mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 199

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

Moreover, it is common to find circuits in literature slightly different from the one suggested by the
recommendations above, but also known as CRC-CCITT circuits (many variations require the message to
be augmented with zeros).
The circuit implemented in the CRC module is exactly the one suggested by the ITU-T V.41
recommendation, with an added flexibility of a programmable SEED. As in ITU-T V.41, no augmentation
is needed and the CRC result is not negated.
a reference. Notice that these are ASCII string messages. For example, message “123456789” is encoded
with bytes 0x31 to 0x39 (see ASCII table).
10.4.2
The CRC module extends its original programming model to allow faster CRC calculations on CF1 cores.
Memory offsets 0x4, 0x5, 0x6 and 0x7 are mapped (aliased) onto the CRCL register, in a way that the
CF1Core can execute 32-bit store instructions to write data to the CRC module. The code must use a single
mov.l store instruction to send four bytes beginning at address 0x4, which will be decomposed by the
platform into four sequential/consecutive byte writes to offsets 0x4-0x7 (all aliased to the CRCL position).
In addition, reads from 0x4-0x7 return the contents of the CRCL register. Reads from 0x2-0x3
(unused/reserved) return 0x00. Writes to 0x2-0x3 have no effect.
Freescale Semiconductor
Programming Model Extension for CF1Core
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
1
A string of 256 upper case “A”
characters with no line breaks
A string of 256 upper case “A”
characters with no line breaks
A string of 256 upper case “A”
characters with no line breaks
One common variation of CRC-CCITT require the message to be
augmented with zeros and a SEED=0xFFFF. The CRC module will
give the same results of this alternative implementation when
SEED=0x1D0F and no message augmentation.
ASCII String Message
“123456789”
“123456789”
“123456789”
Table 10-4. Expected CRC results
“A”
“A”
“A”
Table 10-4
(initial CRC value)
shows some expected results that can be used as
0x1d0f
0x1d0f
0x1d0f
0x0000
0x0000
0x0000
SEED
0xffff
0xffff
0xffff
1
1
1
Cyclic Redundancy Check Generator (CRCV2)
CRC result
0x58e5
0xb915
0x9479
0x31c3
0x29b1
0xabe3
0xea0b
0xe938
0xe5cc
10-5