National Instruments Data Acquisition Device NI USB-621x, User Manual

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Chapter 8 Counters
© National Instruments Corporation 8-7 NI USB-621x User Manual
Buffered Period Measurement
B u ffered period meas u rement is similar to single period meas u rement, b u t
b u ffered period meas u rement meas u res m u ltiple periods.
The co u nter co u nts the n u mber of rising (or falling) edges on the So u rce
inp u t between each pair of active edges on the Gate inp u t. At the end of
each period on the Gate signal, the co u nter stores the co u nt in a hardware
save register. A USB Signal Stream transfers the stored val u es to host
memory.
The co u nter begins on the first active edge of the Gate after it is armed. The
arm u s u ally occ u rs in the middle of a period of the Gate inp u t. The co u nter
does not store a meas u rement for this incomplete period.
Fig u re 8-8 shows an example of a b u ffered period meas u rement. In this
example, a period is defined by two consec u tive rising edges.
Figure 8-8. Buffered Period Measurement
Note that if yo u are u sing an external signal as the So u rce, at least one
So u rce p u lse sho u ld occ u r between each active edge of the Gate signal.
This condition ens u res that correct val u es are ret u rned by the co u nter. If this
t
0
At t
0
, the counter is armed. No measurements are taken until the counter is armed.
t
1
The rising edge of Gate indicates the beginning of the first period to measure. The counter begins counting
rising edges of Source.
t
2
The rising edge of Gate indicates the end of the first period. The USB-621 x device stores the counter value in
the buffer.
t
3
The rising edge of Gate indicates the end of the second period. The USB-621 x device stores the counter value
in the buffer.
S OURCE
GATE
Co u nter V a l u e
B u ffer
1 3
3
3 1 1 2 2
3
3 3
Co u nter Armed
Time N
3
t
0
t
1
t
2
t
3