Whether apocryphal or not, well publicised tales of airbags or ABS systems being activated for no apparent reason bring home to manufacturers and users alike the need to ensure 100percent reliability in systems which can be safety-critical and life-threatening when they fail.

The cost to a manufacturer of faults discovered at the production stage can be enormous: not just in terms of putting right the faults, but also through recalls, loss of sales while the faults are being put right, and – possibly most important of all – the damage to a manufacturer’s brand reputation.

There is thus every incentive for manufacturers to invest in testing and quality-assurance procedures that will detect potential problems at an early stage and allow engineers to take corrective action before designs move on to the manufacturing stage. This applies equally to the suppliers of automotive subsystems, where the situation is complicated by time to market pressures which demand that testing and verification of new designs is completed in the shortest possible time.

System complexity

Automotive market forces have brought about increased system complexity, which in turn raises reliability issues.

The first automotive electronic systems were introduced for combustion control to meet environmental regulations on emissions. This was followed by safety systems, entertainment, navigation and comfort control, all of which require different types of electronic hardware operating at different speeds. This in turn dictates the nature of the communications interface between the electronics and the vehicle controls and output devices.

A key enabler in this scenario is the serial communications Bus, and in particular the CAN (Controller Area Network) and LIN (Local Interconnect Network) buses.

The CAN bus provides two communication services: the sending of a message (data-frame transmission) and the requesting of a message (remote transmission request or RTR). All other services such as error signalling and automatic re-transmission of erroneous frames are user-transparent, which means that the CAN chip automatically performs these services. CAN is operated at data speeds from 10kbit/s to 1Mbit/s, and applications are divided into low-speed (up to 125kbit/s) and high-speed (125kbit/s to 1Mbit/s).

The LIN bus was developed to achieve cost-effective communication for intelligent sensors and actuators in motor vehicles, specifically in applications where the bandwidth and versatility of CAN are not required. The simple protocol, low processing requirements and single-wire bus make LIN ideal for applications that require robust local connectivity. The data rate for the LIN bus is around 20kbit/s.

While the CAN Bus has become accepted as the de facto standard for many in-vehicle functions, it represents ‘overkill’ for activating simple two-wire drives to solenoids or lamp controls, for example.

As a result, cost-conscious motor manufacturers have adopted the LIN Bus for these less complex areas, and modern vehicles use a mixture of CAN and LIN networks, with the CAN network acting as the backbone and subsidiary LIN networks linked to it via gateways.

Reliability issues

There are a number of issues arising from this approach which can have an impact on overall vehicle reliability, and which require monitoring and analysis in order to solve the problems they present:

• Oscillator tolerance and propagation delay: The impact of oscillator tolerance and propagation delay on node arbitration can present problems because the CAN Bus is asynchronous. In a system with few nodes and short separations between nodes the system may perform satisfactorily, but as more nodes are added and separation distances grow larger these timing issues can cause priority problems so that the wrong signals take precedence.

• Bus utilisation and data rate: Operating an automotive bus at more than 30percent full load can cause nodes to generate overload frames or may not allow low priority activity to take place, resulting in improper functioning of the in-vehicle network. To understand and debug these problems, it is important to know the condition which causes the bus to become overloaded.

• Noise problems in the CAN network: Any in-vehicle network operates in an electrically noisy environment. Because CAN is a packet-based system, CAN signals are vulnerable to electrical noise, which can result in aberrations and possible malfunctioning of the network.

• CAN/LIN interface issues: In a typical automotive system, multiple buses are interconnected using a gateway which allows an exchange of data between the different buses. It is important that this gateway allows the accurate transmission of messages between the two networks with the minimum of latency time.

Advanced test tools

The availability of general-purpose high-performance test instruments such as oscilloscopes that are based on an Open Windows platform means that dedicated software can now be combined with the familiar oscilloscope display and controls in order to carry out industry-specific tests.

This is the approach used in the new Tektronix TDSVNM timing, protocol and analysis software, which, when used in conjunction with an external trigger module, turns a Tektronix TDS5000B or TDS7000B oscilloscope into a powerful analysis tool for CAN and LIN Bus networks.

Some of the major features of the software are:

• Measuring oscillator tolerance and propagation delay.

• Measuring bus utilisation and data rate.

• Analysing CAN eye diagrams to help in locating noise problems.

• Capturing CAN and LIN messages based on the content of the CAN frame and viewing it in protocol decoded format.

• Capturing CAN and LIN data simultaneously to help in monitoring the latency time at the gateways.

The software also enables customers to set trigger conditions based on the content of a CAN message. Trigger conditions are then downloaded at run-time to an external automotive trigger module, which triggers on conditions in real time and helps to pinpoint the causes of intermittent problems through the ability to trigger

‘if-then-else’ conditions.

Conclusion

Engineers in the automotive electronics industries who need to design and ensure seamless operation of network nodes based on CAN and LIN serial data buses need tools that provide efficient testing and debugging of serial CAN bus and LIN bus nodes and networks.

Software is now available which adds application-specific features to general-purpose oscilloscopes, and will carry out all the key tests required on CAN and IN Bus networks. 

Hans-Peter Fleischheuer is Oscilloscope Marketing Manager EMEA, Tektronix. For more information, visit www.tektronix.com