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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 inverse assembly failures.
D Ensure that the preprocessor configuration switches are correctly set
for the measurement you are trying
to
make.
Some preprocessors include configuration switches for various features (for
example, to allow dequeueing of the trace list). See Chapter 1 for information
about setting configuration switches.
D Do a full reset of the target system before beginning the measurement.
Some preprocessor designs require a
full
reset to ensure correct
configuration.
D 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.
D Ensure that you have sufficient cooling for the microprocessor.
Microprocessors such as the i486, Pentium™, and MC68040 generate
substantial heat. This is exacerbated by the active circuitry on the
preprocessor board. You should ensure that you have ambient temperature
conditions and airflow that meet or exceed the requirements of the
microprocessor manufacturer.
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.
A-6
E2465A PowerPC 604 PGA Preprocessor Interface
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