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Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
1216
Freescale Semiconductor
cleared by writing SSOVFC to 1, but only in saturated accumulation. Therefore, a non-saturated overflow
that occurs before an SSOVF clear is still flagged in the next output request.
The integrator output request also updates the register DECFILTER_FINTCNT, which holds the number
of samples accumulated into the register DECFILTER_FINTVAL. This internal accumulated sample
counter can operate either in a saturated or “wrapped” count mode, as selected by the
DECFILTER_MXCR bit SCSAT. In both cases, the counter overflow is flagged by the
DECFILTER_MXSR bit SCOVF.
The SCOVF flag is asserted upon an integrator output request, based on the overflow state of the internal
counter. This internal overflow state is negated upon an output request, just after SCOVF is asserted, or
upon an integrator reset (see
Section 26.5.15.3, Integrator reset
”). The internal overflow state is also
negated by writing SCOVFC to 1, but only in saturated count. Therefore, a non-saturated overflow that
occurs before an SCOVF clear is still flagged in the next output request.
An integrator output update can also issue a DMA or interrupt request. The interrupt and DMA requests
are the same ones used for the filter output buffer (see
Section 26.5.11.3, Output buffer interrupt request
and
Section 26.5.12.2, Output buffer DMA request
”). The DECFILTER_MCR bit SDIE is used to enable
integrator interrupts, and the DECFILTER_MXCR bit SDMAE enables the DMA integrator requests. The
integrator DMA request uses the same signal as the filter output DMA request, so one must never use any
configuration that allows both the integrator and filter output to make DMA requests.
Integrator output updates are flagged by the DECFILTER_MXSR bit SDF. The integrator overrun is
detected in the same way as a filter output buffer overrun, and is flagged by DECFILTER_MXSR bit SVR.
An integrator overrun also generates an error interrupt if the DECFILTER_MCR bit ERREN = 1 (see
Section 26.5.4.1, Output buffer overrun
“).
Registers DECFILTER_CINTVAL and DECFILTER_CINTCNT provide a way to poll intermediate
integration values and sample counts, respectively (see
Section 26.4.2.12, Decimation Filter Current
Integration Value Register (DECFILTER_CINTVAL)
Section 26.4.2.13, Decimation Filter Current
Integration Count Value Register (DECFILTER_CINTCNT)
“). DECFILTER_CINTVAL is updated
whenever the integrator is reset or a new sample is accumulated. DECFILTER_CINTCNT is updated only
when DECFILTER_CINTVAL is read, so that coherency between the value and count values is
guaranteed. Therefore, the read access order of that pair of registers must be DECFILTER_CINTVAL first,
followed by DECFILTER_CINTCNT.
NOTE
The flags SSOVF and SCOVF can also asserted when
DECFILTER_CINTVAL is read. The SSOVF and SCOVF set and clearing
rules apply for the DECFILTER_CINTVAL read the same way as for an
integrator output request.
26.5.15.3 Integrator reset
The integration value is reset to the value of zero, in the following ways:
•
by hardware: on hardware reset, or controlled by an external signal; the enabling and the selection
of the zero signal modes is done through the DECFILTER_MXCR field SZROSEL (see
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