Data Translation DT9840 Series, User Manual, Page: 43 / 130

Principles of Operation
43
Repetitive One-Shot
Use repetitive one-shot mode to clean up a poor clock input signal by changing its pulse
width, and then outputting it.
If you specify an external normal or inverted gate type, a single pulse is output each time the
gate signal is active. For example, you could set up the counter to output a single pulse each
time the gate signal changed from a high to low state. Refer to
page 32 for more information
on gate types.
When the operation is enabled with an external gate, the counter begins incrementing. When
the counter increments to the pulse width count, described on
page 35 , the value of the
counter is output. The output stays active until the counter rolls over to 0 (the terminal count).
The output is then deactivated and the counter is automatically reloaded with the period
count, described on
page 35 . The output then stays inactive, and the counter stays disabled,
until the operation is re-enabled with the proper gate signal. All gate signals that occur while
the counter is incrementing are ignored.
You can specify the polarity of the output signal; refer to page 34 for more information.
In repetitive one-shot mode, the internal C/T clock source is more useful than an external C/T
clock source; refer to
page 31 for more information on the internal C/T clock source.
Figure 11 shows an example of a repetitive one-shot operation using an external normal gate; a
period count of FFFFFFFEh, a low-to-high output pulse polarity, and a pulse width count of
FFFFFFFEh to achieve a duty cycle of 66%.
Figure 11: Example of Repetitive One-Shot Mode
Pulse
Output
Signal
External
Gate
Signal
66% duty cycle
Repetitive One-Shot
Operation Starts
66% duty cycle
66% duty
cycle
166.666 ns period
166.666 ns period