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Next, you go to the State Histogram menu and enter the names and
boundaries of the five routines in the state histogram ranges.
State Histogram then displays the relative activity of the five routines. After
several acquisitions, it is apparent that the interrupt routine is being accessed
more often than expected.
The figure below shows the State Histogram display for this example.
SPA State Histogram
You now go to the Time Interval menu and enter the Start and End
conditions for the suspect interrupt routine. Before altering the default Time
Interval ranges, you start the acquisition and observe the Maximum (Max),
Minimum (Min), and Average (Avg) times. From these values, the typical
execution times of the interrupt are apparent, and provide good starting
values for the Time Interval ranges using Auto-range. From the Max time, it
is apparent that the interrupt routine is having problems.
Running the acquisition again, you discover that the interrupt usually takes
the expected 8 microseconds, but occasionally it takes as long as
8 milliseconds. After experimenting with the target system while monitoring
the interrupt with Time Interval mode, a faulty key on the keyboard is
discovered. The key is bouncing excessively, resulting in an extended
interrupt routine call.
The figure on the next page shows the Time Interval display for this example.
The SPA Menu
SPA Measurement Processes
11-19