Data Translation DT9834 Series, User Manual

Page: 98 / 186

Chapter 5
98
C/T Clock Sources
The following clock sources are available for the counter/timers:
• Internal C/T clock – The internal C/T clock always uses an 18 MHz time base. Through
software, specify the clock source as internal, and specify the frequency at which to pace
the operation (this is the frequency of the Counter
n Out signal).
• 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 frequency of the external C/T clock can range from .004
Hz to 9 MHz.
Connect the external clock to the Counter n Clock input signal on the DT9834 Series
module. Counter/timer operations start on the rising edge of the clock input signal.
Using software, specify the clock source as external and specify a clock divider between 2
and 2,147,483,647.
Note: The external C/T clock (the clock connected to the Counter n Clock input signal)
determines how often you want to count events, measure frequency, or measure the time
interval between edges.
If you specify a counter/timer in the analog input channel list, the external A/D clock (the
clock connected to the External ADC Clock input signal) determines how often you want to
read the counter value. Refer to
page 84 for more information about the external A/D clock.
Gate Types
The edge or level of the Counter n Gate signal determines when a counter/timer operation is
enabled. DT9834 Series modules provide the following gate types:
• None – A software command enables any counter/timer operation immediately after
execution.
• Logic-low level external gate input – Enables a counter/timer operation when the
Counter n Gate signal is low, and disables the counter/timer operation when the Counter
n Gate signal is high. Note that this gate type is used for event counting and rate
generation modes; refer to page 99 for more information about these modes.
• Logic-high level external gate input – Enables a counter/timer operation when the
Counter n Gate signal is high, and disables a counter/timer operation when the Counter n
Gate signal is low. Note that this gate type is used for event counting and rate generation
modes; refer to
page 99 for more information about these modes.
• Falling-edge external gate input – Enables a counter/timer operation when a high-to-low
transition is detected on the Counter n Gate signal. In software, this is called a low-edge
gate type. Note that this gate type is used for edge-to-edge measurement, one-shot, and
repetitive one-shot mode; refer to
page 99 for more information about these modes.