NAME
<util/crc16.h>: CRC Computations -
Functions
static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t
__data)
static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc, uint8_t
__data)
static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t
__data)
static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t
__data)
Detailed Description
#include <util/crc16.h>
This header file provides a optimized inline functions for calculating
cyclic redundancy checks (CRC) using common polynomials.
References:
See the Dallas Semiconductor app note 27 for 8051 assembler example and
general CRC optimization suggestions. The table on the last page of the
app note is the key to understanding these implementations.
Jack Crenshaw's 'Implementing CRCs' article in the January 1992 isue of
Embedded Systems Programming. This may be difficult to find, but it
explains CRC's in very clear and concise terms. Well worth the effort
to obtain a copy.
A typical application would look like:
// Dallas iButton test vector.
uint8_t serno[] = { 0x02, 0x1c, 0xb8, 0x01, 0, 0, 0, 0xa2 };
int
checkcrc(void)
{
uint8_t crc = 0, i;
for (i = 0; i < sizeof serno / sizeof serno[0]; i++)
crc = _crc_ibutton_update(crc, serno[i]);
return crc; // must be 0
}
Function Documentation
static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t __data)
[static] Optimized CRC-16 calculation.
Polynomial: x^16 + x^15 + x^2 + 1 (0xa001)
Initial value: 0xffff
This CRC is normally used in disk-drive controllers.
The following is the equivalent functionality written in C.
uint16_t
crc16_update(uint16_t crc, uint8_t a)
{
int i;
crc ^= a;
for (i = 0; i < 8; ++i)
{
if (crc & 1)
crc = (crc >> 1) ^ 0xA001;
else
crc = (crc >> 1);
}
return crc;
}
static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t
__data) [static] Optimized CRC-CCITT calculation.
Polynomial: x^16 + x^12 + x^5 + 1 (0x8408)
Initial value: 0xffff
This is the CRC used by PPP and IrDA.
See RFC1171 (PPP protocol) and IrDA IrLAP 1.1
Note:
Although the CCITT polynomial is the same as that used by the
Xmodem protocol, they are quite different. The difference is in how
the bits are shifted through the alorgithm. Xmodem shifts the MSB
of the CRC and the input first, while CCITT shifts the LSB of the
CRC and the input first.
The following is the equivalent functionality written in C.
uint16_t
crc_ccitt_update (uint16_t crc, uint8_t data)
{
data ^= lo8 (crc);
data ^= data << 4;
return ((((uint16_t)data << 8) | hi8 (crc)) ^ (uint8_t)(data >> 4)
^ ((uint16_t)data << 3));
}
static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t
__data) [static] Optimized Dallas (now Maxim) iButton 8-bit CRC
calculation.
Polynomial: x^8 + x^5 + x^4 + 1 (0x8C)
Initial value: 0x0
See http://www.maxim-ic.com/appnotes.cfm/appnote_number/27
The following is the equivalent functionality written in C.
uint8_t
_crc_ibutton_update(uint8_t crc, uint8_t data)
{
uint8_t i;
crc = crc ^ data;
for (i = 0; i < 8; i++)
{
if (crc & 0x01)
crc = (crc >> 1) ^ 0x8C;
else
crc >>= 1;
}
return crc;
}
static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc, uint8_t
__data) [static] Optimized CRC-XMODEM calculation.
Polynomial: x^16 + x^12 + x^5 + 1 (0x1021)
Initial value: 0x0
This is the CRC used by the Xmodem-CRC protocol.
The following is the equivalent functionality written in C.
uint16_t
crc_xmodem_update (uint16_t crc, uint8_t data)
{
int i;
crc = crc ^ ((uint16_t)data << 8);
for (i=0; i<8; i++)
{
if (crc & 0x8000)
crc = (crc << 1) ^ 0x1021;
else
crc <<= 1;
}
return crc;
}
Author
Generated automatically by Doxygen for avr-libc from the source code.
Version 1.6.8 Thu Aug 12 <util/crc16.h>: CRC Computations(3)