Brochure

Backgrounder
The GREATER THAN trigger can help find “stuck bits” and other signals that fail to return to
their default state after some transaction. This setting causes the oscilloscope to trigger itself if a
trailing edge fails to occur within a specified time. For example, a bus data output signal may
switch to a “1” in response to an Output Enable active, and not transition to a new state
thereafter. This could be caused by the Output Enable signal itself being incorrect; by the
transition time to tri-state being too long on the part being driven; or by the next value to be put
on the data bus not being activated. The GREATER THAN trigger detects this error, again
revealing contributing circumstances on all connected oscilloscope channels. With some
probing, it should be possible to detect which of the above conditions actually created the
difficulty. The time range here, as with the other pulse width trigger settings, extends from the
tens of nanoseconds to multiple seconds. This provides ample time to ensure that the signal is
truly “stuck” and not just delayed.
The EQUAL TO trigger offers an alternative to voltage threshold triggering approaches when the
available trigger signals (such as Output Enables) are plagued by transients or noise, causing
false triggering in the oscilloscope.
The basic embedded microprocessor illustrates this situation. Most of these devices include an
external bus that allows for expansion of the processor’s self-contained memory or peripheral
interfaces. Often this bus allows peripherals to control the timing of their own transactions with
the processor. The processor issues an address, and then sends out an address strobe. The
affected peripheral eventually issues an “acknowledge” signal to confirm receipt of the
processor’s command. This process takes a known (and often user definable) number of delay
cycles often unique to the specific peripheral or set of peripherals.
Setting the pulse width to = 941 ns enables you to capture signals with widths equal to 941 ns
wide on channel 1 using the TDS2000 Series.
The knowledge of this delay time is the key to isolating any peripheral and examining the activity
on various test points when it responds. The solution is simple: use the Address Strobe as the
trigger line and set the pulse width trigger time EQUAL TO the number of clock delays
associated with the specific peripheral. The leading edge of the Address Strobe will initiate the
time countdown; then the trigger circuit will wait for the preset amount of time (say, three cycles’
worth of time expressed in nanoseconds); then the oscilloscope will trigger and capture the
activity on the test points. By definition, this is when the peripheral will be active on the bus.
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