Data Translation DT9800 Series, User Manual

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Principles of Operation
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Each counter corresponds to a counter/timer (C/T) subsystem. To
specify the counter to use in software, specify the appropriate C/T
subsystem. Counter 0 corresponds to C/T subsystem element 0;
counter 1 corresponds to C/T subsystem element 1.
C/T Clock Sources
The following clock sources are available for the user counters:
• Internal C/T clock,
• External C/T clock, and
• Internally cascaded clock.
Refer to the following subsections for more information on these
clock sources.
Internal C/T Clock
The internal C/T clock uses a 12 MHz time base. 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 750 kHz. The minimum
frequency that you can specify for the clock output signal for each
16-bit counter is 183.1 Hz. The rising edge of the clock is the active
edge.
External C/T Clock
An 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. The rising edge of
the external C/T clock input signal is the active edge.