Advantech MIC-3758DIO, Руководство пользователя

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23 Chapter 4
4.2 Digital Filter Function
The digital filter function is used to eliminate glitches on input data and
reduce the number of changes to examine and process. The filter blocks
pulses that are shorter than the specified timing interval and passes pulses
that are twice as long as the specified interval. Intermediate-length pulses,
i.e. pulses longer than half of the interval but less than the interval, may or
may not pass the filter.
The following table lists the pulse widths guaranteed to be passed or
blocked.
You can enable or disable filtering on every input channel as required for
your application. All filtered channels share the same timing interval,
which range from 200 ns to 400 sec.
There are two clocks: the sample clock and the filter clock, in the filter.
The sample clock has a period of 100 ns. The filter clock is generated by
a counter and has a period equal to one half of the specified timing inter-
val. The input signal is sampled on each rising edge of the sample clock.
However, 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 by writing a value to the a 32-bit Filter
Interval Time Preset Register. (high 16-bit register and low 16-bit regis-
ter).
Table 4.1: Pulse Widths Guaranteed
Filter Interval Pulse Width Passed Pulse Width Blocked
Low Pulse High Pulse Low Pulse High Pulse
Tinterval T
interval
T
interval
(T
interval
/2) (T
interval
/2)