User Guide

Other Counter Features

Cascading Counters

You can internally route the Counter n Internal Output and Counter n TC

signals of each counter to the Gate inputs of the other counter. By cascading

two counters together, you can effectively create a 64-bit counter. By

cascading counters, you also can enable other applications. For example,

to improve the accuracy of frequency measurements, use reciprocal

frequency measurement, as described in the Method 3—Measure Large

Range of Frequencies Using Two Counters section.

Counter Filters

You can enable a programmable debouncing filter on each PFI signal.

When the filters are enabled, the NI cDAQ-9172 chassis samples the input

on each rising edge of a filter clock. The NI cDAQ-9172 chassis uses an

onboard oscillator to generate the filter clock with a 40 MHz frequency.

For more information, refer to the NI-DAQmx Help. The NI-DAQmx Help

is available after installation from Start»All Programs»National

Instruments»NI-DAQ»NI-DAQmx Help.

Note NI-DAQmx only supports filters on counter inputs.

The following is an example of low to high transitions of the input signal.

High to low transitions work similarly.

Assume that an input terminal has been low for a long time. The input

terminal then changes from low to high, but glitches several times. When

the filter clock samples the signal high on N consecutive edges, the low to

high transition is propagated to the rest of the circuit. The value of N

depends on the filter setting, as listed in Table 9.

Table 9. Counter Input Filters

Filter Setting

N (Filter Clocks

Needed to

Pass Signal)

Pulse Width

Guaranteed to

Pass Filter

Pulse Width

Guaranteed to

Not Pass Filter

125 ns 5 125 ns 100 ns

6.425 µs 257 6.425 µs 6.400 µs

2.56 ms ~101,800 2.56 ms 2.54 ms

Disabled — — —