CAN FD Eye-Diagram Mask Testing

CAN FD Eye-Diagram Mask Testing

Application Notes

Introduction

Eye-diagram mask testing is used in a broad range of today's serial bus applications. An eye-diagram is basically an infinite persisted overlay of all bits captured by an oscilloscope to show when bits are valid. This provides a composite picture of the overall quality of a system's physical layer characteristics, which includes amplitude variations, timing uncertainties, and infrequent signal anomalies. Eye-diagram testing is especially important for higher speed buses such as the new CAN FD serial bus.

 

Eye-diagram testing can be performed on a CAN FD differential bus using a Keysight Technologies InfiniiVision 3000T or 4000 X-Series oscilloscope licensed with the DSOXT3AUTO or DSOX4AUTO trigger and decode option (CAN, CAN FD, & LIN), along with the DSOX3MASK or DSOX4MASK mask test option. Various CAN FD mask files based on the data phase baud rate and differential probing polarity (dominant-bit high or dominant-bit low) can be downloaded from Keysight's website at no charge. The following CAN FD mask files are available:

  • CAN-FD (L-H) 500k-4M.msk
  • CAN-FD (L-H) 500k-5M.msk
  • CAN-FD (L-H) 500k-6M.msk
  • CAN-FD (L-H) 500k-8M.msk
  • CAN-FD (L-H) 500k-10M.msk
  • CAN-FD (H-L) 500k-4M.msk
  • CAN-FD (H-L) 500k-5M.msk
  • CAN-FD (H-L) 500k-6M.msk
  • CAN-FD (H-L) 500k-8M.msk
  • CAN-FD (H-L) 500k-10M.msk

 

If you are probing the differential CAN FD bus to view dominant-bit low, then you should use one the mask files with "L-H" in the mask file name. If you are probing the bus to view dominant-bit high, then you should use one the mask files with "H-L" in the mask file name. Probing polarity will be discussed in the next section of this document.

 

CAN FD eye-diagram mask testing only supports CAN FD buses with a base-rate (arbitration phase) of 500kbps and FD rates (data phase) ranging from 4Mbps up to 10Mbps. So for example, if your CAN FD data rate is 10Mbps, then you should use one the mask files with "500k-10M" in the file name.

 

CAN FD eye-diagram mask testing does not test every bit. It only tests the first 10 bits in the FD data phase of all CAN FD frames. In other words; only frames with a bit-rate switch (BRS) bit. 10 bits covers worst-case resynchronization, which can happen if two consecutive stuff bits occur. This will test for worst-case clock stability and jitter over 10 bit periods. Although not every bit in the FD data phase are tested, overlaying just the first 10 bits of the FD data phase from all frames will show waveform characteristics of received bits from all nodes in the system. So if one of the FD nodes in your system exhibits physical layer problems, such as reduced amplitudes, slow edge speeds, noise, etc., these characteristics will show up in the eye-diagram by testing just the first 10 bits of the FD data phase.

 

Eye-diagram mask testing can also be performed on "classic" CAN 2.0 signals. Refer to Keysight's application note titled, "CAN 2.0 Eye-diagram Mask Testing" listed at the end of this document to learn more about testing CAN 2.0 signals (non-FD).

 

Probing the Differential CAN FD Bus

CAN FD eye-diagram mask testing is based on capturing and overlaying recessive and dominate bits on the differential bus. The differential bus must be probed using a differential active probe. Keysight recommends using the N2818A 200-MHz differential active probe shown in Figure 1. This probe includes a Keysight AutoProbe interface that automatically sets the oscilloscope's input impedance (50-Ω) and attenuation factor (10:1), and also provides power to the active probe.

 

If you need to connect to SubD-DB9 connectors in your system, Keysight also offers the CAN/FlexRay SubD-DB9 probe head (Part number 0960-2926) shown in the inset photo of Figure 1. This optional probe head adapter allows you to easily connect to your CAN, CAN FD, and/or FlexRay differential buses.

 

A differential active probe allows you to view signals on the differential CAN FD bus in either a dominant-bit high or dominant-bit-low format. And CAN FD eye-diagram mask testing can be performed using either polarity of probing. To observe signals as dominant-bit high, connect the "+" input (red lead) of the differential probe to CAN_H and the "-" input (black lead) of the probe to CAN_L. Figure 2 shows a differential CAN FD waveform in the dominant-bit high format.

 

To observe signals in the dominant-bit low format, connect the "+" input (red lead) of the differential probe to CAN_L and the "-" input (black lead) of the probe to CAN_H. Although connecting the differential probe to the bus in this manner may sound backwards and perhaps unintuitive, timing diagrams of CAN FD signals are typically shown in a dominant-bit low format. In this format, bus idle level is always high (recessive). Also, during transmission of CAN FD frames, high-level signals (recessive bits) will always be interpreted as "1s", while low-level signals (dominant bits) will always be interpreted as "0s". Figure 3 shows a differential CAN waveform in the dominant-bit low format. This is the preferred method of probing by most engineers today.

 

Step-by-Step Instructions to Perform a CAN FD Eye-diagram Mask Test

To perform a CAN FD eye-diagram mask test, first turn off all channels of the oscilloscope except for the input channel that is connected to the CAN FD differential bus. If you begin with a Default Setup, only channel-1 will be turned on. Alternatively, you can begin with the oscilloscope already set up and triggering on the differential CAN FD bus. To begin execution of a CAN FD eye-diagram mask test, do the following:

 

  1. Insert your USB memory device (with the appropriate mask file) into the scope's front panel USB port.
  2. Press the [Save/Recall] front panel key; then press the Recall softkey.
  3. Press the Recall: XXXX softkey; then select Mask (*.msk) as the type of file to recall.
  4. Press the Location (or Press to go, or load from) softkey; then navigate to the appropriate mask file based on FD baud rate and probing polarity (L-H = dominant bit low, H-L = dominant bit high).
  5. Press the Press to Recall softkey to begin a CAN FD eye-diagram mask test.

 

When the mask file is recalled, the scope will automatically set itself up (timebase, vertical, and trigger settings) to display overlaid CAN FD bits across the center five divisions of the scope's display. During this special sequencing test, timebase settings and timing cursors cannot be used. To exit a CAN FD eye-diagram mask test, either turn off mask testing or press Clear Mask in the scope's Analyze-Mask menu. When the test is exited, the scope will restore most oscilloscope settings to the state they were in prior to beginning the test. However, triggering is not restored and will remain set to trigger on the trailing edge of bit-rate switch (BRS) bits, which is the beginning of the FD data rate phase, using the scope's Pulse-width trigger mode.

 

Interpreting the Eye

Figure 4 shows a CAN FD eye-diagram mask test based on an arbitration phase baud rate of 500 kbps, FD data phase baud rate of 10Mbps, and with differential probing established to obverse the waveforms in a dominant-bit low format (L-H). This test basically shows if dominant and recessive bits have settled to valid/specified levels prior to receiver sampling, which typically occurs near the 60% sample point during the FD data phase. In other words, the CAN FD eye-diagram shows what the CAN receiver "sees" by synchronizing the scope's acquisition and display timing to the CAN FD receiver's timing. The result is a single measurement that provides insight into the overall signal integrity of the CAN FD physical layer network to show worst-case timing and worst-case vertical amplitude variations.