of these interfaces was that the transmitter side could be integrated into the baseband application processor and the receiver side in the display. A number of other companies also introduced proprietary solutions, bringing more choice and confusion.
With so many different choices, the systems integrator was faced with how to interconnect the components from different manufacturers. This was a perfect opportunity for the creation of an interface standard (or two). Under the leadership of QUALCOMM, VESA created a special interest group (SIG) to work on standardizing MDDI in April 2002. At that time, VESA had more than 150 members. MIPI was formed shortly after that, when an initial agreement to standardize on interfaces between TI and STMicroelectronics was expanded to include ARM and Nokia. MIPI developed a broad charter, moving beyond just rationalizing the interfaces of the platforms of OMAP (from TI) and Nomadik (from STMicroelectronics) processors. Intel and Motorola later joined as board members and the organization set about defining a number of different standards that could be shared among chip suppliers and systems OEMs. At present, MIPI’s members have grown beyond 100 as well.
QUALCOMM’s work on MDDI brought a silicon-proven interface to VESA. This early start allowed MDDI to get a solid headstart as a practical standard in use today. California Micro Devices, Epson, QuickLogic, Samsung, Toshiba, and others have developed chips using the MDDI standards—some of which are shipping in volume. In VESA, work has continued on MDDI. In July of this past year, Version 1.2 of the MDDI standard was released. The new version of MDDI, which will be available in QuickLogic’s ArcticLink II VX4C Customer Specific Standard Product (CSSP) platform later this year, provides support for audio transducers, keyboards, pointing devices, and other input devices integrated with a mobile display for up to 1-Gbps operation. Some other MDDI features include:
- Full-motion video in the form of full-screen or partial-screen bitmap fields or compressed video, depending on device capability
- Static bitmaps at low rates to conserve power and reduce implementation cost in some portable devices
- PCM or compressed audio data at any resolution or rate compatible with the serial-link speed
- Pointing-device position and selection
- Control and status information in both directions to detect the capability of the opposing device and set its operating parameters
- User-definable data types for capabilities yet to be defined
- Windowless video-stream packet with no X/Y windowing information
- Flexible video-stream packets to allow for the selective inclusion of a video-stream packet
The serial-link speed on each signal pair of the MDDI specification can vary over many orders of magnitude. As a result, the system designer can easily optimize cost, power, implementation complexity, and the update rate across many different cell-phone components. The added attributes of MDDI 1.2 now completely overlap some of MIPI’s standards. So what is the state of these MIPI standards today?
Figure 2: MIPI’s D-PHY and M-PHY work as the physical layer of CSI, DSI, and UniPro. The M-PHY is the physical layer for the new DigRF standard.
After its founding, the MIPI Alliance went right to work converting legacy parallel display and camera interfaces to new serial standards. The standards, called Display Serial Interface (DSI) and Camera Serial Interface (CSI), both use the same common physical standard called the D-PHY. The D-PHY also is the physical-layer (PHY) standard for a universal protocol dubbed UniPro, which is currently being defined. In addition to the D-PHY, which uses source synchronous clocking with differential transceivers, the MIPI organization is defining an M-PHY. This low-power, higher-speed serialized transceiver has an embedded clock. The M-PHY has recently been targeted to act as the interface between the baseband and RF sections of the DigRF standard—another standard that’s now part of the MIPI Alliance. The high-speed roadmap for DSI and CSI includes use of the M-PHY. Figure 2 illustrates the MIPI Alliance vision of plug-and-play PHYs for different protocols.
The MIPI Alliance is organized into working groups focusing on display, camera, PHYs, and other standards. MIPI looks at software issues as well. It has working groups for test and debug. One of the major differences between MIPI and MDDI is the structure and workings of the groups that are defining and controlling the specifications. MDDI is a single specification. QUALCOMM is a major force in its definition and its use model. In fact, many still see MDDI as a QUALCOMM specification even though it has moved into VESA.
In contrast, MIPI has the look and feel of an IEEE standards group. In fact, IEEE-ISTO is the managing organization for MIPI. MIPI organized into multiple working groups with company voting rights based on attendance in addition to the contributor fee that must be paid to join. The specifications are comprehensive within the discipline of each working group. So the MIPI and MDDI standards both exist in the market and target interfaces to cameras and displays on mobile devices. Are these two standards necessary? Should they coexist? A closer look at market dynamics will provide the best answer.
Cellular has had two main competing network technologies: Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA). Both of them are referred to as second-generation (2G) standards. The GSM Association is an international organization founded in 1987 that’s dedicated to providing, developing, and overseeing the worldwide wireless standard of GSM, which is used more outside the United States. CDMA, a proprietary standard designed by QUALCOMM, has been the dominant network standard for North America, Korea, and South America.
In the U.S. today, ATT-Wireless and T-Mobile use GSM while Sprint and Verizon use CDMA. Subscriber usage favors GSM because it is in use in 74% of the cellular markets worldwide. However, the lucrative American markets and the acceleration of CDMA usage in China and India put the market share of CDMA at 14% by 2013 (ABI Research). Both of these standards have evolved over time from 2G to 3G to 3.5G, thereby providing higher data rates. Specifically, GSM has evolved to Wideband CDMA (WCDMA) and then HSxPA technology while CDMA has evolved to CDMA2000 1X EV-DO Rev A to Rev B. Eventually, both camps seem to be converging to a shared Long Term Evolution (LTE) standard.