National Instruments NI USB-621x - Manual (Page 115)

Chapter 9 Counters
© National Instruments Corporation 9-35 NI USB-621x User Manual
Figure 9-31. Prescaling
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) 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.
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1