Keithley KUSB-3100, User Manual

Page: 40 / 129

Chapter 2
26
C/T Clock Sources
The following clock sources are available for the counter/timers:
• Internal clock – The internal clock uses a 24 MHz time base.
Through software, specify the clock source as internal, and
specify the frequency at which to pace the counter/timer
operation. The frequency of the internal C/T clock can range
from 15 Hz to 12 MHz.
• External clock – An external clock is useful when you want to
pace counter/timer operations at rates not available with the
internal clock or if you want to pace at uneven intervals.
Connect an external clock with a maximum recommended
frequency of 6 MHz to the Counter 0 In signal on the KUSB-3100
module. Using software, specify the C/T clock source as external,
and specify a clock divider between 2 and 65536 to determine the
actual frequency at which to pace the counter/timer operation.
For example, if you connect a 6 MHz external C/T clock and use
a clock divider of 2, the resulting C/T output frequency is 3 MHz.
Counter/timer operations start on the falling edge of the Counter
0 In signal.
Gate Types
The edge or level of the Counter 0 Gate signal determines when a
counter/timer operation is enabled. Using software, you can specify
one of the following gate types:
• None – A software command enables any counter/timer
operation immediately after execution.
• Logic-high level external gate input – Enables a counter/timer
operation when Counter 0 Gate is high, and disables a
counter/timer operation when Counter 0 Gate is low. Note that
this gate type is used for event counting and rate generation
modes; refer to
page 28 for more information about these modes.