National Instruments Data Acquisition Device NI USB-621x - Manual (Page 128)

Chapter 8 Counters
© National Instruments Corporation 8-35 NI USB-621x User Manual
Prescaling is not available if the counter Source is one of the internal
timebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
Duplicate Count Prevention
Duplicate count prevention (or synchronous counting mode) is used when
taking frequency or period measurements on USB-621x devices, which
ensures that a counter returns correct data in applications that use a slow or
non-periodic external source. Duplicate count prevention applies only to
buffered counter applications, such as measuring frequency or period. In
such buffered applications, the counter should store the number of times an
external Source pulses between rising edges on the Gate signal.
Example Application That Works Correctly (No
Duplicate Counting)
Figure 8-31 shows an external buffered signal as the period measurement
Source.
Figure 8-31. Duplicate Count Prevention Example
On the first rising edge of Gate, the current count of 7 is stored. On the next
rising edge of Gate, the counter stores a 2 since two Source pulses occurred
after the previous rising edge of Gate.
The counter synchronizes or samples the Gate signal with the Source
signal, so the counter does not detect a rising edge in Gate until the next
Source pulse. In this example, the counter stores the values in the buffer on
Gate
2
7 7
6 7 1 2 1
Rising Edge
of Gate
Counter detects rising edge
of Gate on the next rising
edge of Source.
Source
Counter Value
Buffer