National Instruments DAQ M Series - Manual (Page 96)

Chapter 7 Counters
NI 6236 User Manual 7-34 ni.com
Prescaling is intended to be used for frequency measurement where the
measurement is made on a continuous, repetitive signal. The prescaling
counter cannot be read; therefore, you cannot determine how many edges
have occurred since the previous rollover. Prescaling can be used for event
counting 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 internal
timebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
Duplicate Count Prevention
Duplicate count prevention (or synchronous counting mode) 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 7-31 shows an external buffered signal as the period measurement
Source.
Figure 7-31. Duplicate Count Prevention Example
On the first rising edge of the Gate, the current count of 7 is stored. On the
next rising edge of the Gate, the counter stores a 2 since two Source pulses
occurred after the previous rising edge of Gate.
Gate
Source
Counter Value
Buffer 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.