Epec Programming and Libraries Manual
Click here to see this page in full context

Supported platforms: CODESYS 3.5, CODESYS 3.5 SAFETY, CODESYS 3.5 SP19 SAFETY

 

Calculate16bitCRC (FUN)

Description

This function calculates a 16-bit CRC according to given seed and polynomial.

 

The non-endianness-aware calculation algorithm is following:

  1. Set seed value to register (16-bit)

  2. XOR BYTE data with register’s MSB

  3. While (bits left in BYTE) do

  4. If BYTEs left, get next and go to line 2

  5. The register now contains the remainder.

 

This function is not typically needed by programmer, but is used by higher level checksum functions.

 

Reverse bit order, so called "reflect" bit order, operation not supported.

 

The function does not include any input or output XOR operation. They need to be done outside the function when specified in CRC calculation.

 

This function is NOT endianness-aware. Input data that consists of variables longer than a BYTE has to be converted to specified byte order format before usage.

 

This also means that ARRAY and STRUCT members have to be converted, and then calculated one by one.

 

Inputs

Input variable name

Data type

Default value

Range

Description

i_pData

POINTER TO BYTE

0

≠ 0

Pointer to data of which checksum is to be calculated.

i_Length

UDINT

0

> 0

Length of the data.

i_Seed

WORD

0000h

-

Checksum seed value (i.e. the CRC of previous data if several data has common CRC).

i_Polynomial

WORD

1021h

-

Polynomial value for calculation.

Outputs

Output variable name

Data type

Range

Description

o_OutputValid

BOOL

 

Calculated value is valid.

o_ParameterError

BOOL

 

Error in input parameters.

Return value

Data type

Description

WORD

Calculated checksum.

Diagnostics

Conditions

Return value

o_ValueValid

o_ParameterError

i_pData = 0 OR i_Length = 0

0000h

FALSE

TRUE

i_pData ok AND i_Length > 0

Calculated checksum

TRUE

FALSE

 

Example code

Definitions:

 

table: ARRAY [0..10] OF DWORD;

crc: WORD;

i: INT;

 

 

Code:

 

crc := WORD#0;

 

FOR i := 0 TO 10 DO

    (* Stop if error *)

    IF NOT parametererror THEN

 

        (* Convert pointed data to big-endian *)

        tempDWORD := BigEndianDWORD(i_Data := table[i]);

 

        crc := Calculate16bitCRC(

            i_pData := ADR(tempDWORD),

            i_Length := LibConstants.G_SIZE_OF_U32,

            i_Seed := crc,

            i_Polynomial := 16#1021,

            o_ParameterError => parametererror,

            o_OutputValid => outputvalid

        );

    END_IF

END_FOR

 

 

See also