PXR40RM Datasheet, PDF (1159/1434 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

Decimation Filter
Table 28-18. ADC_ACR1-8 Field Descriptions (continued)
Field
Description
RESSEL[0:1]
PRE_GAIN[0:1]
ADC Resolution Selection.
00 ADC set to 12-bits resolution
01 ADC set to 10-bits resolution
10 ADC set to 8-bits resolution
11 Reserved
ADC Pre-gain control—The PRE_GAIN[0:1] controls the gain of the ADC input stage by changing the
internal ADC iterations in the gain stage.
00 X1 gain
01 X2 gain
10 X4 gain
11 Reserved
The RFIFO number that receives the filtered data from the Decimation Filter block, or directly from the
ADC, is specified in the MESSAGE_TAG[8:11] field in the eQADC CCW (see “Conversion Command
Format for the Standard Configuration section of the reference manual).
28.5 Use Cases
The following use cases provide examples of how to configure the decimation filter for different modes.
One use case describes the mode that filters the conversion results that directly transferred from the ADC.
In this case, the appropriate software initialization of the ADC configuration register is described to enable
the transfer of an ADC result to the Decimation Filter module without CPU intervention. The second use
cases describes the stand-alone mode, where data previously stored in a memory table is transferred into
the filter and subsequent filtered data extracted using CPU interaction. For all uses cases, the Decimation
Filter is configured as an IIR filter, with example filter characteristics. An explanation of the IIR
configuration and coefficient selection is give in the next section.
28.5.1 IIR Filter Configuration
This section describes the topology of the IIR filter that is implemented in the Decimation Filter hardware
and provides a definition of the coefficients for an elliptical low pass, 4th order IIR filter. Figure 28-22
shows the filter functional diagram, consisting of 4 feed-forward stages and 4 feedback stages, to provide
a maximum of a 4th order IIR filter.
Freescale Semiconductor
PXR40 Microcontroller Reference Manual, Rev. 1
28-45