Chapter 7 Counters© National Instruments Corporation 7-33 NI 6238/6239 User ManualOther Counter FeaturesCascading CountersYou can internally route the Counter n Internal Output and Counter n TCsignal of each counter to the Gate inputs of the other counter. By cascadingtwo counters together, you can effectively create a 64-bit counter. Bycascading counters, you also can enable other applications. For example, toimprove the accuracy of frequency measurements, use reciprocal frequencymeasurement, as described in the Method 3—Measure Large Range ofFrequencies Using Two Counters section.Counter FiltersYou can enable a programmable debouncing filter on each PFI, RTSI, orPXI_STAR signal. When the filters are enabled, your device samples theinput on each rising edge of a filter clock. M Series devices use an onboardoscillator to generate the filter clock with a 40 MHz frequency.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 inputterminal then changes from low-to-high, but glitches several times. Whenthe filter clock has sampled the signal high on N consecutive edges, thelow-to-high transition is propagated to the rest of the circuit. The value ofN depends on the filter setting; refer to Table 7-5.Table 7-5. FiltersFilter SettingN (FilterClocks Neededto Pass Signal)Pulse WidthGuaranteed toPass FilterPulse WidthGuaranteed toNot Pass Filter125 ns 5 125 ns 100 ns6.425 μs 257 6.425 μs 6.400 μs2.55 ms ~101,800 2.55 ms 2.54 msDisabled — — —
