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CRC (informative)

M24LR64-R

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Appendix B

CRC (informative)

B.1

CRC error detection method

The cyclic redundancy check (CRC) is calculated on all data contained in a message, from

the start of the flags through to the end of Data. The CRC is used from VCD to M24LR64-R

and from M24LR64-R to VCD.

To add extra protection against shifting errors, a further transformation on the calculated

CRC is made. The One’s Complement of the calculated CRC is the value attached to the

message for transmission.

To check received messages the 2 CRC bytes are often also included in the re-calculation,

for ease of use. In this case, the expected value for the generated CRC is the residue

F0B8h.

B.2

CRC calculation example

This example in C language illustrates one method of calculating the CRC on a given set of

bytes comprising a message.

C-example to calculate or check the CRC16 according to ISO/IEC 13239

#define

POLYNOMIAL0x8408//

x^16 + x^12 + x^5 + 1

#define

PRESET_VALUE0xFFFF

#define

CHECK_VALUE0xF0B8

#define

NUMBER_OF_BYTES4// Example: 4 data bytes

#define

CALC_CRC1

#define

CHECK_CRC0

void main()

{

unsigned int

current_crc_value;

unsigned char array_of_databytes[NUMBER_OF_BYTES + 2] = {1, 2, 3,

4, 0x91, 0x39};

int

number_of_databytes = NUMBER_OF_BYTES;

int

calculate_or_check_crc;

int

i, j;

calculate_or_check_crc = CALC_CRC;

// calculate_or_check_crc = CHECK_CRC;// This could be an other

example

if (calculate_or_check_crc == CALC_CRC)

{

Table 112. CRC definition

CRC definition

CRC type

Length

Polynomial

Direction

Preset

Residue

ISO/IEC 13239

16 bits

X16 + X12 + X5 + 1 = 8408h

Backward

FFFFh

F0B8h

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