National Instruments Data Acquisition Device NI USB-621x User Manual

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National Instruments Data Acquisition Device NI USB-621x User Manual Manual pdf 128 page image

Chapter 8 Counters© National Instruments Corporation 8-35 NI USB-621x User ManualPrescaling is not available if the counter Source is one of the internaltimebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).Duplicate Count PreventionDuplicate count prevention (or synchronous counting mode) is used whentaking frequency or period measurements on USB-621x devices, whichensures that a counter returns correct data in applications that use a slow ornon-periodic external source. Duplicate count prevention applies only tobuffered counter applications, such as measuring frequency or period. Insuch buffered applications, the counter should store the number of times anexternal Source pulses between rising edges on the Gate signal.Example Application That Works Correctly (NoDuplicate Counting)Figure 8-31 shows an external buffered signal as the period measurementSource.Figure 8-31. Duplicate Count Prevention ExampleOn the first rising edge of Gate, the current count of 7 is stored. On the nextrising edge of Gate, the counter stores a 2 since two Source pulses occurredafter the previous rising edge of Gate.The counter synchronizes or samples the Gate signal with the Sourcesignal, so the counter does not detect a rising edge in Gate until the nextSource pulse. In this example, the counter stores the values in the buffer onGate2776 7 1 2 1Rising Edgeof GateCounter detects rising edgeof Gate on the next risingedge of Source.SourceCounter ValueBuffer