ardupilot/libraries/AP_GPS/AP_GPS_MTK16.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: t -*-
//
//  DIYDrones Custom Mediatek GPS driver for ArduPilot and ArduPilotMega.
//	Code by Michael Smith, Jordi Munoz and Jose Julio, DIYDrones.com
//
//	This library is free software; you can redistribute it and / or
//	modify it under the terms of the GNU Lesser General Public
//	License as published by the Free Software Foundation; either
//	version 2.1 of the License, or (at your option) any later version.
//
//	GPS configuration : Custom protocol per "DIYDrones Custom Binary Sentence Specification V1.1"
//

#include "AP_GPS_MTK16.h"
#include <stdint.h>
#if defined(ARDUINO) && ARDUINO >= 100
	#include "Arduino.h"
#else
	#include <wiring.h>
#endif

// Constructors ////////////////////////////////////////////////////////////////
AP_GPS_MTK16::AP_GPS_MTK16(Stream *s) : GPS(s)
{
}

// Public Methods //////////////////////////////////////////////////////////////
void
AP_GPS_MTK16::init(enum GPS_Engine_Setting nav_setting)
{
    _port->flush();

    // initialize serial port for binary protocol use
    // XXX should assume binary, let GPS_AUTO handle dynamic config?
    _port->print(MTK_SET_BINARY);

    // set 4Hz update rate
    _port->print(MTK_OUTPUT_4HZ);

    // set initial epoch code
    _epoch = TIME_OF_DAY;
    _time_offset = 0;
    _offset_calculated = false;
    idleTimeout = 1200;
}

// Process bytes available from the stream
//
// The stream is assumed to contain only our custom message.  If it
// contains other messages, and those messages contain the preamble bytes,
// it is possible for this code to become de-synchronised.  Without
// buffering the entire message and re-processing it from the top,
// this is unavoidable.
//
// The lack of a standard header length field makes it impossible to skip
// unrecognised messages.
//
bool
AP_GPS_MTK16::read(void)
{
    uint8_t 	data;
    int16_t		numc;
    bool		parsed = false;

    numc = _port->available();
    for (int16_t i = 0; i < numc; i++) {	// Process bytes received

        // read the next byte
        data = _port->read();

restart:
        switch(_step) {

            // Message preamble, class, ID detection
            //
            // If we fail to match any of the expected bytes, we
            // reset the state machine and re-consider the failed
            // byte as the first byte of the preamble.  This
            // improves our chances of recovering from a mismatch
            // and makes it less likely that we will be fooled by
            // the preamble appearing as data in some other message.
            //
        case 0:
            if(PREAMBLE1 == data)
                _step++;
            break;
        case 1:
            if (PREAMBLE2 == data) {
                _step++;
                break;
            }
            _step = 0;
            goto restart;
        case 2:
            if (sizeof(_buffer) == data) {
                _step++;
                _ck_b = _ck_a = data;				// reset the checksum accumulators
                _payload_counter = 0;
            } else {
                _step = 0;							// reset and wait for a message of the right class
                goto restart;
            }
            break;

            // Receive message data
            //
        case 3:
            _buffer.bytes[_payload_counter++] = data;
            _ck_b += (_ck_a += data);
            if (_payload_counter == sizeof(_buffer))
                _step++;
            break;

            // Checksum and message processing
            //
        case 4:
            _step++;
            if (_ck_a != data) {
                _step = 0;
            }
            break;
        case 5:
            _step = 0;
            if (_ck_b != data) {
                break;
            }

            fix				= _buffer.msg.fix_type == FIX_3D;
            latitude		= _buffer.msg.latitude  * 10;	// XXX doc says *10e7 but device says otherwise
            longitude		= _buffer.msg.longitude * 10;	// XXX doc says *10e7 but device says otherwise
            altitude		= _buffer.msg.altitude;
            ground_speed	= _buffer.msg.ground_speed;
            ground_course	= _buffer.msg.ground_course;
            num_sats		= _buffer.msg.satellites;
            hdop			= _buffer.msg.hdop;
            date			= _buffer.msg.utc_date;

            // time from gps is UTC, but convert here to msToD
            int32_t time_utc	= _buffer.msg.utc_time;
            int32_t temp = (time_utc/10000000);
            time_utc -= temp*10000000;
            time = temp * 3600000;
            temp = (time_utc/100000);
            time_utc -= temp*100000;
            time += temp * 60000 + time_utc;

            parsed = true;

#ifdef FAKE_GPS_LOCK_TIME
			if (millis() > FAKE_GPS_LOCK_TIME*1000) {
				fix				= true;
				latitude		= -35000000UL;
				longitude		= 149000000UL;
				altitude		= 584;
			}
#endif

            /*	Waiting on clarification of MAVLink protocol!
            if(!_offset_calculated && parsed) {
				int32_t tempd1 = date;
				int32_t day 	= tempd1/10000;
            	tempd1 		-= day * 10000;
				int32_t month	= tempd1/100;
				int32_t year	= tempd1 - month * 100;
            	_time_offset = _calc_epoch_offset(day, month, year);
            	_epoch = UNIX_EPOCH;
            	_offset_calculated = TRUE;
            }
            */

        }
    }
    return parsed;
}