數據包錯誤檢查代碼

Question

我正在PIC微控制器和Linux計算機上編程一個RS-485協議。我最初考慮使用CRC8來檢查傳入數據，但看起來這將是一個處理器密集型任務。

相反，我想更簡單的算法PEC，也許異或所有傳入的字節用種子創建CRC的一個非常簡單的單步執行。

像這樣的算法有什麼缺點？

Answer 1

CRC不是處理器密集型的。所有它增加到你的獨佔 - 或是一個表查找。每個字節的操作很簡單：crc = crc8_table[crc^*data++]。見下文。

只是做的缺點異或的是，有一些相互抵消，造成假陽性的檢查很多簡單的錯誤。 CRC要好得多。

#include <stddef.h> 

/* 8-bit CRC with polynomial x^8+x^6+x^3+x^2+1, 0x14D. 
    Chosen based on Koopman, et al. (0xA6 in his notation = 0x14D >> 1): 
    http://www.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf 
*/ 

static unsigned char crc8_table[] = { 
    0x00, 0x3e, 0x7c, 0x42, 0xf8, 0xc6, 0x84, 0xba, 0x95, 0xab, 0xe9, 0xd7, 
    0x6d, 0x53, 0x11, 0x2f, 0x4f, 0x71, 0x33, 0x0d, 0xb7, 0x89, 0xcb, 0xf5, 
    0xda, 0xe4, 0xa6, 0x98, 0x22, 0x1c, 0x5e, 0x60, 0x9e, 0xa0, 0xe2, 0xdc, 
    0x66, 0x58, 0x1a, 0x24, 0x0b, 0x35, 0x77, 0x49, 0xf3, 0xcd, 0x8f, 0xb1, 
    0xd1, 0xef, 0xad, 0x93, 0x29, 0x17, 0x55, 0x6b, 0x44, 0x7a, 0x38, 0x06, 
    0xbc, 0x82, 0xc0, 0xfe, 0x59, 0x67, 0x25, 0x1b, 0xa1, 0x9f, 0xdd, 0xe3, 
    0xcc, 0xf2, 0xb0, 0x8e, 0x34, 0x0a, 0x48, 0x76, 0x16, 0x28, 0x6a, 0x54, 
    0xee, 0xd0, 0x92, 0xac, 0x83, 0xbd, 0xff, 0xc1, 0x7b, 0x45, 0x07, 0x39, 
    0xc7, 0xf9, 0xbb, 0x85, 0x3f, 0x01, 0x43, 0x7d, 0x52, 0x6c, 0x2e, 0x10, 
    0xaa, 0x94, 0xd6, 0xe8, 0x88, 0xb6, 0xf4, 0xca, 0x70, 0x4e, 0x0c, 0x32, 
    0x1d, 0x23, 0x61, 0x5f, 0xe5, 0xdb, 0x99, 0xa7, 0xb2, 0x8c, 0xce, 0xf0, 
    0x4a, 0x74, 0x36, 0x08, 0x27, 0x19, 0x5b, 0x65, 0xdf, 0xe1, 0xa3, 0x9d, 
    0xfd, 0xc3, 0x81, 0xbf, 0x05, 0x3b, 0x79, 0x47, 0x68, 0x56, 0x14, 0x2a, 
    0x90, 0xae, 0xec, 0xd2, 0x2c, 0x12, 0x50, 0x6e, 0xd4, 0xea, 0xa8, 0x96, 
    0xb9, 0x87, 0xc5, 0xfb, 0x41, 0x7f, 0x3d, 0x03, 0x63, 0x5d, 0x1f, 0x21, 
    0x9b, 0xa5, 0xe7, 0xd9, 0xf6, 0xc8, 0x8a, 0xb4, 0x0e, 0x30, 0x72, 0x4c, 
    0xeb, 0xd5, 0x97, 0xa9, 0x13, 0x2d, 0x6f, 0x51, 0x7e, 0x40, 0x02, 0x3c, 
    0x86, 0xb8, 0xfa, 0xc4, 0xa4, 0x9a, 0xd8, 0xe6, 0x5c, 0x62, 0x20, 0x1e, 
    0x31, 0x0f, 0x4d, 0x73, 0xc9, 0xf7, 0xb5, 0x8b, 0x75, 0x4b, 0x09, 0x37, 
    0x8d, 0xb3, 0xf1, 0xcf, 0xe0, 0xde, 0x9c, 0xa2, 0x18, 0x26, 0x64, 0x5a, 
    0x3a, 0x04, 0x46, 0x78, 0xc2, 0xfc, 0xbe, 0x80, 0xaf, 0x91, 0xd3, 0xed, 
    0x57, 0x69, 0x2b, 0x15}; 

unsigned crc8(unsigned crc, unsigned char *data, size_t len) 
{ 
    unsigned char *end; 

    if (len == 0) 
     return crc; 
    crc ^= 0xff; 
    end = data + len; 
    do { 
     crc = crc8_table[crc^*data++]; 
    } while (data < end); 
    return crc^0xff; 
} 

/* this was used to generate the table and to test the table-version 

#define POLY 0xB2 

unsigned crc8_slow(unsigned crc, unsigned char *data, size_t len) 
{ 
    unsigned char *end; 

    if (len == 0) 
     return crc; 
    crc ^= 0xff; 
    end = data + len; 
    do { 
     crc ^= *data++; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
     crc = crc & 1 ? (crc >> 1)^POLY : crc >> 1; 
    } while (data < end); 
    return crc^0xff; 
} 
*/ 

#include <stdio.h> 

#define SIZE 16384 

int main(void) 
{ 
    unsigned char data[SIZE]; 
    size_t got; 
    unsigned crc; 

    crc = 0; 
    do { 
     got = fread(data, 1, SIZE, stdin); 
     crc = crc8(crc, data, got); 
    } while (got == SIZE); 
    printf("%02x\n", crc); 
    return 0; 
} 

Answer 2

用於生成PEC值的表查找方法肯定更快。我在一個運行在80 MHz下的4字節數據包的PIC32上進行了測試，結果表明表法需要2.8us，而算法需要11.5us。內存要求顯示速度的成本：表方法需要348字節，而算法方法爲：y 216字節。所以如果內存稀缺 - 考慮這裏顯示的算法方法。 （BYTE是一個無符號字符）

/* bit_crc8 FUNCTION DESCRIPTION ************************************ 
* SYNTAX:   BYTE bit_crc8(BYTE *data, BYTE len); 
* KEYWORDS:  PEC, CRC, error checking 
* DESCRIPTION:  Returns the PEC for an array of bytes. This method does 
*     not use a lookup table 
* PARAMETER 1:  BYTE pointer to data array 
* PARAMETER 2:  BYTE - Number of bytes in array 
* RETURN VALUE: BYTE - PEC value 
* NOTES:   SMBus limits the number of bytes in the packet to 256 
*     Primitive polynomial is set by the definition of PEC. 
* END DESCRIPTION **********************************************************/ 

BYTE bit_crc8(BYTE *data, BYTE len) 
{ 
    #define PEC 0x07 // Implements Polynomial X^8 + X^2 + X^1 +1 
    BYTE crc = 0; 
    BYTE loop, b8; 

    while (len--) 
    { 
     crc ^= *data++; 
     for(loop=0; loop <8; loop++) 
     { 
      b8 = crc & 0x80; /* Test for MSB set to 1 */ 
      crc <<= 1;   /* Left shift CRC */ 
      if(b8) 
      { 
       crc ^= PEC;  /* Divide by PEC if bit 8 was set */ 
      } 
     } 
    } 
    return crc; 

}