Oscilloscope Basics: Trigger Holdoff

As discussed in an earlier post, triggering is the means by which we can coax an oscilloscope into showing us what we're looking for in an input signal, and indeed even simply to display it in a stable fashion. Two of the most basic triggering types are edge triggers and pattern triggers. The latter applies to mixed-signal instruments, allowing users to trigger on a logical combination of analog and digital inputs.

A useful feature found in most modern digital oscilloscopes is trigger holdoff, a condition that can be added to edge or pattern triggers. Trigger holdoff enables the user to disable the trigger temporarily, even if the trigger conditions are met, until the holdoff conditions are also met. The trigger fires when the holdoff elapses.

One can also think of trigger holdoff as a function used when there are multiple trigger events in a given acquisition, but you're only interested in certain ones. Rather, it lets users ignore those uninteresting trigger events and stabilize the display as though there were only one trigger event per acquisition.

Now, there are two criteria for trigger holdoff: time and events. Holdoff by time is when you want to wait for some specific period of time to fire the trigger, either since the beginning of the acquisition or since the trigger conditions were met.

Figure 1: Edge trigger with holdoff by time

Sometimes you can achieve a stable display of complex, repetitive waveforms by placing a holdoff condition on the time between each successive edge-trigger event. This time would otherwise be limited only by the input signal, the coupling, and the instrument's bandwidth. Select a positive or negative slope, and a minimum time between triggers.

In Figure 1, the bold edges on the trigger source indicate that a positive slope has been selected. The broken upward-pointing arrows indicate potential triggers, which would occur if other conditions are met. The bold arrows indicate where the triggers actually occur when the holdoff time has been exceeded.

Figure 2: Edge trigger with holdoff by events

For purposes of trigger holdoff, events refers to the number of times the trigger conditions have been met, counted either from the beginning of the acquisition or since the last trigger. For example, if the hold-off number of events is two (counted from the beginning of the acquisition), the trigger fires on the third event. In Figure 2, the bold edges on the trigger source indicate that a positive slope has been selected. The broken, upward-pointing arrows indicate potential triggers, while the bold ones show where triggers actually occur after the holdoff expires.

In general, holdoff can be used in several different applications:

When there are multiple trigger events in a signal.

When triggering on the higher frequency of two harmonically related signals (e.g. output of a PLL frequency multiplier while looking at both the output and input).

Triggering at a fixed time delay. (e.g. trigger the oscilloscope every 10 seconds for a data-logging application).

Triggering on the clock repetition of a pseudo-random process with a known, fixed number of states.

One can envision many applications in which trigger holdoff might be extremely useful, such as debugging the read channel of a disk drive or memory, where only, say, the pulses initiating a specific sector is of interest and you know how far apart in time such pulses occur.