我有以下CRC功能:
#define CRC8INIT 0x00
#define CRC8POLY 0x18 //0X18 = X^8+X^5+X^4+X^0
// ----------------------------------------------------------------------------
uint8_t crc8 (uint8_t *data, uint16_t number_of_bytes_in_data)
{
uint8_t crc;
uint16_t loop_count;
uint8_t bit_counter;
uint8_t b;
uint8_t feedback_bit;
crc = CRC8INIT;
for (loop_count = 0; loop_count != number_of_bytes_in_data; loop_count++) {
b = data[loop_count];
bit_counter = 8;
do {
feedback_bit = (crc ^ b) & 0x01;
if (feedback_bit == 0x01) {
crc = crc ^ CRC8POLY;
}
crc = (crc >> 1) & 0x7F;
if (feedback_bit == 0x01) {
crc = crc | 0x80;
}
b = b >> 1;
bit_counter--;
} while (bit_counter > 0);
}
return crc;
}
如何为0x18涉及多项式X ^ 8 + X ^ 5 + X ^ 4 + X ^ 0?
X ^ 8 + X ^ 5 + X ^ 4 + X ^ 0 = 100110001
为0x18 = 00011000
如果我定义为CRC8POLY代替0xEA(我已经看到了这)是什么,是否会代表什么多项式?
如何为0x18涉及多项式X ^ 8 + X ^ 5 + X ^ 4 + X ^ 0?
由于代码是右每个字节的移位CRC,“最显著位”是0x119位0的位而不是7.聚需要从100110001反转为100011001,这是0x119,右移位后,第0位被移位了,所以量0x118可以用来代替。代码使用第二if语句或(0x100的)>> 1 == 0x80的如果反馈位是1作为替代方案,因为feedback_bit是0或1,则(0-feeback_bit)可以被用作掩模(假设补数学)为聚而不是使用if语句。
typedef unsigned char uint8_t;
typedef unsigned short uint16_t;
#define CRC8INIT 0x00
#define CRC8POLY 0x8c // 0x119 >> 1
uint8_t crc8 (uint8_t *data, uint16_t number_of_bytes_in_data)
{
uint8_t crc;
uint16_t loop_count;
uint8_t bit_counter;
uint8_t b;
uint8_t feedback_bit;
crc = CRC8INIT;
for (loop_count = 0; loop_count != number_of_bytes_in_data; loop_count++) {
b = data[loop_count];
bit_counter = 8;
do {
feedback_bit = (crc ^ b) & 0x01;
crc = (crc >> 1) ^ ((0-feedback_bit) & CRC8POLY);
b = b >> 1;
bit_counter--;
} while (bit_counter > 0);
}
return crc;
}
这CRC代码编写,而奇怪的是,这里的多项式在两个不同的地方应用,打破了位。也就是说,在crc = crc ^ CRCPOLY
和有条件crc = crc | 0x80
。它通常会写更多像这样的:
unsigned crc8(unsigned char const *dat, size_t len) {
unsigned crc = 0;
for (size_t i = 0; i < len; i++) {
crc ^= dat[i];
for (int k = 0; k < 8; k++)
crc = crc & 1 ? (crc >> 1) ^ 0x8c : crc >> 1;
}
return crc;
}
其中0x8c
是多项式逆转,而X8项。
参照Wiki article on CRC,它是作为在达拉斯1线芯片中使用CRC8实现。请注意,polinomial可在常规表示,逆转,互惠和反倒数表示(见Polynomial representations)。看来,这是相反的相互代表与最高位省略。