Oscilloscope Applications in the Automotive Industry LIN, CAN and FlexRay Serial Bus Debugging

Keysight InfiniiVision 2000, 3000, and 4000 X-Series oscilloscopes provide CAN and LIN triggering and decoding, as well as CAN eye-diagram mask testing, helping you debug automotive serial buses faster. The InfiniiVision 3000 and 4000 X-Series also provide FlexRay triggering and decoding, as well as FlexRay eye-diagram mask testing.

In order to improve system communication efficiency and reduce costs, all automotive designs today use a large number of serial bus communication protocols. I2C and SPI protocols are most commonly used for chip-to-chip communication of electronic control units (ECUs). For long-distance serial communication and control between various automotive subsystems (such as comfort control systems, anti-theft locks, transmission systems, and engine controls), CAN, LIN, and FlexRay protocols are the most widely used serial buses in today's automotive industry.

The LIN serial bus based on a master-slave relationship is mainly used in applications that do not require high safety, such as seat and mirror control. The CAN serial bus uses differential event triggering, which has higher noise immunity than the single-ended LIN bus and has been used as the main control bus for automobiles for more than 20 years. The FlexRay serial bus uses differential time triggering and synchronous deterministic schedules. As an emerging serial bus technology, FlexRay is used in some high-end automobiles and is mainly suitable for systems with high performance and safety requirements.

However, serial bus communications are often affected by signal integrity issues caused by the non-ideal environment inside the car, which sometimes introduces errors in critical communication cycles. Although serial bus protocol analyzers are great for testing and monitoring the transmission of serial bus data at higher-level protocol and application layers, they cannot measure the integrity/quality of your car's serial bus signal (physical layer). Some of today's mid- to high-performance digital storage oscilloscopes (DSOs) offer LIN, CAN, and FlexRay bus decoding and triggering capabilities, which can establish a time-correlated link between the protocol layer and the physical layer.

Figure 1 shows a Keysight InfiniiVision 4000 X-Series oscilloscope capturing and decoding a CAN bus symbolic, while simultaneously capturing and decoding a hexadecimal LIN bus. The bottom of the display is a time-correlated decode trace for each bus. The top half of the oscilloscope display shows the oscilloscope industry's only time-interleaved "lister" display.

Figure 1: Keysight InfiniiVision X-Series oscilloscopes simultaneously capture and decode CAN and LIN serial buses.

Keysight InfiniiVision Series oscilloscopes exclusively use hardware decoding, while all other oscilloscopes on the market today use software decoding. Software decoding techniques are slow, especially when using the oscilloscope's deep memory. Hardware decoding provides near real-time updates of serial bus activity without slowing down, even when using deep memory. This increases the probability that the oscilloscope will capture random and infrequent communication errors.

In addition to providing decoded words that are time-correlated with captured waveforms, another very effective tool is the eye-diagram mask test, which can verify the signal integrity of high-speed CAN and FlexRay buses. Eye-diagram testing is widely used in current serial bus applications. The oscilloscope superimposes all the captured bits to obtain an eye diagram to show when these bits are valid and when they are invalid. The eye diagram can provide a composite picture describing the overall quality of the system's physical layer characteristics, including amplitude changes that may be caused by transmission line effects, reflections, system noise, overshoot, ringing, signal edge timing, and jitter.

Figure 2 shows a FlexRay eye mask test based on the TP4-10Mbs standard. The trigger reference for the TP4 mask test is based on each byte start sequence (BSS) event. This is the same reference signal used by the FlexRay receiver to resynchronize and recover the clock used to sample the received data. Keysight InfiniiVision Series oscilloscopes uniquely use repetitive hardware clock recovery technology to capture and overlay every bit of every byte in a synchronized FlexRay system. Note that the oscilloscope can also "filter" the captured and overlaid FlexRay eye data based on a special frame ID.

Figure 2: FlexRay “TP4” eye-diagram mask testing performed using a Keysight InfiniiVision 3000 X-Series oscilloscope.

FlexRay eye diagram mask testing can be performed with Keysight 3000 and 4000 X-Series oscilloscopes equipped with the FlexRay option and the mask test option. You can download various FlexRay mask files for free based on baud rate and test plane (TP#). CAN eye diagram mask testing can be performed with Keysight 2000, 3000, and 4000 X-Series oscilloscopes equipped with the CAN/LIN option and the mask test option. You can download various CAN mask files for free based on baud rate, probing polarity (dominant high or dominant low), and network length.

Keysight InfiniiVision X-Series Oscilloscopes

If you need to buy a new oscilloscope to support automotive serial bus applications, Keysight's InfiniiVision X-Series oscilloscopes, covering bandwidths from 70 MHz to 1.5 GHz, are your best choice. These oscilloscopes come with a standard 3-year warranty and an industry-first 2-year recommended calibration cycle.

To probe the differential CAN bus, Keysight recommends the N2791A 25 MHz differential active probe or the N2818A 200 MHz differential active probe. To probe the differential FlexRay bus, Keysight recommends the N2818A 200 MHz differential active probe.

To learn more about Keysight's InfiniiVision 2000, 3000 and 4000 X-Series oscilloscopes and mixed-signal oscilloscopes for automotive applications, visit: www.keysight.com/find/infiniivision