Renesas M16C FAMILY, User Manual

Page: 262 / 369

Rev.2.00 Oct 16, 2006 page 251 of 354
REJ09B0340-0200
M30245 Group 2. CRC Calculation Circuit
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 using the CRC-CCITT.
Operation (1) Select CRC-CCITT or CRC-16, and LSB first or MSB first by bit 0 and bit 5 of CRC mode
register.
(2) The CRC calculation circuit sets an initial value in the CRC data register.
(3) 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.
(4) When several bytes of CRC calculation is performed in succession, write the following data in
the CRC input register continuously.
(5) 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 (when using CRC-CCITT)
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
1000 1000
LSB
MSB
LSB 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