Chapter 9

Counters

External Signal

Prescaler Rollover

(Used as Source

by Counter)

Counter Value

0

1

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-621xdevices, 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.

© National Instruments Corporation

9-35

NI USB-621x User Manual