National Instruments USB-621 Series, User Manual

Page: 135 / 205

Chapter 8 Counters
© National Instruments Corporation 8-37 NI USB-621x User Manual
Prescaling is not available if the co u nter So u rce is one of the internal
timebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
Duplicate Count Prevention
D u plicate co u nt prevention (or synchrono u s co u nting mode) is u sed when
taking freq u ency or period meas u rements on USB-621 x devices, which
ens u res that a co u nter ret u rns correct data in applications that u se a slow or
non-periodic external so u rce. D u plicate co u nt prevention applies only to
b u ffered co u nter applications, s u ch as meas u ring freq u ency or period. In
s u ch b u ffered applications, the co u nter sho u ld store the n u mber of times an
external So u rce p u lses between rising edges on the Gate signal.
Example Application That Works Correctly
(No Duplicate Counting)
Fig u re 8-33 shows an external b u ffered signal as the period meas u rement
So u rce.
Figure 8-33. Duplicate Count Prevention Example
On the first rising edge of Gate, the c u rrent co u nt of 7 is stored. On the next
rising edge of Gate, the co u nter stores a 2 since two So u rce p u lses occ u rred
after the previo u s rising edge of Gate.
The co u nter synchronizes or samples the Gate signal with the So u rce
signal, so the co u nter does not detect a rising edge in Gate u ntil the next
So u rce p u lse. In this example, the co u nter stores the val u es in the b u ffer on
G a te
2
7 7
6 7 1 2 1
Ri s ing Edge
of G a te
Co u nter detect s ri s ing edge
of G a te on the next ri s ing
edge of S o u rce.
S o u rce
Co u nter V a l u e
B u ffer