Teledyne LeCroy Serial Data Debug Solutions - Manual (Page 37)

Operator's Manual
919586 RevA 37
Isolating and Analyzing Serial Bus Activity
I SOLATING AND A NALYZING S ERIAL B US A CTIVITY O VERVIEW
The combination of Serial Data Triggering, Decoding, and normal oscilloscope features is a powerful mixture of
tools that can make it very easy to find latent Serial Data hardware or software problems in your circuit. No
longer is the oscilloscope a tool just for the hardware engineer. Now, software engineers can also easily visualize
the Serial Data signals and relate them to programming code and operation. Decode and Trigger toolsets allow
hardware and software Engineers to speak the same language when it comes to system debugging and
performance checking.
For more detailed explanations of Decode and Trigger toolsets refer to Technical Explanation of Serial Decode
and Trigger (on page 12). Some common Serial Data analysis needs and methods are discussed in the following
topics.
C APTURING L ONG P RE -T RIGGER T IMES
LeCroy oscilloscopes are available with optional, very long acquisition memory. When decoding low speed (~1
Mb/s) serial data signals, it is possible to set the sample rate to a minimum value (such as 5 MS/s) and capture
and decode seconds of serial data traffic, even with standard 10 Mpt/ch memories. When decoding high speed
(Gb/s) serial data signals, higher sample rates are needed, which reduces capture time, but many LeCroy
oscilloscopes suitable for these serial data rate signals have optional very long acquisition memories that can be
used to achieve long capture times.
If necessary the capture can be 100% pre-trigger, 100% post-trigger, or something in between.
1. First, adjust Pre-Trigger and Post-Trigger time using the Delay knob on the oscilloscope’s front panel.
2. Now, optimize your Sample Rate or Memory Length by accessing the Horizontal Dialog on your
oscilloscope and selecting either Set Maximum Memory mode or Fixed Sample Rate mode.

If you choose to Set Maximum Memory, you can decrease the memory usage so you do not
sample at too high a sample rate (too high a sample rate slows down the decoding algorithm).
Then, adjust your timebase setting to a length sufficient to capture the event.
Note: Make sure your timebase setting and memory length combined do not result in too low a
sample rate. Otherwise, adequate capture and decode is not performed.

More commonly, you will probably choose to fix the sample rate to a specific value providing the
necessary oversampling required to capture your Serial Data messages (at least 4X the bit rate).
Also, it affords a high enough sample rate to capture transients you may want to see on Serial Data
and analog signals (at least 2X the frequency of any expected transients, preferably 10X).
Note: Reference your oscilloscope’s online help for more information about the core oscilloscope settings
mentioned earlier in this topic.
R EPEATEDLY T RIGGERING AND S AVING THE D ATA TO A H ARD D RIVE
You may wish to set up your oscilloscope to capture a short or long memory acquisition for a certain trigger
condition, and then save data to a hard drive or memory stick whenever the trigger condition is met.
Note: While this can be easily done in most LeCroy oscilloscopes, realize there is significant trigger dead time
while using this method. Minimize dead time by using the method described in the following Repeated
Triggering and Storing all Triggers (Sequence Mode) topic.
Repeatedly trigger and save the data to a hard drive by doing the following:
1. First, set up your desired serial data (or other) trigger condition (see corresponding protocol sections
previously covered in this documentation for details).