Develop and troubleshoot hardware and software problems using a digital oscilloscope

Triggering is what really makes an oscilloscope useful. Triggering synchronizes the oscilloscope's display with the input signal and provides the user with a stable waveform display by starting the display at a consistent point on the waveform. Oscilloscopes have had triggering capabilities almost from the beginning, but digital oscilloscope technology takes triggering to the next level. This enhanced functionality makes the oscilloscope even more useful.

The most basic oscilloscope triggering function is to trigger on a specific signal level and slope. That is, the oscilloscope starts displaying the input signal when the signal reaches a certain voltage and the voltage either increases (positive slope) or decreases (negative slope). By starting the waveform display at the same point, the oscilloscope will display a stable waveform, especially if the waveform is periodic (Figure 1).

Figure 1: Setting the trigger point to a specific value and slope stabilizes the waveform display. Additionally, setting the display to show the trigger point in the middle of the screen allows you to see what happens before and after the trigger. (Image source: Rohde & Schwarz)

Digital oscilloscopes also have this basic triggering capability, but give the user more options than just selecting voltage levels and slopes. This enables users to address a wider range of issues and troubleshoot faster and more efficiently than using an analog oscilloscope.

For example, with a digital oscilloscope, you can actually see what happens before the trigger point. This is not possible with an analog oscilloscope because the oscilloscope only displays the waveform after the trigger point is reached. Everything that happened before that point is lost. However, a digital oscilloscope continuously digitizes the input signal and records it in memory, including what happened before the trigger. Typically, all you have to do is scroll back on the display to see where the input signal was before it reached the trigger point.

In addition to this, digital oscilloscopes have many triggering capabilities that analog oscilloscopes do not have because analog oscilloscopes cannot truly analyze the input signal. Many digital oscilloscopes have a range of pulse triggering capabilities. For example, many scopes will allow you to trigger:

The pulse is longer than the programmed reference time.

A pulse shorter than the reference time.

The pulse is equal to a specific reference time (plus or minus a small tolerance).

Pulses that are not equal to a specific reference time (plus or minus a small tolerance).

Pulses with a pulse width between two reference times or pulses with a pulse width outside of these reference times.

Another type of pulse trigger is the "runt" trigger. When this trigger is programmed, the oscilloscope will trigger when the pulse begins to go HIGH or LOW but fails to reach a valid HIGH or LOW level (Figure 2).

Figure 2: Runt triggering allows you to find pulses that do not quite reach the true HIGH value. (Image source: Rohde & Schwarz)

Digital oscilloscopes can also trigger on logic patterns. This is especially useful if you have four or more input channels, but even a good value oscilloscope can be expanded with additional logic inputs. Oscilloscopes can be programmed to trigger on specific logic patterns or channel combinations. For example, you might want to trigger when a set of numeric inputs are all TRUE, so you could set the trigger to AND all those inputs. Likewise, you may want to trigger when at least one input is TRUE, so you can set the trigger to OR the inputs. Some scopes also allow you to apply time limits to these triggers. So, as with pulse triggering, you can set up your oscilloscope to trigger only when a specific logic pattern occurs longer or shorter than a specific time.

Many digital oscilloscopes now offer the ability to analyze serial protocols such as I 2 C, SPI, or RS232/UARTS. Not only can these oscilloscopes decode bus traffic, they can also trigger on specific bus events or data. For example, when troubleshooting the I 2 C bus, you can trigger on:

A frame begins.

One frame stops.

A frame to start over.

Frame not acknowledged.

Various read/write conditions, such as when the bus master starts a transfer.

Specific data is being sent over the bus.

All of these triggers are valuable for debugging hardware and software problems. When it comes to digital oscilloscopes, "trigger happiness" is a good thing. Learning how to use these features can really help you up your game when using an oscilloscope and developing and troubleshooting hardware and software problems.