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Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
1210
Freescale Semiconductor
In Cascade Mode, prefill control is ignored in all cascaded filters except the Tail one: the prefill samples
are filtered, decimated and forwarded to the next block with the prefill indication. The Tail block takes the
samples with prefill indication, filter them and discard the result.
NOTE
The combined decimation count of the cascade combo may not be reset after
the last prefill sample. Unlike in a single (non-cascaded) filter, the number
of non-prefill inputs until an output comes out depends on the number of
past prefill inputs.
26.5.8
Timestamp data transmission
The timestamp information is identified by the master block using the M_CTRL[1:0] bit field of the
register DECFILTER_IOB. For timestamp data, the input data and tag values that come with
M_CTRL[1:0] bits set to 0b10 are sent back to the master block without changing. However, some
additional conditions are considered:
•
The timestamp is additional information that accompanies a sample conversion data. The PSI
master block sends the decimation filter the conversion data with control bits for either prefill or
filter operation. This data may optionally be followed by the corresponding timestamp data. When
the corresponding conversion data is marked for prefill, the timestamp data is not sent to the output
buffer. This occurs because the filter result is not sent to the output buffer.
•
Similarly, when the filter is decimating the results, the timestamp is only sent to the output buffer
if the corresponding received conversion data has generated a filter output that is selected by the
decimation counter to be sent to the output buffer. Other received timestamps that come with data
not selected by the decimator are discarded.
•
Sending two consecutive words of timestamp data is not allowed: there must be at least one
conversion data between two timestamp inputs. In normal operation, the filter should receive only
one timestamp for each word of conversion data to be filtered.
•
Timestamps are not allowed in PSI Input Mixed Mode.
26.5.9
Flush command description
The flush signal is used by the Decimation Filter to execute a partial reset of the filter. This is useful when
the same filter is used on a new set of data samples after finishing the filtering of another set of data.
When the flush control is detected, all filter TAPs are cleared and the DEC_COUNTER[3:0] field in the
status register DECFILTER_MSR is reset.
The flush function does not clear the Coefficient registers file in the Decimation Filter, thus it is not
required to re-write these registers after a flush. The integrator accumulator and sample count are not
cleared either. The output buffer also keeps the last result and may be retrieved until the next output is
posted.
The flush control precedes the input data to be filtered. Therefore, the corresponding sample data is
processed by the block after the flush. When ISEL = 0, the field M_FLUSH in the DECFILTER_IOB
register is processed. When ISEL = 1, the field FLUSH in the DECFILTER_IB register is processed. Note
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