Welch Allyn Duet CO2 Module - User Manual - Manual (Page 65)

Duet CO
2
Module OEM Implementation Manual
Welch Allyn OEM Technologies Confidential Page 65
Software Procedures
CRC Code Calculation
The 8 bit CRC code is based on the CCITT/CRC polynomial which uses the conventional right shifting
(high to low) method. The polynomial is:
G(x) = x^8 + x^7 + x^2 + 1
The feedback constant is 0xA1 (hex). CRC values are initialized to 0xFF (hex).
The CRC update value is implemented by a table look up scheme. The following is an
implementation example in ‘C’ language.
Usage:
#define updatecrc_8(value,crc) (crc_table_8[value^crc])
#define MAX_SIZE 23
/*choose any size*/
unsigned char crc_table_8[256];
unsigned char data[MAX_SIZE];
/*data to be CRC checked*/
unsigned char calc_crc(char data,int n)
{
unsigned char crc;
/*make_table_8( );*/
/*do this at least once*/
crc=0xFF;
for(i=0;i
crc=updatecrc_8(data[i],crc);
}
return(crc);
}