Data Translation DT3000 Series, Руководство пользователя, Страница: 88 / 156

Principles of Operation
87
Frequency Measurement
Use frequency measurement mode to measure the frequency of the signal from counter’s
associated clock input source over a specified duration. In this mode, use an external C/T
clock source; refer to
page 83 for more information on the external C/T clock source.
To perform a frequency measurement on the DT3000 Series, use the same wiring as an event
counting application that does not use an external gate signal.
Use software to specify the counter/timer mode as frequency measurement or event counting,
and specify the duration of the Windows timer over which to measure the frequency. (The
Windows timer uses a resolution of 1 ms.) Frequency is determined using the following
equation:
Frequency Measurement = Number of Events
Duration of the Windows Timer
Figure 25 shows an example of a frequency measurement operation. In this example, three
events are counted during a duration of 300 ms. The frequency is 10 Hz, since 10 Hz = 3/(.3 s).
Figure 25: Example of Frequency Measurement
Rate Generation
Use rate generation mode to generate a continuous pulse output signal from the counter; this
mode is sometimes referred to as continuous pulse output or pulse train output. You can use
this pulse output signal as an external clock to pace other operations, such as analog input.
While the pulse output operation is enabled, the counter outputs a pulse of the specified type
and frequency continuously. As soon as the operation is disabled, rate generation stops.
The period of the output pulse is determined by the clock input signal and the external clock
divider. You can output pulses using a maximum frequency of 5 MHz (this is the frequency of
the clock output signal). In rate generation mode, either the internal or external clock input
source is appropriate, depending on your application; refer to
page 83 for more information
on the C/T clock source.
External C/T
Clock
Input Signal
frequency measurement
starts
frequency
measurement stops
Duration over which the
frequency is measured = 300 ms
3 Events Counted
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