National Instruments NI USB-621x manual 117

Chapter 9 Counters

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.

		No Source edge, so no
		value written to buffer.
Gate
Source
Counter Value 6	7	1
Buffer		7

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.

Counter detects	Counter value
rising Gate edge.
	increments only
Gate	one time for each
	Source pulse.
Source

80 MHz Timebase

Counter Value	6	7	0	1
Buffer			7	0
				7

Figure 9-34.Duplicate Count Prevention Example

© National Instruments Corporation 9-37 NI USB-621x User Manual