NXP Semiconductors MPC5644A, Reference Manual, Page: 1204 / 1735

Decimation Filter
MPC5644A Microcontroller Reference Manual, Rev. 6
1204 Freescale Semiconductor
26.5.4 Output buffer description
The decimation filter has an output buffer to send filtering results to a master block using the PSI, or to the
CPU using the device slave-bus, as selected through the DECFILTER_MCR bits ISEL and MIXM bits.
Filtering of prefill inputs do not update the output buffer, so the flag ODF is not set.
When filter types IIR and FIR are selected and the input source is the PSI master block (normal or PSI
input mixed modes), the output buffer receives data from the MAC sub-block or from the timestamp
storage register. The result from the MAC is written immediately after the processing if the decimation is
enabled. However, the timestamp data is enabled to be written in the output buffer only when the output
buffer is empty, the decimation count is reached, and the corresponding data was also ready to be
transmitted. When a new word of data is available in the output buffer, a read request signal is sent to the
master block. The master block has to send the corresponding read commands to clear the read request
signal. In this configuration, the core can always read the output buffer. The flag ODF is set when the
buffer is updated.
When in filter operation mode with input from the device slave-bus (ISEL = 1), only sample data is
processed by the filter, as there is no way to input timestamps. When the filter result from the MAC is
ready, this is immediately written to the output buffer when the decimation count is reached. The flag ODF
is set when the buffer is updated. It also generates a DMA read request if DSEL = 1 in standalone or PSI
input mixed modes.
When the filter is bypassed (FTYPE = 00), and PSI is selected as output (normal mode or PSI output mixed
mode), the data written into the input buffer waits until the output buffer is empty before passing the data.
This is needed because the master block takes some clock cycles to send the read commands to the
decimation filter after the read request signal is asserted. When the filter is bypassed and the device
slave-bus is selected as output (standalone mode or PSI input mixed mode), the data written into the input
buffer is immediately written into the output buffer. The flag ODF is set when the buffer is updated and a
DMA read request is asserted if DSEL = 1.
Soft reset clears the output buffer, as well as any data read request generated in any mode.
26.5.4.1 Output buffer overrun
An output overrun occurs when the output buffer is holding output data (sample or timestamp) that has not
been read is overwritten with another word of data (sample or timestamp).
Output overruns are flagged by the DECFILTER_MSR bit OVR, if the output buffer is updated when not
empty. The output buffer empty condition depends on the mode and output selection as follows:
• if PSI is selected as output (normal mode or PSI output mixed mode), the output buffer is
considered empty when the last output has been read.
• if cascade mode is selected, the output buffer is considered empty if the last output request was
acknowledged.
• if the device slave-bus is selected (standalone or PSI input mixed mode) for output and DMA is
selected (DSEL = 1), the output buffer is considered empty if the last output has been read.
• if the device slave-bus is selected (standalone or PSI input mixed mode) for output and DMA is
not selected (DSEL = 0), the output buffer is considered empty when the ODF flag is negated.