National-instruments Data Acquisition Device NI USB-621x Manual de usuario descargar pdf (Pagina 125)

Chapter 8 Counters

NI USB-621x User Manual 8-32 ni.com

Other Counter Features

Sample Clock

When taking counter measurements, you can enable a sample clock. When

you use a sample clock, measurements are saved after an active edge of the

sample clock. Figure 8-28 shows an example of using a sample clock with

a buffered period measurement. In this example, a period is defined by

two consecutive rising edges of the Gate.

Figure 8-28. Sample Clock Example

At t

, the counter is armed. No measurements are taken until the counter is armed.

The rising edge of Gate indicates the beginning of the first period to measure. The counter begins counting

rising edges of Source.

The rising edge of Sample Clock indicates that the USB-621x device should store the result of the measurement

of the current period when the period ends.

The rising edge of Gate indicates the end of the first period. The USB-621x device stores the counter value in

the buffer.

The rising edge of Gate indicates the end of the second period. Sample Clock did not assert during this period,

so the counter discards the measurement of the second period.

The rising edge of Gate indicates the end of the third period. Sample Clock asserts during this period, so the

USB-621x device stores the measurement in the buffer.

SOURCE

GATE

Buffer

Counter Armed

Sample Clock

Time N

Counter Value

1 2 ... 120 121 122 123 124 125 126 127 128 129 130 ... 184 185

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National-instruments Data Acquisition Device NI USB-621x Manual de usuario Pagina 125