Chapter 9 Counters

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 9-8.

Filters

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N (Filter Clocks

 

 

 

 

Pulse Width

 

Pulse Width

 

 

 

 

 

 

 

 

 

Needed to Pass

 

 

Guaranteed to Pass

 

Guaranteed to Not

Filter Setting

 

 

 

 

 

 

Signal)

 

 

 

 

 

 

Filter

 

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

 

 

 

 

 

 

 

 

—

 

 

 

 

 

 

 

—

 

—

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The filter setting for each input can be configured independently. On power

 

 

 

 

 

 

 

up, the filters are disabled. Figure 9-30shows an example of a low to high

 

 

 

 

 

 

 

transition on an input that has its filter set to 125 ns (N = 5).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PFI Terminal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Filtered input goes high

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

when terminal is sampled

Filter Clock

 

 

 

1

1 2 3 4

 

 

1

2 3 4 5

 

 

 

 

high on five consecutive

(40 MHz)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

filter clocks.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Filtered Input

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.