Data Translation DT3000 Series, User Manual, Page: 85 / 156

Chapter 6
84
Internal C/T Clock
The internal C/T clock is a 16-bit clock with a 4-bit prescaler; it uses a 10 MHz time base. The
prescaler effectively extends the counter to 20 bits. Counter/timer operations start on the
rising edge of the clock input signal.
Through software, specify the clock source as internal and the frequency at which to pace the
counter/timer operation (this is the frequency of the clock output signal). The maximum
frequency that you can specify for the clock output signal is 5 MHz. The minimum frequency
that you can specify for the clock output signal is 9.54 Hz.
External C/T Clock
The external C/T clock is useful when you want to pace counter/timer operations at rates not
available with the internal C/T clock or if you want to pace at uneven intervals.
Counter/timer operations start on the rising edge of the clock input signal.
Using software, specify the clock source as external and the clock divider used to determine
the frequency at which to pace the operation (this is the frequency of the clock output signal).
The minimum clock divider that you can specify is 2.0; the maximum clock divider that you
can specify is 65,535. For example, if you supply an external C/T clock with a frequency of 2
MHz and specify a clock divider of 2, the resulting frequency of the external C/T clock output
signal is 1 MHz.
Connect the external clock input signal to TB7 of the DT730 or
DT730-T screw terminal panel (pin 7 of connector J1 on the DT3000 Series board).
Gate Types
The active edge or level of the gate input to the counter enables counter/timer operations. The
operation starts when the clock input signal is received. DT3000 Series boards provide the
following gate input types:
• None – A software command enables any specified counter/timer operation immediately
after execution. This gate type is useful for all counter/timer modes.
• Logic-low level external gate input – Enables a counter/timer operation when the
external gate signal is low and disables the counter/timer operation when the external
gate signal is high. Note that this gate type is used only for event counting, frequency
measurement, and rate generation; refer to
page 86 for more information on these modes.
• Logic-high level external gate input – Enables a counter/timer operation when the
external gate signal is high and disables a counter/timer operation when the external gate
signal is low. Note that this gate type is used only for event counting, frequency
measurement, and rate generation; refer to
page 86 for more information on these modes.
• Falling-edge external gate input – Triggers a counter/timer operation on the transition
from the high level to the low level (falling edge). In software, this is called a low-edge
gate type. Note that this gate type is used only for one-shot and repetitive one-shot mode;
refer to
page 86 for more information on these modes.
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