NXP Semiconductors MPC5644A, Reference Manual, Page: 1246 / 1735

Cyclic Redundancy Checker (CRC) Unit
MPC5644A Microcontroller Reference Manual, Rev. 6
1246 Freescale Semiconductor
29.5 Use cases and limitations
Two main use cases are considered:
• Calculation of the CRC of the configuration registers during the process safety time
• Calculation of the CRC on the incoming and outgoing frames for the communication protocols (not
protected with CRC by definition of the protocol itself) used as a safety-relevant peripheral.
29.5.1 Checksums for configuration registers
The checksum (signature) of configuration registers is computed in a correct way only if these registers do
not contain any status bits, i.e., configuration register contents must not dynamically change during, or as
a result of, a CRC checksum calculation.
29.5.2 Calculations on incoming/outgoing protocol frames
The following sections show the sequence for managing CRC checksums as part of a communication
external to the device.
29.5.2.1 Calculating checksums on data to be transmitted
Figure 29-6 illustrates the sequence used to calculate a CRC checksum on a data stream, append the
checksum and transfer it to the peripheral to be used for transmission. The sequence is as follows:
1. Software configures the DMA channel and CRC context to be used.
2. DMA copies the data to be transmitted to the CRC context’s input register (CRC_INP) to calculate
the CRC signature (Phase 1)
3. Software copies the CRC checksum (signature) from the CRC module (CRC_OUTP register) to
the memory location immediately following the transmission data. (Phase 2)
4. DMA transfers the data block (p CRC checksum from memory to the peripheral module
(e.g., SPI Tx FIFO) (Phase 3)