FROM EDN EUROPE: Can MOST escape from the car?
On the timescales that prevail in the automotive world, the MOST bus is experiencing a rapid deployment, routing multi-media data around a number of (admittedly, high-end) vehicles already in volume production. Its proponents envisage a wider application scope for the bus: are they being realistic, or will in-car use mark the limits of MOST's domain?
By Graham Prophet, Editor -- EDN, September 1, 2005
| AT A GLANCE |
| MOST brings fibre-optic bandwidth and noise immunity to automotive designBus has achieved volume deployment in a short time—by automotive standards—since its conception in 1997Now supported by comprehensive and growing list of suppliers from high-end DSPs through to plastic-fibre-connector makersOptimised for media data types but flexible and versatile in definitionPlanned extension to 150 Mbits/sec will help ensure longevityUnlikely to penetrate the home multimedia market but offers a potentially useful low-cost fibre bus for some industrial environments |
The MOST (Media-Oriented Systems Transport) bus is an attempt to bring the many advantages of fibre-optic data transmission into the mass-market, automotive environment. As numerous car makers now deploy it in multiple vehicle models, it is clearly successful in that ambition. You might have, personally, so far resisted the temptations of a car with an OEM DVD player. But a trip along the autobahn, autoroute or motorway, with occasional sideways glances into the up-scale family cars you pass will demonstrate that specifying that particular option to keep the children distracted in the back seats is now a far-from-exclusive purchasing decision. The seat-back DVD screen is only the most visible manifestation of fitment of a MOST bus in a car—the fibre bus also handles digital audio, both for entertainment and for hands-free telephony, and information-system data such as GPS. Vehicles such as Porsche's Cayenne 4x4, and the BMW 7-Series epitomise the class of car on which such multimedia systems might be expected, and which do indeed incorporate MOST. However, current model-year vehicle specifications show the technology diffusing, following the usual automotive pattern, from high-end into mid-range models.
Nor is MOST the only fibre-optic channel car makers are now deploying in production vehicles; BMW also employs plastic optical fibre to link the sensors and control module of the airbag system in the 7-Series, using the ByteFlight protocol.
Goodbye to EMI
The benefits of fibre optics for the automotive environment are obvious to design engineers. The medium provides a means of routing high-bit-rate data through the noisy environment of the car, providing immunity for that data stream from electrical interference. Reciprocally, it also removes any risk of that data, with its fast edge rates, generating interference that might find its way into the electrically-bused signals in the car, especially the power train and other safety-critical systems.
MOST is an automotive industry co-operation, rather than having the status of a formal standard. It was conceived in the mid-1990s, as a joint venture between BMW, DaimlerChrysler, Harman/Becker (audio systems manufacturer) and Oasis Silicon Systems. The participants set up an autonomous body, the MOST Cooperation, shortly thereafter (1998), and it is this body that controls the definition of the bus. Oasis retains rights to the MOST name itself. A product qualification process is operated by independent test houses, for example Ruetz Technologies. In addition to conformance testing, Ruetz offers hardware and software analysis tools for use in MOST-bus-system development, and training in MOST systems.
While the potential benefits of optical fibre may have been obvious, with experience from telecommunications confirming them, the costs associated with telecoms were definitely not compatible with automotive manufacturing. A major part of the MOST story has been the effort to reduce component costs.
MOST is defined across all seven layers of the standard OSI model. It is, as might be expected, highly optimised for the automotive multimedia environment, defining signal classes for the main traffic types it has to handle (such as CD stereo audio, and DVD video).
Low-cost plastic fibre
At the physical level, the transmission medium itself is a 1-mm core, PMMA (poly-methyl-methacrylate) fibre with an outer plastic jacket, that OEM suppliers can incorporate into a wiring harness together with all of the electrical cables of the loom. Transmission is at 650 nm (red) using LED emitters (650 nm is a low-loss "window" in the spectral response of the PMMA). Data is sent at 50 Mbaud, bi-phase coded, to give a maximum data rate of 24.8 Mbits/sec.
The MOST definition is very general and allows a number of topologies including star and ring—the majority of vehicle installations appear to use ring layouts. There can be up to 64 nodes in a MOST network. As long as the vehicle's power is on, all of the MOST nodes in the network are active, so there is great emphasis on low-power, power-down modes, both in terms of how they are entered, and of the amount of power used while the system is in that state. The default condition of a MOST node at power-up is pass-through; incoming data passes from receiver to transmitter, maintaining the ring.
MOST data is sent in frames of 512 bits, and in blocks of 16 frames (Figure 1). Frames are repeated at a 44.1-kHz rate (22.67 msec per frame) and each frame contains—in addition to pre-amble and other housekeeping bits—payload of synchronous, asynchronous and control data. The bus is fully synchronous, the designer assigning any single device in a net as the master, from which all other nodes derive their clocks. Networks are fully plug-and-play, with a device-discovery process that runs at power-up or when connectivity changes. The master node maintains a central registry of connected devices.
Progress of the bus is demonstrated by the increasing level of product support. Key elements in implementing a MOST node include the electrical/optical interfaces—the transmitter and receiver modules. The dominant supplier in this niche has been Infineon, which has recently released specifications for its ODIN MIT/MIR4 transmit/receive modules. These conform to a defined, 4-pin physical outline and provide the 650-nm transmission/reception functions. The receiver module combines a photodiode with signal-processing and active wake-up functions, on an IC that Infineon now produces—on previous generations, the company sourced it from Oasis Silicon Systems. The director of marketing for plastic optical fibres at Infineon, Martin Weigert, confirms that the normal pressures of supplying the automotive industry apply—stringent requirements on quality and reliability, with continuous price pressure. A transmitter/receiver module pair has a target price in the region of €3—the starting point, when fibre technology was first adapted for the automotive environment was, "much higher" says Weigert. Infineon also manufactures transceivers for ByteFlight and its "SFH" series for short-range industrial links.
This generation of transceiver elements takes temperature ratings beyond the standard industrial upper limit of 85°C, to 95°C—a figure which has been a challenge, and which Agilent first achieved earlier this year with the announcement of its AFBR1010/2010 device pair. 95°C is also somewhat beyond the ideal operating range of the PMMA fibre material, a limitation that may ultimately see it superseded by a silica-cored fibre.
Also due to enter the market for MOST transceivers is Melexis, under the part numbers MLX75600/02, which are also 95°C compliant (Figure 2). Piet De Pauw, optical product bus manager at Melexis, confirms that the target pricing would be in the €3-3.5 region, and also notes that the company comes to this sector with pre-existing expertise in the automotive sector through its sensors products. De Pauw sees this as an advantage over companies adapting optical expertise gained in the telecoms sectors.
All of the optical/electrical and electrical/optical modules marry to a connector-shell pattern that accepts a low-cost fibre termination. A variety of suppliers, including FCI and Furukawa, offer connectors in a format that mixes optical fibre pairs with electrical connections in various combinations.
Interface controller
On the other side of the transceiver module, the essential item of a MOST node is the network controller IC, available from Oasis Silicon Systems, now part of SMSC. Oasis supplies the OS 8104, a chip that integrates network interface, data routing engine, host and management interfaces, with full software and the related development tools. The 81050 adds a higher level of intelligence to supervise MOST functions, while the 8805 is a system-level integrated device that includes ADCs and DACs for direct input and output of audio signal streams.
The bus is also increasingly supported by the major microprocessor and DSP vendors. Analog Devices, in its Blackfin DSP configurations for automotive video display and for hands-free cellular-phone connections, offers connectivity to the car's networks via MOST. Texas Instruments has just announced a version of its OMAP processor optimised for automotive infotainment products. Under a licence from SMSC, TI has integrated the MediaLB bus, an on-chip and inter-chip communications bus that transports a local de-multiplexed version of MOST network data. This, TI says, future-proofs users' designs for use with any forth-coming MOST upgrade. An on-chip digital-audio-routing engine processes and directs audio data between audio ports, the DSP core and the MediaLB interface—this engine is equivalent to 100 MHz DSP bandwidth in its own right, TI says.
Fujitsu was an early player in MOST-enabled processors, announcing as early as the beginning of 2004 a cooperation between itself, Oasis and the R&D company mycable to develop reference designs for multimedia car systems. mycable focuses on embedded multimedia designs; Fujitsu MPEG chips contributed the video decode to the reference designs.
Applications beyond the car
The MOST Cooperation presents a picture of the bus being used in diversifying applications beyond the strict automotive environment. One possible scenario is the transfer of the bus into the domestic environment, but current trends would seem to make this unlikely. Analysts Gartner Dataquest are on record as forecasting the multi-media-equipped home will require both wired and wireless, high-speed data-transmission media. MOST might appear to be a candidate for the wired portion of that scenario, as Oasis defined the bus specifically for the appropriate content (video and audio). But Gartner Analyst Paul O'Donovan says that any significant penetration into the home is unlikely. Even in the anticipated higher speed version of MOST, "It's unlikely simply due to the 'no-new-wires' principle: we expect power-line and other pre-existing physical wiring to predominate in the home, and transfer of content—rather than players—to be the norm between home and car."
Nevertheless, manufacturers must address the issue of providing access to a vehicle's MOST network for one specific purpose, namely that of system upgrades. In many vehicles, the standard rectangular slot for radios, CD players and the like has all but disappeared, with designers distributing the functions of radio, CD, phone, GPS and other systems across hardware resident somewhere behind the dash, rather than in a specific box. Vendors such as Analog Devices have proposed an ultimate version of the concept in which each of these functions becomes merely a software thread running on one of its Blackfin DSPs. Not the least benefit is that the radio becomes more-or-less impossible to steal—in fact, the alarm software could be bundled onto the same processor.
A brief glance at any one of a number of car-enthusiast websites will reveal that a great deal of angst already exists on the subject. Scenarios include there being no physical slot available to house an upgrade for radio or some other item of in-car equipment. Or, the car's software doesn't recognise the upgraded item and it will not function properly (or at all). Or, and possibly worst of all, the processor in the replaced item was hosting some body-electronics software in addition to its entertainment function—the replacement audio unit may function but (for example) the climate control stops working. As MOST-equipped cars proliferate, the potential for such interface problems can only increase.
The US-based Consumer Electronics Association is working with the MOST Cooperation to define a consumer-electronics gateway function through which such upgrades could be connected, with a draft document proposing the gateway's specification due for release by the time we publish this article.
Further developments in train include a revision to MOST that would increase its basic speed from 50 to 150 Mbits/sec, to provide the bandwidth necessary for multiple video streams, and to interface to a proposed in-car wireless network. The optical standard applied to ByteFlight is likely to see further development—its designers structured ByteFlight very differently from the multimedia-oriented MOST, with (at present) only a 10 Mbit/sec data rate. Because of its use in safety-critical systems, they organised it to be fully deterministic and fail-safe, and, says Infineon's Wiegert, ByteFlight demands full traceability of product, with data stored indefinitely. Those guiding the FlexRay bus are also contemplating an optical transport solution.
Infineon's Weigert notes an increased level of interest from industrial systems designers in MOST. For designs that do not require more than the 64 nodes (although bus extenders are available), it could offer a ready-made, and low-cost, way of moving to a fibre-based data collection or control network for a noisy environment. Although the MOST Cooperation designed it for use over the few metres of a typical car's wiring loom, there is margin in hand to cover somewhat larger distances. Typical plastic optical fibre might have an attenuation of under 0.2 dB/m, while transmitter/receiver pairs such as Agilent's AFBR1010/2010 specify an optical budget from the output (optical) face of the transmitting device to the input face of the receiver of 14 dB. Allowing for the fact that low-cost, mass-market connectors will impose a certain minimum transmission loss, and for the fact that losses in the first few metres of fibre are higher than in the (relatively) long-haul, a range of tens-of-metres would appear to be feasible.
Companies supporting the MOST specification with products are too numerous to list here; the following URLs are provided for the specific examples mentioned in this article. The membership of the MOST Cooperation is listed on its website, but sources of relevant products are not limited to that list.
Agilent Technologies
www.agilent.com
Analog Devices
www.analog.com
Consumer Electronics Association
www.ce.org
FCI (Framatome)
www.fciconnect.com
Fujitsu Microelectronics Europe
www.fme.fujitsu.com
Furukawa
www.furukawa.co.jp
Infineon
www.infineon.com
Melexis
www.melexis.com
MOST Cooperation
www.mostnet.de
mycable
www.mycable.de
Oasis Silicon Systems
www.oasis.de
Ruetz Technologies
www.testerlyzer.com
SMSC (Standard Microsystems)
www.smsc.com
Texas Instruments
www.ti.com
