Chapter 8
Counters
©
National Instruments Corporation
8-7
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
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
