Before the mid 1980s electronic content on passenger vehicles was minimal and Electronic Control Units (ECUs) were few and far between. Most functionality was simple on / off control activated via point-to-point connections. The increase in ECUs and more distributed functionality created a need for some form of multiplexed communications. Initially individual car makers went it alone in developing their own ad-hoc networks. After the mid 1990s the need was spread across most car manufacturers and a gradual shift towards standardised networks began.
These networks brought benefits to the car makers not only in terms of their technical capability, but also lower costs through economies of scale, reduced development time, and the freedom to use them with a variety of supplier products.
The different requirements of various vehicle functions in terms of data rate and the degree to which applications were safety critical led to several standard networks emerging and evolving.
Different functions with different network requirements
Networks on a vehicle can be broken into clear groups each with different needs.
Powertrain (engine and transmission) – These are typically low latency closed loop control systems with a continuous flow of high amounts of data – typically around 100kbps. They are required to exchange data with other same car domains and have a high redundancy requirement.
Chassis (suspension, steering and braking) – Again these are low latency closed loop control systems that exchange data with other systems in the vehicle. Data flows in long heavy bursts at around 100kbps. Safety is key so good network reliability and continuity is essential.
Body (comfort functions) – Although these represent the greatest number of functions on the vehicle, they are the least demanding in terms of network performance. They transport short bursts of slower data, usually triggered by driver and passenger inputs to comfort function controls. The main demand on body control networks is that they have flexibility (to accommodate car upgrades for example), have a high degree of compatibility and are low cost.
Active and passive safety (airbags, tyre pressure monitoring etc.) – These networks have low latency coupled to very high redundancy and safety. Airbags in particular have become widely adopted with the driver’s airbag standard on almost every vehicle. This is now widely being complemented by passenger air bags and complex side curtain and other airbags in higher specification cars.
Telematics (wireless, navigation, entertainment, diagnostics) – Uniquely for IVNs, these systems are required to exchange data with the external world and therefore must be compatible with non-automotive standards. Whilst latency is not especially important, telematics networks have to be able to exchange large amounts of multimedia data both inside and outside the vehicle. Information integrity and confidentiality can both be important.
The primary network protocols that exist to meet the needs of the five application groups are local interconnect network (LIN) and Controller Area Network (CAN). Others used to a lesser extent are Media Orientated Systems Transport (MOST) and FlexRay. Numerous manufacturer specific protocols also exist.
Commonly used and with the lowest cost per node is single-wire (LIN). With a data rate of 20kbps (over 40 metre cable) plus good flexibility and extendibility, LIN is well suited to the body electronics functions for which it is commonly adopted.
Dual-wire CAN is currently the most dominant bus system in the automotive market. It was developed by Bosch in the early 1980s and first used in Mercedes cars in 1992. Although it has the potential to achieve a data rate of 1Mbps (over 40 metres of cable) most current powertrain and chassis systems on which it is prevalent use a 500kbps set-up.
MOST uses fibre optics in a point-to-point network with ring, star or daisy chain topology. Specifically developed to serve rapidly evolving vehicle telematics, audio and multimedia applications, MOST runs at 24Mbps on 64 nodes.
As steer-by-wire, brake-by-wire and active safety systems move closer to being adopted for mainstream vehicles, FlexRay is positioned as an ideal network for these types of safety critical powertrain and chassis applications. Using dual-wire optical fibre it is able to handle a gross data rate of 500kbps to 10Mbps – significantly higher than CAN, but also much more expensive.
SBCs provide an integrated bus interface
All IVNs require transceiver functionality to sit between the protocol controller and the physical bus. In order to reduce cost and space requirements and at the same time improve robustness and long-term reliability, it is important that this circuitry be as integrated as possible. Thanks in recent years to the advent of mixed signal processes such as ON Semiconductor’s Smart Power, high-voltage, 0.35µm CMOS technology, high-voltage analog circuitry is now able to co-exist with digital functionality in the same device.
Further integration has allowed SBCs to be developed. These incorporate a transceiver, voltage regulator and a host of other features such as a watchdog mode and wake-up circuitry to save power, plus protection through thermal shutdown and ESD measures. SBCs effectively provide a one-chip solution that reduces the number of external components required to just a few de-coupling capacitors. Housed in small SOIC packages, SBCs occupy minimal board space and simplify the design process in what can often be a cramped installation. Add to this the well proven benefits on reliability of hard-wired connections inside an encapsulated package as opposed to external ones made on a printed circuit board, and its easy to see why SBCs for IVNs are a key development.
As LIN, CAN, FlexRay and MOST installations proliferate down through manufacturers vehicle ranges over the coming few years, SBCs born out of the ability to combine analog and digital functions with these standards based transceivers will have increasing relevance.
Captions
Figure 1: A current generation SBC: Block diagram of ON Semiconductor’s LIN AMIS-40616 LIN transceiver.
Figure 2: A typical application for the AMIS-40616
Figure 3: Comparison chart of the most popular IVN protocols
Click here for the illustrations:
Figure 1, Figure 2, Figure 3
