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CRC Calculation Circuit
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2.11.2 Operation of CRC Calculation Circuit
The following describes the operation of the CRC calculation. Figure 2.11.3 shows an example of calcu-
lation data 0123
16
using the CRC calculation circuit.
Operation (1) The CRC calculation circuit sets an initial value in the CRC data register.
(2) Writing 1 byte data to the CRC input register generates CRC code based on the data register.
CRC code generation for 1 byte data finishes in two machine cycles.
(3) The CRC calculation circuit detects an error by means of comparing the CRC-checking data
with the content of the CRC data register, after the next data is written to the CRC input
register.
(4) The content of CRC data register after all data is written becomes CRC code.
Figure 2.11.3. Calculation example using the CRC calculation circuit
b15
b0
(1) Setting 0000
16
CRC data register
CRCD
[03BD
16
, 03BC
16
]
b0
b7
b15
b0
(2) Setting 01
16
CRC input register
CRCIN
[03BE
16
]
2 cycles
After CRC calculation is complete
CRC data register
CRCD
[03BD
16
, 03BC
16
]
1189
16
Stores CRC code
b0
b7
b15
b0
(3) Setting 23
16
CRC input register
CRCIN
[03BE
16
]
After CRC calculation is complete
CRC data register
CRCD
[03BD
16
, 03BC
16
]
0A41
16
Stores CRC code
The code resulting from sending 01
16
in LSB first mode is (1000 0000). Thus the CRC code in the generating polynomial,
(X
16
+ X
12
+ X
5
+ 1), becomes the remainder resulting from dividing (1000 0000) X
16
by (1 0001 0000 0010 0001) in
conformity with the modulo-2 operation.
Thus the CRC code becomes (1001 0001 1000 1000). Since the operation is in LSB first mode, the (1001 0001 1000 1000)
corresponds to 1189
16
in hexadecimal notation. If the CRC operation in MSB first mode is necessary in the CRC operation
circuit built in the M16C, switch between the LSB side and the MSB side of the input-holding bits, and carry out the CRC
operation. Also switch between the MSB and LSB of the result as stored in CRC data.
1 0001 0000 0010 0001
1000 0000 0000 0000 0000 0000
1000 1000 0001 0000 1
1000 0001 0000 1000 0
1000 1000 0001 0000 1
1001 0001 1000 1000
LSB
1000 1000
LSB
MSB
MSB
9
8
1
1
Modulo-2 operation is
operation that complies
with the law given below.
0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0
-1 = 1