Chapter 8 Counters
NI USB-621x User Manual 8-36 ni.com
Figure 8-31. Filter Example
Enabling filters introduces jitter on the input signal. For the 125 ns and
6.425 μs filter settings, the jitter is up to 25 ns. On the 2.56 ms setting, the
jitter is up to 10.025 μs.
Refer to the KnowledgeBase document, Digital Filtering with M Series and
CompactDAQ, for more information about digital filters and counters. To
access this KnowledgeBase, go to
ni.com/info and enter the info code
rddfms.
Prescaling
Prescaling allows the counter to count a signal that is faster than the
maximum timebase of the counter. USB-621x devices offer 8X and 2X
prescaling on each counter (prescaling can be disabled). Each prescaler
consists of a small, simple counter that counts to eight (or two) and rolls
over. This counter can run faster than the larger counters, which simply
count the rollovers of this smaller counter. Thus, the prescaler acts as a
frequency divider on the Source and puts out a frequency that is one-eighth
(or one-half) of what it is accepting.
Figure 8-32. Prescaling
Prescaling is intended to be used for frequency measurement where the
measurement is made on a continuous, repetitive signal. The prescaling
counter cannot be read; therefore, you cannot determine how many edges
have occurred since the previous rollover. Prescaling can be used for event
counting provided it is acceptable to have an error of up to seven (or one).
Prescaling can be used when the counter Source is an external signal.
1 2 3 1 4 1 2 3 4 5
PFI Terminal
Filter Clock
(40 MHz)
Filtered Input
Filtered input goes high
when terminal is sampled
high on five consecutive
filter clocks.
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1
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