The automotive industry is entering an exciting yet challenging time for electronics system designers. Applications such as infotainment, telemetry, safety, and control require the use of several networking standards. Design engineers are faced with the daunting task of selecting the right buses for a multitude of applications while enabling feature sets that customers want. Additionally, the topology must support the rapid pace of evolving bus standards and future applications.
The automotive electronics market segment is not that dissimilar to many others such as data-com, telecom, and consumer electronics. There are a vast array of networking protocols to choose fromeach with advantages and disadvantages. No one protocol satisfies the requirements of all automotive applications.
Automotive networks can be general classified into 3 main areas:
Body control: rRequires relatively low bandwidth networks, but high reliability and data integrity.
Infotainment: r Requires high bandwidth and real time processing for audio, video.
Safety: tTraditional hydraulics and sensors are giving way to by-wire approaches for steering and braking.
There is a need to consolidate data from these networks and perform processing in a central location. As such, a gateway is used as a central hub to interconnect and process data from a vehicle's embedded networks. A typical gateway is depicted below. The gateway is composed of several automotive networking interfaces such as CAN, MOST and FlexRay in addition to embedded micro controllers and peripheral functions.
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CAN (Controller Area Network) is well suite for body control applications due to its low cost and high reliability. Typically, automobiles consist of several CAN networks for functions such as engine management, instrument clusters, and body control. CAN does not have the bandwidth to process video and audio data as it supports data rates of up to a maximum of 1 Mb/s. However, CAN is a very inexpensive, fault-tolerant, and essential network protocol that is here for the foreseeable future. According to ABI Research's In-Vehicle Networking Report (4Q 2004) there will be approximately 528 million automotive CAN nodes shipped worldwide by 2010.
MOST (Media-Oriented Systems Transport) serves the needs of video and audio functions quite well. It supports a 24 Mb/s stream over plastic fiber that enables both synchronous and asynchronous data transfer. MOST transceivers are found in devices that require real time processing, such as DVD players, head units, GPS devices, and displays. The MOST Consortium has defined standards not just for interconnect but for software application interfaces as well. It is a scalable yet highly reliable networking standard.
FlexRay is architected for X-by-wire applications, such as brake-by-wire and steer-by-wire. Today, these functions are typically addressed by mechanical methods employing hydraulics. In the automobiles of the future hydraulics will be replaced by FlexRay based controllers. This wire-based network approach is a high speed, fault tolerant bus. FlexRay also supports both synchronous and asynchronous data transfers with a data rate of approximately 10 Mb/s, and has deterministic data transmission, fault tolerance, and guaranteed message latency. Provision for redundancy is provided in a dual channel mode. The FlexRay standard is developed and maintained by a consortium of automotive OEMs, electronics suppliers and semiconductor manufacturers.
The typical gateway contains several other interfaces in addition to the automotive networking standards described above. Ethernet is a widely used networking standard that is suitable for diagnostics, and, as a service interface, is a logical choice. The hardware is inexpensive and a plethora of software applications are available. Because the interface is used for diagnostics, the regular automotive requirements of fault tolerance and noise immunity do not apply. Embedded processors are required to process data content from the CAN, MOST, and FlexRay networks. The processor will strip headers, aggregate data and transform data from one type to another. The Ethernet interface requires an embedded processor running the TCP/IP stack. Off-chip memory is required to store program code. Additional memory may be used for temporary data storage from the embedded networks.
Design factorsWhich networks need to be bridged?
What is the bridge topology?
Is DMA (direct memory access) required?
How should data buffers be sized?
What bus should be used for internal data transfer?
How wide should this bus be?
What arbitration mechanisms should be employed?
How much processing power is required?
The system designer has many decisions to make when architecting a gateway to serve the needs of today's automobile. There are vast array of design choices that must be made, including:
Answers to these questions are very system and application dependent. However, there are some common issues that must be dealt with. Obviously, CAN, MOST, and FlexRay are different protocols. They each have different payload sizes, data rates, and needs for real time processing. The gateway must be able to efficiently process all of the incoming and outgoing data from these interfaces.