USB-6525 User Guide and Specifications
6
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edges. The optocouplers can therefore subtract up to 150
μ
s from a low
pulse.
Table 3 lists the pulse widths guaranteed to be passed and blocked.
You can enable filtering on as many input lines as is necessary for your
application. All filtered lines share the same timing interval, which ranges
from 120
μ
s to 2600 ms.
Internally, the filter uses two clocks: the sample clock and the filter clock.
The sample clock has a 40
μ
s period
1
. The filter clock is generated by a
counter and has a period equal to one half of the specified timing interval.
The input signal is sampled on each rising edge of the sample clock, which
is every 40
μ
s
1
. A change in the input signal is recognized only if it
maintains its new state for at least two consecutive rising edges of the filter
clock.
The filter clock is programmable and allows you to control how long a
pulse must last to be recognized. The sample clock provides a fast sample
rate to ensure that input pulses remain constant between filter clocks.
Table 3.
NI 6525 Digital Filtering
Filter Interval
Pulse Width Passed
Pulse Width Blocked
Low Pulse
High Pulse
Low Pulse
High Pulse
t
interval
t
interval
+ 150
μ
s
t
interval
t
interval
/2
(t
interval
/2) – 150
μ
s
1
The accuracy of the sample clock is ±1.5%.