Renesas M16C/62P Group, Hardware Manual

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M16C/62P Group (M16C/62P, M16C/62PT) 20. CRC Calculation
Rev.2.41 Jan 10, 2006 Page 254 of 390
REJ09B0185-0241
Figure 20.3 CRC Calculation
(2) Write 0000h (initial value)
b15 b0
CRCD register
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
Generator polynomial
Data
CRC code
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
Setup procedure and CRC operation when generating CRC code “80C4h”
• CRC operation performed by the M16C
CRC code: Remainder of a division in which the value written to the CRCIN register with its bit positions reversed is
divided by the generator polynomial
(1) Reverse the bit positions of the value “80C4h” by program in 1-byte units.
(3) Write 01h
b0 b7
b15
b0
CRCIN register
CRCD register
1189h
Two cycles later, the CRC code for “80h”, i.e.,
9188h, has its bit positions reversed to become
“1189h” which is stored in the CRCD register.
• Details of CRC operation
As shown in (3) above, bit position of “01h” (00000001b) written to the CRCIN register is inversed and becomes
“10000000b”.
Add “1000 0000 0000 0000 0000 0000b”, as “10000000b” plus 16 digits, to “0000 0000 0000 0000 0000 0000b”,
as “0000 0000 0000 0000b” plus 8 digits as the default value of the CRCD register to perform the modulo-2 division.
“0001 0001 1000 1001b (1189h)”, the remainder “1001 0001 1000 1000b (9188h)” with inversed bit position, can
be read from the CRCD register.
When going on to (4) above, “23h (00100011b)” written in the CRCIN register is inversed and becomes
“11000100b”.
Add “1100 0100 0000 0000 0000 0000b”, as “11000100b” plus 16 digits, to “1001 0001 1000 1000 0000 0000b”, as
“1001 0001 1000 1000b” plus 8 digits as a remainder of (3) left in the CRCD register to perform the modulo-2 division.
“0000 1010 0100 0001b (0A41h)”, the remainder with inversed bit position, can be read from CRCD register.
• Setting procedure
Generator polynomial: X
16
+ X
12
+ X
5
+ 1 (1 0001 0000 0010 0001b)
(4) Write 23h
b0 b7
b15 b0
0A41h
CRCIN register
CRCD register
Two cycles later, the CRC code for “80C4h”, i.e.,
8250h, has its bit positions reversed to become
“0A41h” which is stored in the CRCD register.
“80h” “01h”, “C4h” “23h”
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