Chapter 7 Counters
X Series User Manual 7-52 ni.com
Synchronization Modes
The 32-bit counter counts up or down synchronously with the Source
signal. The Gate signal and other counter inputs are asynchronous to the
Source signal, so X Series devices synchronize these signals before
presenting them to the internal counter.
Depending on how you configure your device, X Series devices use one of
three synchronization methods:
• 100 MHz Source Mode
• External Source Greater than 25 MHz
• External or Internal Source Less than 25 MHz
100 MHz Source Mode
In 100 MHz source mode, the device synchronizes signals on the rising
edge of the source, and counts on the third rising edge of the source. Edges
are pipelined so no counts are lost, as shown in Figure 7-40.
Figure 7-40. 100 MHz Source Mode
External Source Greater than 25 MHz
With an external source greater than 25 MHz, the device synchronizes
signals on the rising edge of the source, and counts on the third rising edge
of the source. Edges are pipelined so no counts are lost, as shown in
Figure 7-41.
Figure 7-41. External Source Greater than 25 MHz
100 MHz Source
Synchronize Count
External Source >25 MHz
Synchronize Count
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