Freescale Semiconductor HCS08 Series, Reference Manual

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Cyclic Redundancy Check Generator (S08CRCV2)
MC9S08JS16 MCU Series Reference Manual, Rev. 4
Freescale Semiconductor 281
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.
Table 16-4 shows some expected results that can be used as
a reference. Notice that these are ASCII string messages. For example, message “123456789” is encoded
with bytes 0x31 to 0x39 (see ASCII table).
Table 16-4. Expected CRC Results
16.5 Initialization Information
To initialize the CRC Module and initiate a CRC16-CCITT calculation, follow this procedure:
1. Write high byte of initial seed value to CRCH.
2. Write low byte of initial seed value to CRCL.
3. Write first byte of data on which CRC is to be calculated to CRCL (an alternative option is offered
for CF1Cores. See Section 16.5, “Initialization Information” ).
4. In the next bus cycle after step 3, if desired, the CRC result from the first byte can be read from
CRCH:CRCL.
5. Repeat steps 3-4 until the end of all data to be checked.
ASCII String Message
SEED
(initial CRC value)
CRC result
“A” 0x0000 0x58e5
“A” 0xffff 0xb915
“A” 0x1d0f
1
1
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.
0x9479
“123456789” 0x0000 0x31c3
“123456789” 0xffff 0x29b1
“123456789” 0x1d0f
1
0xe5cc
A string of 256 upper case “A”
characters with no line breaks
0x0000 0xabe3
A string of 256 upper case “A”
characters with no line breaks
0xffff 0xea0b
A string of 256 upper case “A”
characters with no line breaks
0x1d0f
1
0xe938