PXR40RM Datasheet, PDF (1141/1434 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

Decimation Filter
When bypass mode is selected, an output overrun cannot occur because the data written into the input
buffer (DECFILT_x_IB) is written into the output buffer (DECFILT_x_OB) only when this buffer is
empty, but an input overrun may still occur (see Section 28.3.1.1, Input Buffer Overrun).
28.3.3 Bypass Operation
Bypass operation is configured by setting the field FTYPE[1:0] of the Module Configuration Register
DECFILT_x_MCR to 00. In this case, the input sample is sent to the output with no change. This behavior
is independent of input data source or output result destination selections. The following applies to the
bypass configuration:
• flush is ignored
• prefill is ignored
• decimation is ignored
• BSY bit is not set
• the input flag DECFILT_x_MSR[IDF] and output flag DECFILT_x_MSR[ODF] are set
28.3.4 Filter Prefill Control
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).
When the input data source is an eQADC (DECFILT_x_MCR[IO_SEL[1]] = 0), prefill is enabled/disabled
in the eQADC conversion command word (CCW). See Section 28.4.1, eQADC Configuration for
Decimation Filter Operation for more information on an eQADC configuration of decimation filter
functionality. When the input data source is the CPU/DMA (DECFILT_x_MCR[IO_SEL[1] = 1]), prefill
is controlled by the PREFILL field in the DECFILT_x_IB register.
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 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.
Freescale Semiconductor
PXR40 Microcontroller Reference Manual, Rev. 1
28-27