Digital Filtering Considerations for TIO-Based Devices

There are two methods for filtering and synchronizing digital signals. One method is to synchronize the input signal to the maximum onboard timebase on the device. To do this, set Digital Synchronization Enable to true.

The other method is to pass the input of any PFI line through a digital debouncing filter. Each PFI line can independently select from four fixed values (5 µs, 1 µs, 500 ns, 100 ns) and one custom filter value. The custom filter value must be the same for each PFI line. That is, if you choose a filter value of 2 µs for a PFI line, other PFI lines on the device at the same time can only choose from the four fixed values and the 2 µs value selected as the custom filter value. For each counter input property, there are four attributes/properties associated with digital debounce filtering: Digital Filter Enable, Digital Filter Minimum Pulse Width, Digital Filter Timebase Source, and Digital Filter Timebase Rate.

When you set the Digital Filter Enable to true, you must also configure the Digital Filter Minimum Pulse Width attribute/property. This value represents the minimum value that is guaranteed to pass into the TIO. The minimum pulse width guaranteed to be blocked is one-half of the Digital Filter Minimum Pulse Width attribute/property. When you select a custom filter value with the Minimum Pulse Width attribute/property, NI-DAQmx uses an internal 32-bit utility counter to generate the desired filter value. If you would like to generate the filter clock using your own external signal, you can use the Digital Filter Timebase Source and Digital Filter Timebase Rate attributes/properties. You must configure both to use an external signal as the source for the digital filter.

You cannot set both Digital Filter Enable and Digital Synchronization Enable to true at the same time. You can use only one of these digital filtering methods at a time.

The following table lists the counter input terminals that can be digitally filtered.