Streamlining interconnections for high-performance, compact designs

Engineers must address numerous concurrent trends as they strive to craft innovative designs for mobile and mobile-influenced devices. Miniaturization of silicon chips, gigabit-per-second data speeds, the Internet of Things and new interest in modular architectures, for example, are opening up compelling opportunities to improve functionalities and create new design efficiencies.

One challenge associated with these trends is the need to simplify the electronic interfaces that interconnect device chipsets and peripheral components. I’d like to use this article to discuss MIPI UniPro (Unified Protocol), a versatile transport layer that can address these important needs. MIPI UniPro simplifies interconnections for smartphones, tablets, laptops, cameras and multimedia devices, as well as the various types of smart watches, health monitors and other innovative products that are entering the market as part of the Internet of Things.

MIPI UniPro: Its purpose and applications in the market
MIPI UniPro, offered by MIPI Alliance, specifies a transport layer protocol stack for switched packet transfer. Implemented on top of the MIPI M-PHY physical layer, it forms the MIPI UniPort-M interface.

As its name indicates, MIPI UniPro is a unified protocol. It is not dedicated to specific applications, as protocols often are. It was designed from the outset as a general-purpose, lightweight protocol with excellent protocol efficiency that is dedicated to mobile devices. It does not suffer from the heavy overhead imposed by other well-known legacy protocols that have been developed for the PC industry, where power consumption is not always a concern. It can be used as a standalone interface for inter-processor communications (IPC) or as a building block for multimedia interfaces. It can be used with other MIPI or non-MIPI application layer protocols.

MIPI UniPro’s and MIPI M-PHY’s versatility are illustrated by the types of organizations that have adopted the specifications. The JEDEC organization uses UniPort-M to provide the basis for its Universal Flash Storage (UFS) specification. Google is using UniPort-M to connect the modules that will make up its Project Ara smartphone platform. The MIPI Camera Serial Interface (MIPI CSI-3) uses the UniPort-M interface to connect cameras with the application processor.

The specification’s history
MIPI UniPro has been available since 2007. The current release, v1.6, has been available since September 2013.

The role for MIPI UniPro in the industry began in 2004, when MIPI Alliance formed the MIPI UniPro Working Group to develop an interoperable interface that could meet a wide range of needs. The group’s objectives were to support a variety of component types and data traffic in a single protocol stack; reduce and standardize the number of physical links between devices and thus pave the way for design modularity to shorten the design and manufacturing cycle of new phones; simplify system integration of key components; and ensure backward compatibility of all versions to future-proof the specification.

Highlights of key technical features with use case scenarios
Following are summaries of some of the technical features that enable MIPI UniPort-M to meet so many diverse interface needs.

Versatile physical link: MIPI UniPort-M makes it possible to use multiple logical channels on one physical link. A so-called CPort serves as a bi-directional application entry and exit point for each of the up to 2047 logical channels. Application messages on each logical channel are fragmented, dispatched with a UniPro frame header and footer, and travel time-multiplexed through the physical M-PHY channel, as illustrated in Figure 1.

Figure 1. Example of a UniPro Frame Multiplex.

Therefore a single physical channel can be used to support the logic needed to interface multiple applications, such as display, audio, control, storage, and others. Alternatively, a single application can share a bi-directional CPort channel for multiple purposes: For example, a display device can use a CPort channel to connect to the display and use the reverse channel for touch screen data (see Figure 2).

Figure 2. Using MIPI UniPort-M as a common transport channel into a media module.

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Scaling link speeds and changing power modes: MIPI UniPort-Mprovides scalable link speeds and the ability to adjust thesedynamically to optimize performance while managing power consumption. Abi-directional UniPort-M link is characterized mainly by its mode ofoperation, its speed gear, and the number of lanes in use. The SLOW MODEis optimized for low power transfer; lanes are typically unterminated,and seven different speed gears are supported, ranging from 3Mbps to576Mbps per lane. In FAST MODE, three different speed gears aresupported starting from 1.5Gbps per lane, doubling with every gear andreaching to 6Gbps per lane. Up to four lanes in parallel are supportedon outbound or inbound link offering asymmetric settings in bothdirections. The SLOW and FAST modes can be combined with the AUTOfeature, which controls M-PHY BURST closure and thus link power in thecase of gaps in data traffic. In the complete absence of data traffic,HIBERNATE mode drives the entire link into ultra-low power consumption.

UniPro offers a lower level link protocol, managing the linkdynamically without the need for side-band signaling (see Figure 3). Anapplication at one end controls the entire bi-directional link settingusing a so called POWERMODE message requesting the new link settings forMode, Gear and number of used lanes, each for the inbound and outboundlink. The application request is checked for consistency andcommunicated to the peer via UniPro’s unique PHY Adapter ConfigurationProtocol (PACP). Here again a consistency check is carried out and theapproval for the new settings is returned to the requester. Settings arethen applied simultaneously for local and peer device. Application datatraffic is temporarily paused during the configuration process, whichitself takes only a couple of micro seconds allowing frequent dynamicchanges to adapt to the power plan of the application.

Figure 3. Dynamic in-band Link Management via PACP.

Media converter: MIPI UniPort-M allows data transfer fromone node to another via optical or copper wires. Based on MIPI M-PHY,UniPro supports the control of a media converter (MC) using in-bandM-PHY configuration. MCs help to improve signal integrity over longerdistances and improve EMI characteristics.

Forthcoming advancements
The MIPI UniPro WorkingGroup continues to advance MIPI UniPro. The next version of thespecification, expected for release by mid-2016, will integrate withMIPI M-PHY v4.1, the next release of the MIPI M-PHY physical layer.M-PHY v4.1 will introduce High Speed GEAR 4 capabilities, doublingmaximum data rate compared to the previous specification to enableoperation at 11.6 Gbps/lane or 46.4 Gbps over 4 lanes. This double datarate will enable designers to deliver higher throughput and/or reducethe number of lanes to lower costs.

The current release of MIPI UniPro, v1.6, is a peer-to-peerinterface. However, UniPro was designed with basic support for networksin mind and conditions are present in the market to facilitate its usefor networking. Google Project Ara, for example, developed a networkswitch based on MIPI UniPro v1.6 to transport packets among multipleMIPI UniPro peer-to-peer connections. Because the MIPI UniPro transportlayer does not define a master/slave relationship between the peers,such a network may have more than one application master. Thisexperience is likely the first to create a module network in a mobilephone and could spur interest in defining related capabilities in futureversions of the MIPI UniPro specification.

Invitation to participate in MIPI UniPro’s future development
MIPIUniPro offers many practical design advantages because it provides asimple but highly capable interface that is easy to implement, flexiblefor use in traditional or modular architectures.

From a business standpoint, MIPI Alliance believes its global,collaborative structure is tailored to meet and respond to ongoingmarket needs. As an independent, standalone standards developmentorganization, the Alliance welcomes new ideas and participation byanyone who wants to help ensure the continued advancement and relevanceof its specifications. As chair of the MIPI UniPro Working Group, Iencourage interested members of the engineering community to join MIPI Alliance and contribute to our important work.

Jürgen Urban chairs the MIPI Alliance UniPro Working Group and is chief engineer forToshiba Corporation where he works in the field of mobile devices. Hisspecial interest lies in high speed interfaces and the associated(network) protocols built on top of them. Urban has been associated withMIPI for seven years, participating in several relatedactivities. Prior to Toshiba, he was with Nokia and Austria MicroSystems. Urban studied microelectronics at the University of Karlsruhe,later at the University of Southampton(UK), and did his diploma thesisat ESIEE, Paris. During his leisure time, he enjoys all kind of watersports, swimming, scuba diving, sailing, traveling, and exploring othercultures.

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