GPS NMEA protocol 單晶片資訊解析與定時校正

GPS 通訊模組系統，透過RS232通訊介面可獲得衛星時間與方位
NMEA（(National Marine Electronics Association)0183 ASCII 的格式
本人將使用收到的 RMC 資料串進行解析，取出衛星當下的時間

Message Structure:                                                                           

$GPRMC,hhmmss,status,latitude,N,longitude,E,spd,cog,ddmmyy,mv,mvE,mode*cs<CR><LF>

Example:                                                                                            

$GPRMC,083559.00,A,4717.11437,N,00833.91522,E,0.004,77.52,091202,,,A*57

Field No.	Example	Format	Name	Unit	Description
0	$GPRMC	string	$GPRMC	-	Message ID, RMC protocol header
1	083559.00	hhmmss.sss	hhmmss.	-	UTC Time, Time of position fix
2	A	character	Status	-	Status, V = Navigation receiver warning, A = Data valid, see Position Fix Flags description
3	4717.11437	ddmm.mmmm	Latitude	-	Latitude, Degrees + minutes, see Format description
4	N	character	N	-	N/S Indicator, hemisphere N=north or S=south
5	00833.91522	dddmm. Mmmm	Longitude	-	Longitude, Degrees + minutes, see Format description
6	E	character	E	-	E/W indicator, E=east or W=west
7	0.004	numeric	Spd	knots	Speed over ground
8	77.52	numeric	Cog	degrees	Course over ground
9	091202	ddmmyy	date	-	Date in day, month, year format
10	-	numeric	mv	degrees	Magnetic variation value, not being output by receiver
11	-	character	mvE	-	Magnetic variation E/W indicator, not being output by receiver
12	-	character	mode	-	Mode Indicator, see Position Fix Flags description
13	*57	hexadecimal	cs	-	Checksum
14	-	character	<CR><LF>	-	Carriage Return and Line Feed

  

程式撰寫                                                                                          

• 使用編譯器: CCS PCWHD
• 使用單晶片:Microchip PIC18F45K22

透過剛剛介紹的$GPSRMC 接收來自衛星訊號的ASCII內容，裡面包含了有UTC標準時間，本文透過解析內容並轉為資料串再回傳給單晶片修正RTC時間

副程式除了解析出時間外，同時也將轉換經緯度座標位置

char *srg: 傳入接收來自於RS232資料串

TDateTime *DT: 傳入欲擺放時間日期等資訊的結構串

TCoordinate *Coordinate:傳入欲擺放經緯度等資訊的結構串

程序完成後成功傳為0

int GPS_GPRMC_stream_decoder(char *srg, TDateTime *DT, TCoordinate *Coordinate)

{   

  char st = 0, ed = 0, index = 0;

  //step1: find $GPRMC position，not found reutn value = -1

  if (strFind(srg, (char*)"$GPRMC", 0) == -1)

    return -1;

  

  //step2: find first ',' position in string

  st = charFind(srg, ',', 0);

  if (st == -1) 

    return -1;

    

  char str[16];

  char str_length = strlen(srg);

  while (st != -1 && index < str_length)

  {

    ed = charFind(srg, ',', st+1);

    switch (++index)

    {

      

      case 1 : DT->hour = atoi(StrmnCpy(str, srg, st+1, 2));

               DT->min  = atoi(StrmnCpy(str, srg, st+3, 2));

               DT->sec  = atoi(StrmnCpy(str, srg, st+5, 2));

               break;

      case 2 : if (*StrmnCpy(str, srg, st+1, 1) != 'A')

                 return -1;

               break;

      case 3 : StrmnCpy(str, srg, st+1, ed-st-1);

               Coordinate->Latitude = GPS_StrtoLatitude(&str, ed-st-1); 

               break;

      case 4 : StrmnCpy(str, srg, st+1, 1);

               Coordinate->N_S = str[0];

               break;        

      case 5 : StrmnCpy(str, srg, st+1, ed-st-1);

               Coordinate->Longitude = GPS_StrtoLongitude(&str, ed-st-1);

               break;

      case 6 : StrmnCpy(str, srg, st+1, 1);

               Coordinate->E_W = str[0];

               break; 

      case 7 : StrmnCpy(str, srg, st+1, ed-st-1);

               Coordinate->speed  = strtof(str, '0');

               break; 

      case 9 : DT->m_day   = atoi(StrmnCpy(str, srg, st+1, 2));

               DT->month   = atoi(StrmnCpy(str, srg, st+3, 2));

               DT->year    = atoi(StrmnCpy(str, srg, st+5, 2));

               DT->w_day   = RTC_Weekend_Transfer(20, DT->year, DT->month, DT->m_day); 

               break;

    }

    st = ed;

  }

  return 0;

}     

尋找字串中指定的字串內容處在位置

int strFind(char *srg, char *s, size_t st)

{

  BYTE pos; 

  for (pos=st, srg+=st; *srg != '\0'; srg++, pos++)

    if (strspn(srg, s) == strlen(s))  

      return pos;

  return -1;

}

尋找字元中指定的字串內容處在位置

int charFind(char *s, char c, size_t st)

{

  BYTE pos;

  for (pos=st, s+=st; *s != '\0'; s++, pos++)

    if (*s == c)  

      return pos;

  return -1; 

}

複製字串
char *StrmnCpy(char *str, char *srg, size_t m, size_t n)
{
  int i=0;
  char *s;
 
  for(s=str, i=m, srg+=m; i<m+n; i++)
    *s++ = *srg++;
  *s++ = '\0';
   
  return str;
}