Chapter 7 Counters© National Instruments Corporation 7-35 NI 6232/6233 User Manualprescaling 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.Figure 7-30. PrescalingPrescaling is intended to be used for frequency measurement where themeasurement is made on a continuous, repetitive signal. The prescalingcounter cannot be read; therefore, you cannot determine how many edgeshave occurred since the previous rollover. Prescaling can be used for eventcounting provided it is acceptable to have an error of up to seven (or one).Prescaling can be used when the counter Source is an external signal.Prescaling 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) ensures that acounter returns correct data in applications that use a slow or non-periodicexternal source. Duplicate count prevention applies only to bufferedcounter applications such as measuring frequency or period. In suchbuffered applications, the counter should store the number of times anexternal Source pulses between rising edges on the Gate signal.Duplicate Count Prevention ExampleFigure 7-31 shows an external buffered signal as the period measurementSource.External SignalCounter ValuePrescaler Rollover(Used as Sourceby Counter)0 1
