Iir And Fir Filter - STMicroelectronics SPC572L series Reference Manual

RM0400
output with no change. This behavior is independent of the ISEL setting. The following
applies to the bypass configuration:
•
flush is ignored
•
prefill is ignored
•
counted decimation is ignored
•
BSY bit is not set
•
the input and output flags are set
37.4.6

IIR and FIR Filter

This section describes the IIR filters implemented in the Decimation Filter block.
shows the filters functional diagrams. The figure shows a IIR filter of fourth order (4poles).
The filter topology, not the hardware, is represented. The hardware implementation is based
on a MAC unit controlled by an FSM which implements the filter algorithm.
x(n)
-1
Z
x(n-1)
-1
Z
x(n-2)
-1
Z
x(n-3)
-1
Z
x(n-4)
The difference equation for the IIR filter of
Equation 18
where x(n) is the filter input at time n, y(n) is the filter output at time n, N is the number of
feed-forward filter coefficients minus one, B
number of feed-back filter coefficients, and A
Equation 18
Figure 372. 1 x 4 Poles IIR Filter Functional Diagram
B
+
0
B
+
1
B
+
2
B
+
3
B
4
can be written as:
DocID027809 Rev 4
+
+
+
+
Figure 372 can be written as:
N

y n ( ) B x n i – ( )
= +
i
i = 0 j
are the feed-forward filter coefficients, M is the
i
are the feedback filter coefficients.
j
Decimation Filter
Figure 372
-1
Z
y(n-1)
A
1
-1
Z
y(n-2)
A
2
-1
Z
y(n-3)
A
3
-1
Z
A
y(n-4)
4
M

A y n j – ( )
j
= 1
y(n)
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