Pra conferir os dados eu prefiro CRC.
Vou tentar fazer um resuminho de um método simples.
Se vc tiver memória livre pode usar um algoritmo baseado em tabelas e não em calculos.
Por exemplo:
declara como global:
unsigned int crc_tx;
unsigned int crc_rx;
const unsigned int tabela_crc[256] = {
0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241, 0xC601, 0x06C0,
0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440, 0xCC01, 0x0CC0, 0x0D80, 0xCD41,
0x0F00, 0xCFC1, 0xCE81, 0x0E40, 0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0,
0x0880, 0xC841, 0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41, 0x1400, 0xD4C1,
0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641, 0xD201, 0x12C0, 0x1380, 0xD341,
0x1100, 0xD1C1, 0xD081, 0x1040, 0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1,
0xF281, 0x3240, 0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41, 0xFA01, 0x3AC0,
0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840, 0x2800, 0xE8C1, 0xE981, 0x2940,
0xEB01, 0x2BC0, 0x2A80, 0xEA41, 0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1,
0xEC81, 0x2C40, 0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041, 0xA001, 0x60C0,
0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240, 0x6600, 0xA6C1, 0xA781, 0x6740,
0xA501, 0x65C0, 0x6480, 0xA441, 0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0,
0x6E80, 0xAE41, 0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41, 0xBE01, 0x7EC0,
0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40, 0xB401, 0x74C0, 0x7580, 0xB541,
0x7700, 0xB7C1, 0xB681, 0x7640, 0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0,
0x7080, 0xB041, 0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440, 0x9C01, 0x5CC0,
0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40, 0x5A00, 0x9AC1, 0x9B81, 0x5B40,
0x9901, 0x59C0, 0x5880, 0x9841, 0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1,
0x8A81, 0x4A40, 0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641, 0x8201, 0x42C0,
0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040 };
usa essas funções:
void calculo_crc(unsigned int *crc, unsigned int byte_in) {
(*crc) = ((*crc) >> 8) ^ tabela_crc[((*crc) ^ byte_in) & 0x00FF];
}
unsigned char char_to_hex(unsigned char caracter, unsigned char H1L0) {
unsigned char saida;
saida = caracter;
if (H1L0) { saida >>=4; }
saida &= 0x0F;
if ((saida >= 10) && (saida <=15)) { saida += 7; }
saida += 48;
return saida;
}
unsigned int rx_hex_to_int(unsigned char b3, unsigned char b2, unsigned char b1, unsigned
char b0) {
unsigned int i;
i = hex_to_char(b3,b2);
i = i << 8;
i += hex_to_char(b1,b0);
return i;
}
na hora de enviar faz algo do tipo:
void serial_tx_pacote(char *pacote) {
unsigned int i=0;
crc_tx = 0xffff;
while(i < 11){
tx_byte(pacote[i]);
calculo_crc(&crc_tx,(unsigned int)pacote[i]);
i++;
}
tx_byte(char_to_hex((char)(crc_tx >> 8),1));
tx_byte(char_to_hex((char)(crc_tx >> 8),0));
tx_byte(char_to_hex((char)(crc_tx & 0x00ff),1));
tx_byte(char_to_hex((char)(crc_tx & 0x00ff),0));
tx_byte('%');
}
tx_byte é a função que vc usa pra enviar pela serial seu byte.
na hora de receber usa tipo isso:
crc_rx=0xffff;
for (i=0;i<11;i++) {
calculo_crc(&crc_rx,(unsigned int)rx_chegada[i]);
}
l=rx_hex_to_int(0,0,rx_chegada[11],rx_chegada[12]);
m=rx_hex_to_int(0,0,rx_chegada[13],rx_chegada[14]);
calculo_crc(&crc_rx,l);
calculo_crc(&crc_rx,m);
se o resultado for zero em crc_rx, o pacote tá ok.
Abraços!