mavlink协议从入门到放弃（二）

r囧r小猫 2023-01-23 13:58 207阅读 0赞

### mavlink协议从入门到放弃（二） ###

*  姿态消息帧
 *  GPS消息帧
 *  mavlink校验位的计算

因为这个项目没有说要完全移植mavlink协议，所以只是提取了有用的信息包成mavlink协议然后发给上位机，所以就将用到的消息帧提取出来。

# 姿态消息帧 #

可以直接从PIX中捕捉到这一帧，同样的可以在mavlink协议中查找到这一帧，拿一个例子来看一下

> FE 1C 31 01 01 1E 53 CB 00 00 5C 9F 05 BE 33 30 52 BE 08 5D 3A C0 D4 7A F1 BC 8A CE F6 3B 58 D7 67 BD 81 A6

对于这里的1C和心跳帧一样，都是表示了负载信息的长度，详细的可以看一下mavlink协议中怎么描述，通过数据转化再进行打包，计算CRC校验。

# GPS消息帧 #

也那个例子看一下就可以了

> FE 1E 9F 01 01 18 5D 00 F0 50 00 00 00 00 1E 40 FC 11 A7 B8 9B 47 9E 70 00 00 82 00 8E 00 0E 00 D6 3A 04 0B 48 36

每个消息帧在计算校验位的时候都会有一个类似于隐藏的CRC，对于GPS数据来说需要校验的数据后面是加一个0X18，姿态后面是加一个0x27，这个其实可以去看ardupilot的代码。

# mavlink校验位的计算 #

直接上代码吧，直接可以用的

unsigned short crc16_ccitt_table[256] =   
    {    
    0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,   
    0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,   
    0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,   
    0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,   
    0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,   
    0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,   
    0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,   
    0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,   
    0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,   
    0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,   
    0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,   
    0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,   
    0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,   
    0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,   
    0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,   
    0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,   
    0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,   
    0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,   
    0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,   
    0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,   
    0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,   
    0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,   
    0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,   
    0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,   
    0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,   
    0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,   
    0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,   
    0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,   
    0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,   
    0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,   
    0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,   
    0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78  
    };  
    
    
    unsigned short do_crc(unsigned short reg_init, unsigned short *message, unsigned short len)   
    {    
            unsigned short crc_reg = reg_init;   
            while (len--)   
                    crc_reg = (crc_reg >> 8) ^ crc16_ccitt_table[(crc_reg ^ *message++) & 0xff];   
            return crc_reg;   
    }
    
    int main(void)
    { 
    	unsigned short CRC;
    	
    	CRC = do_crc(0xffff,a,15);
    	printf("%x\r\n",CRC);
    	
    	return 1;
    }