This guide explores the use of external triggers for use with various WaveForms tools. Note that there are many applications and approaches to using external triggers beyond what will be covered in this guide.
Before discussing any examples, it is important to understand what a trigger is and why a trigger is helpful. The following sections will define triggers as an acquisition tool and describe what triggers do to make signal analysis more effective.
What is a trigger? How is one set up?
Once set up, what does the trigger do to incoming data? How does this help the user?
When the acquisition instrument sees this pulse from the trigger system, it begins to digitize, process, store, and measure this data. Before this waveform data can be displayed on the screen, the time-based interpolator figures out what address in the waveform buffer matches the defined trigger point and tells the acquisition instrument to display this data at time t=0.0 seconds.
By repeatedly displaying similar sections of the input signal at a specified trigger point, a trigger helps display a dynamic waveform as a static image. Otherwise, the waveform would just race across the display in a way that would be hard to interpret and impossible to take measurements with.
Triggers can also be helpful for capturing data around or locating an infrequent event. Some examples include digital pulses that are: missing, irregular in width, or not reaching valid logic high or low levels.
What makes a trigger “external”?
Below are a few examples of applications using an external trigger. These are not the only uses for external triggers but merely a showcase of how an external trigger might be used in practice and to allude to further applications. Two examples are presented:
This is an example of using an analog signal to trigger the oscilloscope. Once set up, when pushed, the button in this example will generate a trigger.
Set up the following hardware as shown in the schematic and diagram to the right. Trig 1 refers to the External Trigger 1 pin on the Digilent Test and Measurement Device. The resistors are for limiting the current into and out of the WaveGen, Scope, and Trigger pins. While a push button was used, its behavior could be emulated by connecting and disconnecting a resistor or jumper wire. Components:
See the dropdowns below for additional figures of the circuit setup, with BNC and MTE cables. Note: If using BNC cables for analog input, refer to the Using the Oscilloscope guide for additional information on setup, including selecting an attenuation factor in the WaveForms application. |
This software setup section is streamlined to focus on the external trigger application. For more detailed help with any of the following WaveForms instruments, refer to their “Using” guides, linked below.
Note: In order to show each of the opened instruments on the screen at once, click on the Docking Windows button () at the top right corner of the window. To return to the default tabbed view later, click on the Tabbed Windows button () adjacent to the Docking Windows button.
Refer to the image to the right to see what the workspace will look like after setup is complete.
Once the hardware and software are set up properly, pushing down on the pushbutton will send 3.3V to the External Trigger 1 pin, generating a trigger for acquisition. The trigger point should change each time the button is pressed since no condition is set to be displayed at t=0.0 seconds and the WaveGen is constantly running. The sine wave should be displayed statically in the Scope plot plane and update with every push of the button.
This is an example of using a digital signal to trigger the logic analyzer. A clock signal from a DIO pin is used to periodically generate a trigger.
The hardware used in this example is a simple loopback circuit. Digital input/output pins will be used to stimulate the external trigger and to provide some data to be read.
Connect the External Trigger 1 to the DIO0 pin.
DIO1 will also be used, but since Digilent Test and Measurement devices support capturing the state of a digital output pin as if it was a digital input, it will not be connected to anything.
These connections are detailed in the diagram to the right.
The following software setup is streamlined to focus on the External Trigger application. For more detailed help with the following instruments, refer to their “Using” guides, linked below.
Note: In order to show each of the opened instruments on the screen at once, click on the Docking Windows button () at the top right corner of the window. To return to the default tabbed view later, click on the Tabbed Windows button () adjacent to the Docking Windows button.
Refer the image to the right to see what the workspace will look like after setup is complete.
Viewing the Logic instrument, the Patterns instrument is generating a clock signal with a two-second period on DIO0 which is being sent to the External Trigger 1 pin. When DIO0 is driven to a logic high level, a trigger is generated.
The Patterns instrument is generating a random logic signal at a rate of 10kHz on DIO1 which is being observed in the Logic instrument on the respective DIO1 signal line. DIO1 should update every two seconds in the Logic instrument with every rising edge of the clock signal from DIO0 being sent to the External Trigger 1 pin.
Note: This is not a manually operated trigger whereas the previous example is.
More examples of External Triggering being used are covered in the Using Cross Triggering guide
For more guides on how to use the Digilent Test and Measurement Device, return to the device's Resource Center, linked from the Test and Measurement (Redirect) page of this wiki.
For more information on WaveForms visit the WaveForms Reference Manual.
For technical support, please visit the Test and Measurement section of the Digilent Forums.