National Instruments NI USB-621x - Manual (Page 87)

Chapter 9 Counters
© National Instruments Corporation 9-7 NI USB-621x User Manual
Single Period Measurement
With single period measurement, the counter counts the number of rising
(or falling) edges on the Source input occurring between two active edges
of the Gate input. On the second active edge of the Gate input, the counter
stores the count in a hardware save register and ignores other edges on the
Gate and Source inputs. Software then reads the stored count.
Figure 9-8 shows an example of a single period measurement.
Figure 9-8. Single Period Measurement
Buffered Period Measurement
Buffered period measurement is similar to single period measurement, but
buffered period measurement measures multiple periods.
The counter counts the number of rising (or falling) edges on the Source
input between each pair of active edges on the Gate input. At the end of
each period on the Gate signal, the counter stores the count in a hardware
save register. A USB Signal Stream transfers the stored values to host
memory.
The counter begins on the first active edge of the Gate after it is armed. The
arm usually occurs in the middle of a period of the Gate input. The counter
does not store a measurement for this incomplete period.
Figure 9-9 shows an example of a buffered period measurement. In this
example, a period is defined by two consecutive rising edges.
SOURCE
GATE
Counter Value
HW Save Register
1 0 3
5
4 5 2