آموزش تابع GPS_NMEA_newFrame در MultiWii

مثال شماره 1

#if defined(NMEA)
  /* This is a light implementation of a GPS frame decoding
     This should work with most of modern GPS devices configured to output NMEA frames.
     It assumes there are some NMEA GGA frames to decode on the serial bus
     Here we use only the following data :
       - latitude
       - longitude
       - GPS fix is/is not ok
       - GPS num sat (4 is enough to be +/- reliable)
       - GPS altitude
       - GPS speed
  */
  #define FRAME_GGA  1
  #define FRAME_RMC  2
  
  bool GPS_NMEA_newFrame(char c) {
    uint8_t frameOK = 0;
    static uint8_t param = 0, offset = 0, parity = 0;
    static char string[15];
    static uint8_t checksum_param, frame = 0;
  
    if (c == '$') {
      param = 0; offset = 0; parity = 0;
    } else if (c == ',' || c == '*') {
      string[offset] = 0;
      if (param == 0) { //frame identification
        frame = 0;
        if (string[0] == 'G' && string[1] == 'P' && string[2] == 'G' && string[3] == 'G' && string[4] == 'A') frame = FRAME_GGA;
        if (string[0] == 'G' && string[1] == 'P' && string[2] == 'R' && string[3] == 'M' && string[4] == 'C') frame = FRAME_RMC;
      } else if (frame == FRAME_GGA) {
        if      (param == 2)                     {GPS_coord[LAT] = GPS_coord_to_degrees(string);}
        else if (param == 3 && string[0] == 'S') GPS_coord[LAT] = -GPS_coord[LAT];
        else if (param == 4)                     {GPS_coord[LON] = GPS_coord_to_degrees(string);}
        else if (param == 5 && string[0] == 'W') GPS_coord[LON] = -GPS_coord[LON];
        else if (param == 6)                     {f.GPS_FIX = (string[0]  > '0');}
        else if (param == 7)                     {GPS_numSat = grab_fields(string,0);}
        else if (param == 9)                     {GPS_altitude = grab_fields(string,0);}  // altitude in meters added by Mis
      } else if (frame == FRAME_RMC) {
        if      (param == 7)                     {GPS_speed = ((uint32_t)grab_fields(string,1)*5144L)/1000L;}  //gps speed in cm/s will be used for navigation
        else if (param == 8)                     {GPS_ground_course = grab_fields(string,1); }                 //ground course deg*10 
      }
      param++; offset = 0;
      if (c == '*') checksum_param=1;
      else parity ^= c;
    } else if (c == '\r' || c == '\n') {
      if (checksum_param) { //parity checksum
        uint8_t checksum = hex_c(string[0]);
        checksum <<= 4;
        checksum += hex_c(string[1]);
        if (checksum == parity) frameOK = 1;
      }
      checksum_param=0;
    } else {
       if (offset < 15) string[offset++] = c;
       if (!checksum_param) parity ^= c;
    }
    if (frame) GPS_Present = 1;
    return frameOK && (frame==FRAME_GGA);
  }
#endif //NMEA