A long time a go , i interfaced an GPS receiver to an 16c74.
Because the 16c74 didn't had enough ram space i picked out of the
data stream from the GPS receiver only the bytes i needed.
Hope this code will help you to interface your gps receiver, sorry
for the Dutch comment and the not very structered way of programming.


// GPS-communication & synchronisation //

// prototyping
void gpscom(void);
void GPS1Hz_handler(void);
// void transmitbuffer_empty(void);

#use delay(clock=CRYSTAL)
#use RS232(BAUD=9600,XMIT=PIN_C6,RCV=PIN_C7)

  struct gps_data{
    byte sats;
    long utc_year;
    byte utc_month;
    byte utc_day;
    byte utc_hr;
    byte utc_min;
    byte utc_sec;
    long utc_nsec;
  };

  byte byte_cnt;
  byte word_cnt;

  union {
    byte _byte[2];
    long _word;
  } rx;

  long ck_sum;
  long msg_id;
  long msg_len;
  struct gps_data gpd;

//  byte last_byte ; // voor simulatie debugging

/*
#int_tbe
transmitbuffer_empty()
{

}
*/

#int_rda
gpscom()
{

  byte c;

  c = getc();

  if (byte_cnt == 0) {  // init structures;nieuwe message
    msg_id = 0;
    ck_sum = 0;
    if (c!=0xff) return;
  }


  byte_cnt++;       // received bytes #

  if (byte_cnt & 1) // oneven bytes (eerste, derde ...)
  {
    rx._byte[0] = c;    // alleen onthouden
  }
  else
  { word_cnt = byte_cnt >>1;
    rx._byte[1] = c;
    if (byte_cnt)
      ck_sum += rx._word;
    switch (word_cnt) {
        case 1 : if (rx._word != 0x81ff)
                     byte_cnt = 0;  // foute sync
         break;

        case 2 : msg_id = rx._word;
             break;

        case 3:  msg_len = rx._word;
             break;

        case 5 : // header compleet. controleer checksum
            if (ck_sum != 0)
                    byte_cnt =0;        // niet ok --> wacht op sync
                break;

        case 6 : ck_sum = 0;  // reset checksum voor data van message
    }

    if (word_cnt >= 6) // na header
    {
        if (msg_id == 1000)
          switch(word_cnt) {
            case 12: gpd.sats = rx._word;
                     break;
            case 19: gpd.utc_day =rx._word;
                     break;
            case 20: gpd.utc_month=rx._word;
                     break;
            case 21: gpd.utc_year=rx._word;
                     break;
            case 22: gpd.utc_hr  =rx._word;
                     break;
            case 23: gpd.utc_min =rx._word;
                     break;
            case 24: gpd.utc_sec =rx._word;
                     break;
            case 26: gpd.utc_nsec=rx._word;
                     break;
          }

        if ((word_cnt - 5) == msg_len)      // aantal woorden -
header = msg_len ?
        {
          if (ck_sum == 0)
          { // alles ok. doe er wat mee.


             if ((msg_id == 1000) && (gpd.sats >1)) // velden uit msg
1000 geldig, >1 sat.
             {
             }

           } else
           {
         byte_cnt = 0; // structure is niet geldig.
           }
        }
    }
  }
}

#int_ext
GPS1Hz_handler()
{
  timer1_interupt_flag=off;         // clear flag from previous
overflow and to enable next int.
  set_timer1(0xbfff+ (((gpd.utc_nsec + 0x7ff)/100)*93) );
//(0xbfff + 0x83e + (gpd.utc_nsec + (73*gpd.utc_nsec)/1000) );
  enable_interrupts(INT_TIMER1);
}


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