National Instruments NI USB-621x - Manual (Page 117)

Chapter 9 Counters
© National Instruments Corporation 9-37 NI USB-621x User Manual
Example Application That Works Incorrectly
(Duplicate Counting)
In Figure 9-33, after the first rising edge of Gate, no Source pulses occur,
so the counter does not write the correct data to the buffer.
Figure 9-33. 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 9-34.
Figure 9-34. Duplicate Count Prevention Example
Gate
Source
Counter Value
Buffer 7
6 7 1
No Source edge, so no
value written to buffer.
Gate
Source
80 MHz Timebase
Counter Value
Buffer 0
7 7
6 7 0 1
Counter detects
rising Gate edge.
Counter value
increments only
one time for each
Source pulse.