Keysight's InfiniiVision X-Series Oscilloscopes offer a broad range of application specific
licensed options to help you debug your automotive serial buses more efficiently, such as, CAN,
CAN FD, LIN, and FlexRay.
In this short video, I'll give the demonstration of performing an eye-diagram mask test on
the CAN FD bus.
Eye-diagram mask testing is an important test for many of today's higher speed buses such
as the new CAN FD bus.
So lets get started: here we show an InfiniiVision X-Series Oscilloscope triggering on and decoding
a differential CAN FD bus.
This bus has a base rate of 500 kilobits per second, and an FD rate of 10 megabits per
second.
Now we can clearly see the FD fields here where you see the speed really kick up.
Now we can zoom in on one particular frame and see it closer.
Or we can even zoom in closer to see just the empty bits where they begin to take off.
Now this right here is the bit rate switch bit and all the bits following it until you
get to the CRC field are the FD bits.
In this view of the waveform we can see our amplitudes look pretty solid - not a lot of
noise or ringing.
But it's much more difficult to get information about the timing: are these edges in the right
location?
And that's where a CAN FD eye diagram is really going to help us out.
So let's go ahead and perform an eye diagram mask test.
First, I'll insert my USB memory device that has stored copies of CAN FD mask files that
I downloaded from Keysight's website.
Then I'll recall the appropriate mask file.
There are a variety of mask files, some specifically for dominant bit low probing, some for dominant
bit high probing, and various FD baud rates.
I'm probing dominant bit low and I have a 10 megabit per second FD rate, so I'm going
to select that file.
When you recall the file the scope will automatically set itself up vertically, horizontally and
triggering, and overlay all dominant and recessive bits in what looks like an eye.
This is called an eye diagram.
Now, CAN FD eye diagram mask testing tests just the first ten bits in the FD data phase,
but it tested from all CAN FD frames from all nodes.
Ten bits covers the worst-case re-synchronization time which can happen if two consecutive stop
bits occur.
This will test for worst-case clock stability, and jitter over ten bit periods.
Overlaying just the first ten bits of the FD data phase of all frames will show waveform
characteristics of all FD bits from all nodes in the system.
Let's now take a closer look at the mask itself.
The mask defines a fail zone, which is shaded in gray here.
This is the area on the scope's display where signals should not enter for this pass-fail
eye test.
The mask is based on a six point polygon, which is common for many of today's serial
bus standards.
The upper boundary is set at -.5 V. The lower boundary at -.9V.
Now this is for if you're probing to view dominant bit low.
If I was probing to view dominant bit high and I recalled that particular mask file,
it would be +.5 to +.9.
The left point on the mask is set at 30% of a bit time and the right point on the mask
is set at 80% of the bit time.
A bit time in this case, since it's 10 megabits per second, is 100 nanoseconds.
Now, the sample point on most FD buses is typically somewhere above 50%.
Where my mouse is pointing now is approximately 60%.
Sometimes it's as high as 75%, which is at the apex of the polygon mask in that corner
right there.
So that is why the mask is shifted to the right rather than right in the center of the
screen.
Let's now zoom back out and interpret what we're seeing here.
In this particular CAN FD eye diagram mask test we can see that one of the system's nodes
generates a recessive bit which is high in this case, and it rings and it dips down into
our mask and violates it.
So you can see where it's highlighted in red indicating failures.
This test reveals that there may be a risk at this dip in the differential signal from
that particular node could be interpreted in other CAN FD transceivers as a dominant
bit.
We can also see that this CAN FD system generates approximately 20-25 nanoseconds of timing
uncertainty or jitter.
And if we look at the statistics here we have a failure rate of about 1.7% after testing
over 750,000 CAN FD bits.
Now watch what happens if we add noise to the signal, which might be more typical of
a real automotive system.
You can see the eye is beginning to close, which means that we have a smaller valid data
window, and it even begins with this ringing and dip.
It begins to look like the eye is winking at us.
Now to exit a CAN FD eye diagram mask test, all you have to do is press "clear mask" and
it goes back to the sequential timing mode that we were looking at before.
To learn more about Keysight's IInfiniiVision X-Series oscilloscopes and how they might
be able to help you test and debug your automotive designs, contact a Keysight authorized distributor,
and ask for a demonstration, thank you.