Change Sampling Rate When Using LabVIEW FPGA I/O Node

Issue Details

I have used an I/O Node in LabVIEW FPGA application that is acquiring data at it's maximum sampling rate.

The code above demonstrates how acquisition loop is set up. The loop runs at approximately the maximum rate the FPGA clock allows the loop to run.

How can I change the frequency of this acquisition to a predetermined value?

Solution

To define the sampling rate of a specific device in LabVIEW FPGA, the maximum sampling rate of the device needs to be considered, which can be found in the module specifications. This maximum rate cannot be exceeded and LabVIEW will not be able to acquire the data any faster.

Most of the NI C Series modules can use the following software architecture for setting predetermined sampling rates in the FPGA code. However, there are some modules which would require a different architecture, for example the Sound and Vibration modules. 

Note: This image is a LabVIEW snippet, which includes LabVIEW code that you can reuse in your project. To use a snippet, right-click the image, save it to your computer, and drag the file onto your LabVIEW block diagram.

The code above shows the minimum functions required to change the sampling rate of the module accessing its I/O with the FPGA I/O Node. The Loop Timer VI is used to define the time between each loop iteration, which is equal to the acquisition period. It can be calculated using the following equation:

T = 1/f, where T is the period, and f is acquisition frequency

The sequence structure is used to prevent the FPGA I/O Node from executing before the Loop Timer VI, thus enforcing the timing.

Additional Information

There are some specific modules that can reach faster sampling rates by using different functions. For more information, please refer to Why Does User Controlled I/O Sampling Acquire Faster than an I/O Node?