nmea.c

Go to the documentation of this file.

/*! \file nmea.c \brief NMEA protocol function library. */
//*****************************************************************************
//
// File Name    : 'nmea.c'
// Title        : NMEA protocol function library
// Author       : Pascal Stang - Copyright (C) 2002
// Created      : 2002.08.27
// Revised      : 2002.08.27
// Version      : 0.1
// Target MCU   : Atmel AVR Series
// Editor Tabs  : 4
//
// NOTE: This code is currently below version 1.0, and therefore is considered
// to be lacking in some functionality or documentation, or may not be fully
// tested.  Nonetheless, you can expect most functions to work.
//
// This code is distributed under the GNU Public License
//      which can be found at http://www.gnu.org/licenses/gpl.txt
//
//*****************************************************************************

#ifndef WIN32
    #include <avr/io.h>
    #include <avr/interrupt.h>
    #include <avr/pgmspace.h>
#endif
#include <string.h>
#include <stdlib.h>
#include <math.h>

#include "global.h"
#include "buffer.h"
#include "rprintf.h"
#include "gps.h"

#include "nmea.h"

// Program ROM constants

// Global variables
extern GpsInfoType GpsInfo;
u08 NmeaPacket[NMEA_BUFFERSIZE];

void nmeaInit(void)
{
}

u08* nmeaGetPacketBuffer(void)
{
    return NmeaPacket;
}

u08 nmeaProcess(cBuffer* rxBuffer)
{
    u08 foundpacket = NMEA_NODATA;
    u08 startFlag = FALSE;
    //u08 data;
    u16 i,j;

    // process the receive buffer
    // go through buffer looking for packets
    while(rxBuffer->datalength)
    {
        // look for a start of NMEA packet
        if(bufferGetAtIndex(rxBuffer,0) == '$')
        {
            // found start
            startFlag = TRUE;
            // when start is found, we leave it intact in the receive buffer
            // in case the full NMEA string is not completely received.  The
            // start will be detected in the next nmeaProcess iteration.

            // done looking for start
            break;
        }
        else
            bufferGetFromFront(rxBuffer);
    }
    
    // if we detected a start, look for end of packet
    if(startFlag)
    {
        for(i=1; i<(rxBuffer->datalength)-1; i++)
        {
            // check for end of NMEA packet <CR><LF>
            if((bufferGetAtIndex(rxBuffer,i) == '\r') && (bufferGetAtIndex(rxBuffer,i+1) == '\n'))
            {
                // have a packet end
                // dump initial '$'
                bufferGetFromFront(rxBuffer);
                // copy packet to NmeaPacket
                for(j=0; j<(i-1); j++)
                {
                    // although NMEA strings should be 80 characters or less,
                    // receive buffer errors can generate erroneous packets.
                    // Protect against packet buffer overflow
                    if(j<(NMEA_BUFFERSIZE-1))
                        NmeaPacket[j] = bufferGetFromFront(rxBuffer);
                    else
                        bufferGetFromFront(rxBuffer);
                }
                // null terminate it
                NmeaPacket[j] = 0;
                // dump <CR><LF> from rxBuffer
                bufferGetFromFront(rxBuffer);
                bufferGetFromFront(rxBuffer);

                #ifdef NMEA_DEBUG_PKT
                rprintf("Rx NMEA packet type: ");
                rprintfStrLen(NmeaPacket, 0, 5);
                rprintfStrLen(NmeaPacket, 5, (i-1)-5);
                rprintfCRLF();
                #endif
                // found a packet
                // done with this processing session
                foundpacket = NMEA_UNKNOWN;
                break;
            }
        }
    }

    if(foundpacket)
    {
        // check message type and process appropriately
        if(!strncmp(NmeaPacket, "GPGGA", 5))
        {
            // process packet of this type
            nmeaProcessGPGGA(NmeaPacket);
            // report packet type
            foundpacket = NMEA_GPGGA;
        }
        else if(!strncmp(NmeaPacket, "GPVTG", 5))
        {
            // process packet of this type
            nmeaProcessGPVTG(NmeaPacket);
            // report packet type
            foundpacket = NMEA_GPVTG;
        }
    }
    else if(rxBuffer->datalength >= rxBuffer->size)
    {
        // if we found no packet, and the buffer is full
        // we're logjammed, flush entire buffer
        bufferFlush(rxBuffer);
    }
    return foundpacket;
}

void nmeaProcessGPGGA(u08* packet)
{
    u08 i;
    char* endptr;
    double degrees, minutesfrac;

    #ifdef NMEA_DEBUG_GGA
    rprintf("NMEA: ");
    rprintfStr(packet);
    rprintfCRLF();
    #endif

    // start parsing just after "GPGGA,"
    i = 6;
    // attempt to reject empty packets right away
    if(packet[i]==',' && packet[i+1]==',')
        return;

    // get UTC time [hhmmss.sss]
    GpsInfo.PosLLA.TimeOfFix.f = strtod(&packet[i], &endptr);
    while(packet[i++] != ',');              // next field: latitude
    
    // get latitude [ddmm.mmmmm]
    GpsInfo.PosLLA.lat.f = strtod(&packet[i], &endptr);
    // convert to pure degrees [dd.dddd] format
    minutesfrac = modf(GpsInfo.PosLLA.lat.f/100, &degrees);
    GpsInfo.PosLLA.lat.f = degrees + (minutesfrac*100)/60;
    // convert to radians
    GpsInfo.PosLLA.lat.f *= (M_PI/180);
    while(packet[i++] != ',');              // next field: N/S indicator
    
    // correct latitute for N/S
    if(packet[i] == 'S') GpsInfo.PosLLA.lat.f = -GpsInfo.PosLLA.lat.f;
    while(packet[i++] != ',');              // next field: longitude
    
    // get longitude [ddmm.mmmmm]
    GpsInfo.PosLLA.lon.f = strtod(&packet[i], &endptr);
    // convert to pure degrees [dd.dddd] format
    minutesfrac = modf(GpsInfo.PosLLA.lon.f/100, &degrees);
    GpsInfo.PosLLA.lon.f = degrees + (minutesfrac*100)/60;
    // convert to radians
    GpsInfo.PosLLA.lon.f *= (M_PI/180);
    while(packet[i++] != ',');              // next field: E/W indicator

    // correct latitute for E/W
    if(packet[i] == 'W') GpsInfo.PosLLA.lon.f = -GpsInfo.PosLLA.lon.f;
    while(packet[i++] != ',');              // next field: position fix status

    // position fix status
    // 0 = Invalid, 1 = Valid SPS, 2 = Valid DGPS, 3 = Valid PPS
    // check for good position fix
    if( (packet[i] != '0') && (packet[i] != ',') )
        GpsInfo.PosLLA.updates++;
    while(packet[i++] != ',');              // next field: satellites used
    
    // get number of satellites used in GPS solution
    GpsInfo.numSVs = atoi(&packet[i]);
    while(packet[i++] != ',');              // next field: HDOP (horizontal dilution of precision)
    while(packet[i++] != ',');              // next field: altitude
    
    // get altitude (in meters)
    GpsInfo.PosLLA.alt.f = strtod(&packet[i], &endptr);

    while(packet[i++] != ',');              // next field: altitude units, always 'M'
    while(packet[i++] != ',');              // next field: geoid seperation
    while(packet[i++] != ',');              // next field: seperation units
    while(packet[i++] != ',');              // next field: DGPS age
    while(packet[i++] != ',');              // next field: DGPS station ID
    while(packet[i++] != '*');              // next field: checksum
}

void nmeaProcessGPVTG(u08* packet)
{
    u08 i;
    char* endptr;

    #ifdef NMEA_DEBUG_VTG
    rprintf("NMEA: ");
    rprintfStr(packet);
    rprintfCRLF();
    #endif

    // start parsing just after "GPVTG,"
    i = 6;
    // attempt to reject empty packets right away
    if(packet[i]==',' && packet[i+1]==',')
        return;

    // get course (true north ref) in degrees [ddd.dd]
    GpsInfo.VelHS.heading.f = strtod(&packet[i], &endptr);
    while(packet[i++] != ',');              // next field: 'T'
    while(packet[i++] != ',');              // next field: course (magnetic north)

    // get course (magnetic north ref) in degrees [ddd.dd]
    //GpsInfo.VelHS.heading.f = strtod(&packet[i], &endptr);
    while(packet[i++] != ',');              // next field: 'M'
    while(packet[i++] != ',');              // next field: speed (knots)

    // get speed in knots
    //GpsInfo.VelHS.speed.f = strtod(&packet[i], &endptr);
    while(packet[i++] != ',');              // next field: 'N'
    while(packet[i++] != ',');              // next field: speed (km/h)

    // get speed in km/h
    GpsInfo.VelHS.speed.f = strtod(&packet[i], &endptr);
    while(packet[i++] != ',');              // next field: 'K'
    while(packet[i++] != '*');              // next field: checksum

    GpsInfo.VelHS.updates++;
}

---