crcCalculate (Function)

This function calculates, by standard, the CRC-CCITT 16 (16 bit polynomial) checksum of a buffer.

By using the CRC-CCITT 16 polynomial, an overall error detection rate of 99.955% is achieved.

By adjusting the parameters, a wide range of CRC polynomials/algorithms can be realised.

Please consult relevant literature for further information.

Input:

buffer : PTR

Pointer to the buffer that is used to calculate the CRC.

length : DINT

Number of bytes in the buffer.

polynom : DINT (Default: 16#00001021)

The polynom used to calculate the CRC.

Default is X^16 + X^12 + X^5 + 1.

preset : DINT (Default: 16#0000FFFF)

The preset CRC value.

finalxor : DINT (Default: 16#00000000)

This value is XOR'ed with the CRC calculated from the buffer just before it is returned.

order : SINT (1..32, Default: 16)

The number of bits used in the CRC.

direct : BOOL (Default: TRUE)

Calculates the CRC with (FALSE) or without (TRUE) augmented zero bits.

reversedata : BOOL (Default: FALSE)

Reverses data bytes (TRUE) before CRC is calculated.

reversecrc : BOOL (Default: FALSE)

Reverse calculated CRC (TRUE) before finalxor is applied.

Returns: DINT

The calculated CRC of the buffer.

Declaration:

FUNCTION crcCalculate : DINT;
VAR_INPUT
   // Data buffer
   buffer      : PTR;
   length      : DINT;
   // CRC algorithm parameters
   polynom     : DINT := 16#00001021;
   preset      : DINT := 16#0000FFFF;
   finalxor    : DINT := 16#00000000;
   order       : SINT := 16;
   direct      : BOOL := TRUE;
   reversedata : BOOL := FALSE;
   reversecrc  : BOOL := FALSE;
END_VAR;

Example:

INCLUDE rtcu.inc
 
PROGRAM test;
VAR
   buffer : ARRAY[1..9] OF SINT;
   crc16  : INT;
   crc32  : DINT;
END_VAR;
 
   // Test data
   strToMemory(dst:=ADDR(buffer), str:="123456789", len:=9);
 
   // Calculate CRC-CCITT16
   DebugMsg(message:="****************************************");
   DebugMsg(message:="CALC_CRC: (CRC-CCITT16)");
   crc16 := INT( crcCalculate(buffer:=ADDR(buffer), length:=9) );
   DebugFmt(message:="-CRC = \1", v1:=crc16);
 
   DebugMsg(message:="****************************************");
   DebugMsg(message:="CALC_CRC: (CRC16)");
   DebugMsg(message:="-Order = 16");
   DebugMsg(message:="-Polynom = 16#00008005");
   DebugMsg(message:="-Preset = 16#00000000");
   DebugMsg(message:="-Direct = TRUE");
   DebugMsg(message:="-Finalxor = 16#00000000");
   DebugMsg(message:="-ReverseData = TRUE");
   DebugMsg(message:="-ReverseCRC = TRUE");
   crc16 := INT(
                crcCalculate(
                             buffer:=ADDR(buffer),
                             length:=9,
                             Order:=16,
                             Polynom:=16#00008005,
                             Preset:=0,
                             Direct:=TRUE,
                             Finalxor:=0,
                             ReverseData:=TRUE,
                             ReverseCRC:=TRUE
                            )
               );
   DebugFmt(message:="-CRC = \1", v1:=crc16);
 
   DebugMsg(message:="****************************************");
   DebugMsg(message:="CALC_CRC: (CRC32)");
   DebugMsg(message:="-Order = 32");
   DebugMsg(message:="-Polynom = 16#04C11DB7");
   DebugMsg(message:="-Preset = 16#FFFFFFFF");
   DebugMsg(message:="-Direct = TRUE");
   DebugMsg(message:="-Finalxor = 16#FFFFFFFF");
   DebugMsg(message:="-ReverseData = TRUE");
   DebugMsg(message:="-ReverseCRC = TRUE");
   crc32 := crcCalculate(
                         buffer:=ADDR(buffer),
                         length:=9,
                         Order:=32,
                         Polynom:=16#04C11DB7,
                         Preset:=16#FFFFFFFF,
                         Direct:=TRUE,
                         Finalxor:=16#FFFFFFFF,
                         ReverseData:=TRUE,
                         ReverseCRC:=TRUE
                        );
   DebugFmt(message:="-CRC = \4", v4:=crc32);
 
BEGIN
END;
 
END_PROGRAM;