Glitch Detection - HP 54620A User's And Service Manual

Ensuring Accurate Measurements

Glitch Detection

Glitch Detection
In digital system design, a glitch is an unintentional or unexpected signal
transition, which may or may not pass through the logic threshold. The
HP 54620A/C Logic Analyzer provides support for capturing glitches during
acquisition. However, because the analyzer cannot determine whether the
transition was valid, it defines the concept of a glitch differently. The
analyzer considers a glitch to be any set of two or more edges that pass
through the logic threshold and fall between logic analyzer samples.
Why Glitch Capture?
When the analyzer is sampling at its maximum rate (2-ns sample period), all
pulses within the bandwidth of the probes will be captured by the analyzer.
As the sweep speed is decreased, the sample period is increased to make best
use of acquisition memory. (See "Time base and Acquisition" in this chapter.)
The longer sample period increases the probability that a pulse will fall
between samples, and will therefore be missed.
To prevent missing these pulses, the analyzer automatically enables glitch
capture once the Time/Div setting is 1 µs/div or slower. Auto Glitch detection
is disabled for time base settings where the sampling interval is 4 ns or faster.
At these sweep speeds, the analyzer can reliably sample all signals that are
within the bandwidth of its probing system, thus preventing aliases. In glitch
capture, the analyzer uses memory resources to record an event where at
least two transitions occurred between sample periods. (Those transitions
must pass through the logic threshold.) Thus, because of the glitch capture
circuitry, the analyzer can capture pulses as narrow as 3.5 ns.
Glitch Display
Glitches displayed by the analyzer fall into one of four categories, depending
on the relationship between input waveform events and acquisition samples.
Figure 46 shows input waveforms and the resulting display for each of these
categories.
In category 1, the pulse transitions high and then low between two samples.
The reconstructed waveform simply shows this pulse as a glitch. In category
2, the pulse transitions high, low, then back to high again, and is still high
when the next sample occurs. The reconstructed waveform shows the
transition as a disjoint line, indicating that a glitch and some other transition
occurred some time before the sample. Categories 3 and 4 are simply the
inversions of categories 1 and 2.
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