Triggering
TDS5000 Series User Manual
3-65
Setup/Hold Trigger. A setup/hold trigger occurs when a logic input change s state
inside of the setup and hold times rela tive to the clock. When you use setup/hold
trigger ing, you define:
H The channel containing the logic input (the data source) and the channel
containing the clock (the clock source)
H The direction of the clock edge to use
H The clocking level and data level that the oscilloscope uses to determine if a
clock or data transition has occurred
H The setup and hold times that togethe r define a time range rela tive to the clock
Data that changes state within the setup/hold violation zone triggers the
oscilloscope. Figure 3--24 shows how the setup and hold times you choose
positions this zone relative to the clock.
Setup/hold triggering uses the setup/hold violation zone to detect when data is
unstable too near the time it is clocked. Each time trigger holdoff ends, the
oscilloscope monitors the data and clock sources. When a clock edge occurs, the
oscilloscope checks the data stream it is processing (from the data source) for
transitions occurring within the setup/hold violation zone. If any occur, the
oscilloscope triggers with the trigger point located at the clock edge.
Positive settings for both setup and hold times (the most common application)
locate the setup/hold violation zone so it spans the clocking edge. (See the top
waveform in Figure 3--24.) The oscilloscope detects and triggers on data that
does not become stable long enough before the clock (setup time violation) or
that does not stay stable long enough after the clock (hold time violation).
Negative settings for setup or hold times skew the setup/hold violation zone to
locate it before or after the clocking edge. (See the bottom and center waveforms
of Figure 3 --24.) The oscilloscope can then detect and trigger on violations of a
time range that occurs before or one that occurs after the clock.
NOTE. Keep the hold-time setting to no more than 2.5 ns less than one-half the
clock period (hold time ≤ (period/2) -- 2.5 ns) or the oscilloscope cannot trigger
(this assumes a 50% duty cycle clock).
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