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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