National Instruments NI USB-621x - Manual (Page 114)

Chapter 9 Counters
NI USB-621x User Manual 9-34 ni.com
The filter setting for each input can be configured independently. On power
up, the filters are disabled. Figure 9-30 shows an example of a low to high
transition on an input that has its filter set to 125 ns (N = 5).
Figure 9-30. 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 2.55 ms setting, the
jitter is up to 10.025 µs.
Refer to the KnowledgeBase document, Digital Filtering with M Series, 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. M Series 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.
Table 9-8. Filters
Filter Setting
N (Filter Clocks
Needed to Pass
Signal)
Pulse Width
Guaranteed to Pass
Filter
Pulse Width
Guaranteed to Not
Pass Filter
125 ns 5 125 ns 100 ns
6.425 µs 257 6.425 µs 6.400 µs
2.55 ms ~101,800 2.55 ms 2.54 ms
Disabled — — —
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.