CAN基础知识
标准的CAN 数据为8字节，即64位，但是CAN FD的最大数据可为64字节，为512位，其中的帧ID分为标准帧和扩展帧，其中用11位标准帧，用29位表示扩展帧。
CAN 信号
信号具体指的是CAN数据的多少位到多少位间代表一个具体的信号，如5位到16位表示车辆的行驶速度，即完整的CAN数据可以表示多个信号。
can信号获取:
#include <iostream>
#include <array>
unsigned char msbmask[] = {
0xFF, 0xFE, 0xFC, 0xF8,
0xF0, 0xE0, 0xC0, 0x80
};
unsigned char lsbmask[] = {
0x01, 0x03, 0x07, 0x0F,
0x1F, 0x3F, 0x7F, 0xFF
};
#define BITSET(p,n) ((p) |= (1u <<(n)))
#define BITCLR(p,n) ((p) &= ~(1u <<(n)))
#define BITGET(i,n) ((i) & (1u << (n)))
typedef struct {
unsigned char* can_data_ptr;
int len;
int msb_pos;
int lsb_pos;
}can_signal;
static can_signal cansingal;
int can_data_assignment(unsigned char* candata, int msbpos, int lsbpos, int lens)
{
cansingal.can_data_ptr = (unsigned char*)malloc(lens);
memcpy((void *)cansingal.can_data_ptr, (const void *)candata,lens);
cansingal.len = lens;
cansingal.msb_pos = msbpos;
cansingal.lsb_pos = lsbpos;
return 0;
}
unsigned int can_data_transfer_signal()
{
int a = 0;
int b = 0;
int c = 0;
int d = 0;
unsigned int singal = 0;
printf("%d %d\n", cansingal.lsb_pos, cansingal.msb_pos);
printf("%02x %02x %02x %02x\n", cansingal.can_data_ptr[0], cansingal.can_data_ptr[1], cansingal.can_data_ptr[2], cansingal.can_data_ptr[3]);
a = cansingal.lsb_pos / 8;
b = cansingal.lsb_pos % 8;
printf("a %d b %d\n", a, b);
cansingal.can_data_ptr[a] = cansingal.can_data_ptr[a] & msbmask[b];
c= cansingal.msb_pos / 8;
d = cansingal.msb_pos % 8;
printf("c %d d %d\n", c, d);
cansingal.can_data_ptr[c] = cansingal.can_data_ptr[c] & lsbmask[d];
printf("%02x %02x %02x %02x\n", cansingal.can_data_ptr[0], cansingal.can_data_ptr[1], cansingal.can_data_ptr[2], cansingal.can_data_ptr[3]);
for (int i = cansingal.lsb_pos, j = 0; i <= cansingal.msb_pos; ++i, ++j)
{
a = i / 8;
b = i % 8;
if ( BITGET(cansingal.can_data_ptr[a], b) )
{
BITSET(singal, j);
}
else
{
BITCLR(singal,j);
}
}
return singal;
}
void can_data_free(void)
{
free(cansingal.can_data_ptr);
cansingal.len = 0;
cansingal.lsb_pos = 0;
cansingal.msb_pos = 0;
return;
}
int main(int argc, char* argv[])
{
unsigned char candata[4] = { 0x44, 0xFE, 0x23, 0x81};
printf("%02x %02x %02x %02x\n", candata[0], candata[1], candata[2], candata[3]);
can_data_assignment(candata,31,14,4);
unsigned int c = can_data_transfer_signal();
can_data_free();
printf("%d\n", c);
system("pause");
return 0;
}如上图，can数据的其中4字节为0x44,0xFE，0x23,0x81, 分别对应0到32位的数据，现在获取14位到31位的数据，形成具体的信号值。
运行结果：
C语言涉及到知识
位操作、指针与数组的操作、MSB LSB的表索引。
数组与指针关系：
指针操作 +1 即 p + 1是指向下一位的地址，若p指向的类型为int类型，则p+1 指向下一个int类型数据的地址，若p指向的是个结构体，则p+1指向相对应结构体下一个元素的地址。
其中p[i] = *(p+i)
#include <stdio.h>
int main(int argc, char *argv[]){
int a[] = {1, 3, 5, 7, 9};
int *p, i, n;
p = a;
n = sizeof(a) / sizeof(int);
printf("%p %p %p\n", a, a+1, a+2);
for(i = 0; i < n; i++){
printf("%d %d %d\n", a[i], *(p+i), *(a+i), p[i]);
}
puts("");
return 0;
}
//打印出来的结果如下
0xbf92c324 0xbf92c328 0xbf92c32c
1 1 1
3 3 3
5 5 5
7 7 7
9 9 9
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