Chapter 7 CountersNI 6238/6239 User Manual 7-34 ni.comThe filter setting for each input can be configured independently. On powerup, the filters are disabled. Figure 7-29 shows an example of a low-to-hightransition on an input that has its filter set to 125 ns (N = 5).Figure 7-29. Filter ExampleEnabling filters introduces jitter on the input signal. For the 125 ns and6.425 μs filter settings, the jitter is up to 25 ns. On the 2.55 ms setting, thejitter is up to 10.025 μs.When a PFI input is routed directly to RTSI, or a RTSI input is routeddirectly to PFI, the M Series device does not use the filtered version of theinput signal.Refer to the KnowledgeBase document, Digital Filtering with M SeriesandCompactDAQ, for more information about digital filters and counters. Toaccess this KnowledgeBase, go to ni.com/info and enter the info coderddfms.PrescalingPrescaling allows the counter to count a signal that is faster than themaximum timebase of the counter. M Series devices offer 8X and 2Xprescaling on each counter (prescaling can be disabled). Each prescalerconsists of a small, simple counter that counts to eight (or two) and rollsover. This counter can run faster than the larger counters, which simplycount the rollovers of this smaller counter. Thus, the prescaler acts as afrequency divider on the Source and puts out a frequency that is one-eighth(or one-half) of what it is accepting.1 2 31 4 1 2 3 4 5RTSI, PFI, orPXI_STAR TerminalFilter Clock(40 MHz)Filtered InputFiltered input goes highwhen terminal is sampledhigh on five consecutivefilter clocks.
