National Instruments NI 6602 User Manual

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National Instruments NI 6602 User Manual Manual pdf 13 page image

Chapter 2 Device OverviewNI 660x User Manual 2-2 ni.comFigure 2-1 shows an example of prescaling.Figure 2-1. Prescaling ExamplePrescaling is intended for use with two counter period and frequencymeasurements where the measurement is made on a continuous, repetitivesignal. The prescaling counter cannot be read, so you cannot determine howmany edges have occurred since the previous roll-over. You can also useprescaling for counting edges if it is acceptable to have an error of up toseven when using 8X prescaling or one when using 2X prescaling.Pad SynchronizationThe NI 660x devices allow synchronization of their PFI lines and RTSIlines at the I/O pads. This is called pad synchronization in this document,and digital synchronization in the NI-DAQmx API. You cannot use digitalfiltering while enabling this feature.Pad synchronization is useful when several counters are measuring oroperating off the same external signal. For example, supposecounters 0 and 1 are configured for triggered pulse generation and eachcounter uses the same external trigger (this external signal is connected toPFI 38 on the I/O connector and both counters have PFI 38 selected as theirGATE). After the trigger signal propagates through the I/O pad of theASIC, the time for the signal to reach the GATE of each counter within theASIC may differ by a few nanoseconds.This signal is sampled at the counters’ GATEs using the selected SOURCE.Because of different propagation times for the paths to the two GATEs,it is possible for the counters to detect the trigger on different edges onSOURCE. Thus, one counter could see the trigger one SOURCE periodafter the other. If you want to allow the counters to see the changes in thesignal at the same instance, you should use pad synchronization. Duringpad synchronization, the signal is offset by one clock cycle.External SignalCounter ValuePrescaler Rollover(Used as Sourceby Counter)0 1