31.3.10.13 CRCCTRL SFR Description

CRCRST reset the CRC SFR bits and returns the FSM to its idle state. The FSM clears CRCRST before returning to idle.

CRCEN is essentially the “enable” and “go” bit. All other CRC related SFR bits and fields must be setup prior to writing CRCEN to one. When CRCEN = 1 all other CRC bits are write protected except CRCEN, CRCRST, and PAUSE.

The PLEN32 bit determines the Polynomial Length, either CRC-16 (when zero) or CRC-32 (when one). PLEN32 also defines the effective length of the bits in CRCIV, CRCACC, CRCPOLY, CRCFXOR, and CRCSUM.

Other CRC functions may have a fully programmable PLEN from 0 to 31. These values define CRC lengths from 1 to 32. So PLEN32=0 and PLEN32=1 are equivalent to PLEN=15 and PLEN=31, respectively, on a fully programmable CRC.

The AUTOR selects between Manual mode and Auto Repeat mode. Manual mode requires software to start the CRC engine for each check of the message allowing. Manual mode allows software to check several different messages. Auto Repeat mode does not require software to restart the CRC engine to check the message. However, this mode is limited to checking the same message repeatedly, only stopping on an error or by software disable of the CRC.

The ROUT, Reflected Output, controls the bit order output of the CRC Accumulator. When ROUT = 0, the FSM XORS the CRCACC with CRCFXOR directly. When ROUT = 1, the FSM reverses the bit order of the CRCACC before it XORS it with CRCFXOR.

The RIN, Reflected Input, controls the shift direction of each byte. When RIN = 0, the FSM feeds the LSFR LSbit first. When RIN = 1, the MSbit is first into the LFSR. RIN does not swap bytes in a (32-bit) word or a Flash word, bytes flow from lowest addressed to highest addressed.

The PerCLK is an 8MHz fixed frequency clock. The PERIOD field defines the number of PerCLK counts that the CRC FSM waits from the last bit shifted in to the LFSR until the next read from Flash. The request to count is synchronized between the PerCLK domain and the AHB clock domain. Therefore, up to two PerCLK and two AHB clock periods are added to the period.