National Instruments Data Acquisition Device NI USB-621x - Manual (Page 129)

Chapter 8 Counters
NI USB-621x User Manual 8-36 ni.com
the first rising Source edge after the rising edge of Gate. The details of
when exactly the counter synchronizes the Gate signal vary depending
on the synchronization mode.
Example Application That Works Incorrectly
(Duplicate Counting)
In Figure 8-32, after the first rising edge of Gate, no Source pulses occur,
so the counter does not write the correct data to the buffer.
Figure 8-32. Duplicate Count Example
Example Application That Prevents Duplicate Count
With duplicate count prevention, the counter synchronizes both the Source
and Gate signals to the 80 MHz Timebase. By synchronizing to the
timebase, the counter detects edges on Gate even if the Source does not
pulse. This enables the correct current count to be stored in the buffer even
if no Source edges occur between Gate signals, as shown in Figure 8-33.
Gate
Source
Counter Value
Buffer 7
6 7 1
No Source edge, so no
value written to buffer.