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MPC563XM Reference Manual, Rev. 1
Freescale Semiconductor
1137
Preliminary—Subject to Change Without Notice
Figure 25-16. Convergent Rounding Methodology
25.5.6.2
Saturation
The filter output saturation occurs when an overflow or underflow of the filter is detected by a dedicated
logic and if enabled by the SAT control bit of the configuration register DECFILTER_MCR. In this
condition, the filter output is set to a saturated value equals to the maximum or minimal value that can be
represented by the 16-bits output port. Also for IIR filter, an equivalent logic is used to assert the saturation
for the feedback result that has 24 bits.
25.5.7
Filter Prefill Control Description
A prefill indicates that the input data should be accepted by the Decimation Filter but no decimated output
should be generated while the control field indicates prefill. Therefore the prefill function is used in the
beginning of the filter operation to initialize and stabilize the Decimation Filter without generating
decimated samples.
The prefill control comes together with the input data to be filtered. When ISEL=0, it is considered the
field M_CTRL[1:0]=00 in the DECFILTER_IOB register. Or the field PREFILL=1 in the
DECFILTER_IB register when ISEL=1. The prefill control is usually activated only in a certain number
of sample data in the beginning of the input data sequence.
When the prefill control is set, the decimation filter block operates as follows:
•
Input data is processed by the digital filter normally and tap values are updated.
•
Decimation counter is maintained in reset value.
•
Output buffer is not updated and no output interrupt or read request are generated.
•
The accompanying timestamp for the identified prefill conversion data is not bypassed.
•
The overflow detector/flag operate normally and the interrupt request is set if enabled.
xx...xx xx.......xx.......00100 1000.....00......00
+0
xx...xx xx.......xx.......00100 0000.....00......00
rounded down
before rounding
xx...xx xx.......xx.......0010
1
1000.....00......00
+1
xx...xx xx.......xx.......00110 0000.....00......00
rounded up
before rounding
MS_WORD
LS_WORD