Understanding FlexRay Designers' Oscilloscope Test Needs

March 3, 2008

FlexRay is the next-generation higher-performance, time-triggered, and deterministic serial bus used in higher-end automobiles for many safety-critical and "x-by-wire" systems. X-by-wire may include systems such as steer-by-wire, brake-by-wire, and navigation/collision-avoidance-by-wire. These types of automotive systems, which you will see in future automobiles, must be "error-free." Although today's computers can tolerate low bit-error-ratios, safety-critical automotive systems must induce no errors -- for obvious reasons. This is why this new serial bus has been selected for these types of applications.

"Time-triggered/deterministic" simply means that digital packets of information are always transmitted within designated time-slots according to a FlexRay system global timing schedule. This timing mechanism eliminates the possibility of arbitration, which is common with the current CAN bus technology, and makes this new technology an attractive alternative for some of tomorrow's safety-critical applications.

In addition to safety and reliability issues, "x-by-wire" technology will enable automotive manufacturers to eliminate some bulky mechanical linkages throughout the automobile, such as the steering column as a possible future example. Not only will this reduce weight and improve efficiency, but it opens up a myriad of future possibilities for applying new state-of-the-art electronic systems in the automobile such as navigation, entertainment, and comfort controls.

FlexRay technology is being developed first by German automotive companies and system providers, followed by the Japanese market. Prior to the introduction of Agilent's FlexRay measurement option for Mixed Signal Oscilloscope, Agilent conducted extensive customer research with leading vendors and their embedded hardware and software designers who are working with FlexRay technology today, including:

• Bosch
• BMW
• Audi
• Siemens VDO
• Renesas

Key Learnings from FlexRay Designers

During early market research efforts, Agilent discovered that early adopters of FlexRay technology needed advanced oscilloscope measurements beyond the capabilities offered by today's scopes or protocol analyzers alone. The technology required by FlexRay designers consists of leading-edge measurement technologies -- primarily mixed signal oscilloscopes and FlexRay protocol analyzers.

In addition to being able to accurately measure the signal integrity of the physical layer of the differential FlexRay bus, FlexRay designers expressed a strong desire to be able to time-correlate actual signal transmissions with the ideal global timing schedule. Although synchronizing to the FlexRay global timing schedule is a fundamental capability of a FlexRay protocol analyzer, this in not an inherent capability of a scope. Nor is performing signal integrity measurements an inherent capability of a protocol analyzer.

Secondly, because of the required "error-free" nature of FlexRay technology, designers said they need to be able to quickly debug their systems in order to detect any possible errors conditions, and if they exist, relate them back to the differential FlexRay signal to see if errors are a result of signal integrity issues. Simply triggering on and decoding CRC errors would be insufficient. FlexRay designers need a robust set of error triggering and decoding in their scopes in order to quickly find problems.

FlexRay designers also asked Agilent to include the ability to view multiple analog and signals simultaneously so they can correlate mixed-signal activity, including digital signal activity within ECUs correlated to the FlexRay signal, as well as to analog sensor inputs. This request means that most FlexRay designers would prefer to use a mixed signal oscilloscope (MSO) rather than a conventional digital storage oscilloscope (DSO), which only provides a limited number of analog channels of acquisition and display. FlexRay designers also need to be to test their systems/devices in temperature chambers at extreme temperatures -- often beyond the temperature range of today's off-the-shelf measurement equipment.

Because of the data multiplexing communication possibilities that the FlexRay protocol allows, FlexRay designers want to be able to trigger oscilloscope acquisitions on particular communication cycles -- not just cycle number, but cycles based on a base and repetition rate.

Lastly, Agilent learned that most FlexRay designers prefer a portable and non-Windows-based scope without a PC "in-the-loop" for their FlexRay measurements. FlexRay measurements often need to be performed inside the automobile under driving conditions, and battery operation is highly desired. It is hard for test engineers to handle large, bulky, and power-hungry Windows-based machines when they are sitting in an automobile's passenger seat.

FlexRay designers presented quite a challenge to Agilent's oscilloscope designers especially since a common theme was that they wanted an advanced oscilloscope FlexRay measurement solution today, and they didn't want to pay too much for it.

Meeting the FlexRay Challenge

How did Agilent satisfy most of the measurement needs of FlexRay designers? In 2007, Agilent introduced an advanced oscilloscope measurement system for automotive embedded designers working with the new FlexRay serial bus technology. This new measurement system, which combines an Agilent 6000 Series mixed signal oscilloscope (MSO) with the Agilent VPT1000 Vehicle Protocol Tester for FlexRay, provides many advanced features not currently available in other oscilloscope solutions on the market including:

• First oscilloscope with time-correlated measurements relative to the FlexRay global timing schedule
• First oscilloscope with extensive FlexRay error triggering and decode
• First oscilloscope FlexRay cycle-base and repetition-cycle triggering
• First oscilloscope with hardware-based real-time FlexRay decode update rates
• First oscilloscope with mixed-signal measurements using MegaZoom technology for FlexRay applications

Why use an MSO for FlexRay measurements?

Automotive electronic systems are by definition mixed-signal systems consisting of multiple analog, digital, and serial signals. FlexRay designers said they need the ability to correlate multiple mixed-signal activity within their automotive systems. A mixed signal oscilloscope is a natural fit for these types of measurements. With an MSO, designers can easily view and time-correlate their analog sensor inputs, their differential serial signals, such as FlexRay, and their digital control and I/O signals within their ECUs using a single instrument. And with advanced FlexRay triggering and decoding, they can also correlate all of the mixed signals to the FlexRay bus.

The Importance of the FlexRay Global Timing Trace

The theoretical error-free nature of the FlexRay protocol is partially attributable to its time-triggered architecture. FlexRay signal transmissions must occur within a specified window of time. Verifying the timing of physical-layer FlexRay signals to the global timing schedule is important to FlexRay designers so they can eliminate possible timing errors. One unique capability of Agilent's MSO FlexRay measurement solution is the time-correlated global timing trace displayed on the MSO's display. This feature makes it easy for designers to verify correct signal timing of FlexRay slots along with color-coded static and dynamic segments.

Meeting the Need for Robust FlexRay Error Analysis

For safety-critical FlexRay systems, it is extremely important to eliminate all possible errors. After all, when a driver turns the steering wheel to avoid a collision, the electronic steering system must respond immediately. Identifying errors goes beyond just detecting CRC errors. What about possible timing errors such as slot boundary violations? Agilent's oscilloscope FlexRay measurement solution provides the most robust set of error triggering and error decoding in an oscilloscope measurement system to help FlexRay designers debug their systems faster.

Hardware-Based Decoding

Waveform and FlexRay decode update rates are important for two reasons. First, fast waveform/decode update rates provide for responsive measurements, making the oscilloscope easier to use. Secondly, and probably most importantly, real-time decode update rates enhance the probability that the scope will capture random and infrequent error conditions. Most oscilloscopes on the market today use software decoding techniques. Using software decoding, waveform and decode updates can take seconds -- especially when an engineer is using deep memory to capture multiple packets of FlexRay transmissions. If random errors occur, they will probably occur during the scope's data-processing dead-time, which is often orders of magnitude longer than the scope's acquisition time.

Rather than using software decoding techniques, Agilent's oscilloscope FlexRay measurement system uses hardware-based decoding. Hardware decoding provides fast waveform and decode update rates to enhance the probability of catching random and infrequent error conditions.

Overcoming Design Challenges

Developing an oscilloscope FlexRay measurement solution consisting of a protocol analyzer and mixed signal oscilloscope (MSO) was not without its design challenges. One particularly difficult challenge concerned real-time responsiveness. How could two independent instruments communicate with one another without significant post-processing delays? This design challenge was overcome by taking advantage of one of the unique capabilities of an MSO. Eight of the sixteen logic channels of the MSO were re-defined into a high-speed communication bus. Using this 8-lane communication port, the Agilent VPT1000 is able to transfer 10 Mbps FlexRay timing and frame protocol data to the MSO at a net rate of 320 Mbps. This provides real-time responsiveness when coupled with the MSO's hardware/FPGA-based decoding. Although data transferred from the VPT1000 to the MSO exhibits approximately 300 ns latency, the scope displays decoded data with precise time-alignment to the physical-layer differential FlexRay signal utilizing a fixed de-skew factor.

Another design challenge was satisfying FlexRay designers' requirement that the oscilloscope FlexRay measurement solution must be able operate in a synchronous mode without using a PC in the measurement loop. Customers wanted a "standalone" measurement system. Synchronous FlexRay protocol analysis normally requires a PC and a software driver to establish and synchronize to the FlexRay global timing schedule. This design challenge was overcome by parsing a FIBEX file within the MSO to "strip-out" important setup parameters that define the global timing schedule. These parameters are then downloaded to the VPT1000 Vehicle Protocol Tester via a direct LAN connection from the MSO without using a PC in the loop.

Testing in Extreme Environmental Conditions

Signal integrity measurements on automotive differential signals such as the physical layer of the FlexRay serial bus requires differential active probing. In addition, FlexRay designers must often test their embedded designs under very extreme conditions in environmental chambers. These extreme conditions may include testing ECUs and differential serial buses at temperatures exceeding 150 degrees Celsius. Unfortunately, the active circuitry in today's typical active probes cannot tolerate temperatures exceeding 55 degrees C. However, with the unique electrical and physical architecture of the 1130 Series InfiniiMax active probes, the Extreme Temperature Cable Extension Kit (N5450A) can be used to extend and displace the probe's active amplifier to be outside of an environmental chamber. With this configuration, InfiniiMax' passive probe heads can be connected to test points within the chamber with temperatures ranging from -55 degree C up to 155 degrees C.

Summary

Providing advanced oscilloscope FlexRay measurements was achieved by Agilent by combining a Mixed Signal Oscilloscope (MSO) and FlexRay Vehicle Protocol Tester to take advantage of each products core competency. The result of this synergistic design effort is that automotive FlexRay designers now have at their disposal the most advanced oscilloscope FlexRay measurement system on the market. This new measurement solution, consisting of an Agilent 6000 Series mixed signal oscilloscope and the Agilent VPT1000 Vehicle Protocol Tester for FlexRay provides important and critical FlexRay measurements currently not available in other oscilloscopes, including time-correlated global timing trace, real-time responsiveness, cycle-base and repetitive-cycle triggering, and extensive error triggering and decoding.