Erratic Trace Measurements; Capacitive Loading - HP E2466C User Manual

If You Have a Problem

Erratic trace measurements

Erratic trace measurements
There are several general problems that can cause erratic variations in trace
lists and transaction tracker failures.
Ensure that the preprocessor operating mode switches are correctly set
for the measurement you are trying to make.
Do a full reset of the target system before beginning the measurement.
Some preprocessor designs require a full reset to ensure correct
configuration.
Ensure that your target system meets the timing requirements of the
processor with the preprocessor probe installed.
See "Capacitive Loading" in this chapter. While preprocessor loading is slight,
pin protectors, extenders, and adapters may increase it to unacceptable
levels. If the target system design has close timing margins, such loading may
cause incorrect processor functioning and give erratic trace results.
Ensure that you have sufficient cooling for the microprocessor.

Capacitive loading

Excessive capacitive loading can degrade signals, resulting in incorrect
capture by the preprocessor interface, or system lockup in the
microprocessor. All preprocessor interfaces add additional capacitive loading,
as can custom probe fixtures you design for your application.
Careful layout of your target system can minimize loading problems and
result in better margins for your design. This is especially important for
systems that are running at frequencies greater than 50 MHz, or that have
fast edge speeds.
A–6 Preprocessor Interface for the Pentium II Processor