Data Translation DT9857, Руководство пользователя, Страница: 95 / 192

Principles of Operation
95
• One-shot
• Repetitive one-shot
Note: The active polarity for each counter/timer operation mode is software-selectable.
The following subsections describe these modes in more detail.
Event Counting
Use event counting mode if you want to count the number of rising edges that occur on the
counter’s clock input when the counter’s gate signal is active (low-level or high-level). Refer
to
page 93 for information about specifying the active gate type.
You can count a maximum of 4,294,967,296 events before the counter rolls over to 0 and starts
counting again.
Using software, specify the counter/timer mode as event counting (count), the C/T clock
source as external, and the active gate type as low-level or high-level.
Make sure that the signals are wired appropriately. Refer to page 53 for an example of
connecting an event counting application.
Up/Down Counting
Use up/down counting mode if you want to increment or decrement the number of rising
edges that occur on the counter’s clock input, depending on the level of the counter’s gate
signal.
If the gate signal is high, the C/T increments; if the gate signal is low, the C/T decrements.
Using software, specify the counter/timer mode as up/down counting (up/down), and the
C/T clock source as external. Note that you do not specify the gate type in software.
Make sure that the signals are wired appropriately. Refer to page 54 for an example of
connecting an up/down counting application.
Note: Initialize the counter/timer so that the C/T never increments above FFFFFFFFh or
decrements below 0.