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MPC563XM Reference Manual, Rev. 1
1136
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
Figure 25-15. Filter Configuration Paths
25.5.6.1
Rounding
The Decimation Filter implements the Convergent Rounding methodology, also known as
round-to-nearest even number, in two different locations as shown in
decision on rounding up or down is based on the value of the lower portion of data to be rounded
(LS_WORD). The rounding up/down condition is equal to the traditional rounding except when the
LS_WORD has the format {1000...00}. In this particular case, the rounding procedure is like the example
of
. If the MS_WORD is odd, the value is rounded up. Otherwise the word is rounded down.
There are two locations where the rounding is applied. One is to obtain the filter output result with 16 bits,
and the other is to obtain the IIR feedback result to be stored in tap4 register with 24 bits.
+
+
Z
-1
Z
-1
Z
-1
Z
-1
Z
-1
Z
-1
Z
-1
Z
-1
coefficient 1
coefficient 2
coefficient 3
coefficient 4
coefficient 5
coefficient 6
coefficient 7
coefficient 8
coefficient 0
scale factor S
x(n)
y(n)
FIR section
IIR section
FTYPE[1:0]
10
01
00
bypass
FIR
IIR
A
B
Reg
tap0
tap1
tap2
tap3
tap4
tap5
tap6
tap7
Round/
Sat
Round/ Sat