Measurement Computing Data Translation DT9857, User Manual

Page: 93 / 192

Principles of Operation
93
C/T Clock Sources
The following clock sources are available for the general-purpose counter/timer:
• Internal C/T clock – The internal C/T clock always uses a 48 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’s output 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
0.0112 Hz to 10 MHz.
Connect the external clock to the C/T 0 clock input signal of the Digital connector on the
DT9857 or DT9857E 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 4,924,967,296. Internally, the base frequency of C/T clock, which is 48 MHz, is divided
by the specified clock divider to program the frequency of the external C/T clock.
Note: You typically use the external C/T clock (the clock connected to the C/T 0 Clock
input signal of the Digital connector) to measure frequency (event counting), or to
measure the time interval between edges (measure mode).
If you specify a counter/timer in the analog input channel list, the A/D clock determines
how often you want to read the counter value. Refer to
page 77 for more information
about the A/D clock.
Gate Types
The edge or level of the counter gate signal determines when a counter/timer operation is
enabled. DT9857 and DT9857E 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’s gate signal is low, and disables the counter/timer operation when the counter’s
gate signal is high. Note that this gate type is used for event counting and rate generation
modes; refer to
page 94 for more information about these modes.
• Logic-high level external gate input – Enables a counter/timer operation when the
counter’s gate signal is high, and disables a counter/timer operation when the counter’s
gate signal is low. Note that this gate type is used for event counting and rate generation
modes; refer to
page 94 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’s 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 94 for more information about these modes.