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Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
1209
Figure 26-19. Convergent rounding methodology
26.5.6.2
Saturation
Filter output saturation occurs when an overflow or underflow condition of the filter is detected by
dedicated logic, and if it is enabled by the SAT control bit of the configuration register
DECFILTER_MCR. In this condition, the filter output is set to a saturated value equal to the maximum or
minimum value that can be represented by the 16-bit output port. Also, for the IIR filter an equivalent logic
is used to assert the saturation for the 24-bit feedback result.
26.5.7
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. In addition, the prefill does not operate when the filter is in bypass (FTYPE = 0b00).
The prefill is controlled by the value in the M_CTRL[1:0] field in the DECFILTER_IOB register. When
ISEL = 0, the M_CTRL[1:0] field in the DECFILTER_IOB register controls the prefill. When ISEL = 1,
it is controlled by the PREFILL field in the DECFILTER_IB register. The prefill control is usually
activated only in a certain number of words 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 normally by the digital filter and tap values are updated.
•
The decimation counter is maintained in reset value.
•
The output buffer is not updated and no output interrupt or read request is generated.
•
The accompanying timestamp for the identified prefill conversion data is not bypassed.
•
The overflow detector/flag operates normally and the error interrupt request is set if enabled.
xx.......xx.......0010
1
xx.......xx.......00100
xx...xx
1000.....00......00
Before Rounding
xx.......xx.......00100
xx...xx
Rounded Down
MS_WORD
LS_WORD
+0
xx...xx
1000.....00......00
Before Rounding
xx.......xx.......00110
xx...xx
Rounded Up
+1
Содержание MPC5644A
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