NI cDAQ-9178 - Page 34

NI cDAQ-9178/9174 User Guide and Specifications 34 ni.com
• Frequency Measurement
• Period Measurement
• Position Measurement
Counting Edges
In edge counting applications, the counter counts edges on its Source after the counter is armed. You can
configure the counter to count rising or falling edges on its Source input. You also can control the
direction of counting (up or down), as described in the Controlling the Direction of Counting section.
The counter values can be read on demand or with a sample clock.
Refer to the following sections for more information about edge counting options:
• Single Point (On-Demand) Edge Counting
• Buffered (Sample Clock) Edge Counting
Single Point (On-Demand) Edge Counting
With single point (on-demand) edge counting, the counter counts the number of edges on the Source
input after the counter is armed. On-demand refers to the fact that software can read the counter contents
at any time without disturbing the counting process. Figure 21 shows an example of single point edge
counting.
Figure 21. Single Point (On-Demand) Edge Counting
You also can use a pause trigger to pause (or gate) the counter. When the pause trigger is active, the
counter ignores edges on its Source input. When the pause trigger is inactive, the counter counts edges
normally.
You can route the pause trigger to the Gate input of the counter. You can configure the counter to pause
counting when the pause trigger is high or when it is low. Figure 22 shows an example of on-demand
edge counting with a pause trigger.
Figure 22. Single Point (On-Demand) Edge Counting with Pause Trigger
Buffered (Sample Clock) Edge Counting
With buffered edge counting (edge counting using a sample clock), the counter counts the number of
edges on the Source input after the counter is armed. The value of the counter is sampled on each active
edge of a sample clock and stored in the FIFO. The USB-STC3 transfers the sampled values to host
memory using a high-speed data stream.
Counter Armed
SOURCE
Counter Value105432
Counter Armed
SOURCE
Pause Trigger
(Pause When Low)
Counter Value
100 5432