AP_GPS_MTK.ino

// -*- 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_MTK.h"
#include <stdint.h>

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

// Public Methods //////////////////////////////////////////////////////////////
void
AP_GPS_MTK::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 5Hz update rate
    _port->print(MTK_OUTPUT_5HZ);

    // set SBAS on
    _port->print(SBAS_ON);

    // set WAAS on
    _port->print(WAAS_ON);

    // Set Nav Threshold to 0 m/s
    _port->print(MTK_NAVTHRES_OFF);

    // set initial epoch code
    _epoch = TIME_OF_DAY;

    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_MTK::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 (MESSAGE_CLASS == data) {
                _step++;
                _ck_b = _ck_a = data;                                   // reset the checksum accumulators
            } else {
                _step = 0;                                                      // reset and wait for a message of the right class
                goto restart;
            }
            break;
        case 3:
            if (MESSAGE_ID == data) {
                _step++;
                _ck_b += (_ck_a += data);
                _payload_counter = 0;
            } else {
                _step = 0;
                goto restart;
            }
            break;

        // Receive message data
        //
        case 4:
            _buffer.bytes[_payload_counter++] = data;
            _ck_b += (_ck_a += data);
            if (_payload_counter == sizeof(_buffer))
                _step++;
            break;

        // Checksum and message processing
        //
        case 5:
            _step++;
            if (_ck_a != data) {
                _error("GPS_MTK: checksum error\n");
                _step = 0;
            }
            break;
        case 6:
            _step = 0;
            if (_ck_b != data) {
                _error("GPS_MTK: checksum error\n");
                break;
            }

            fix                 = _buffer.msg.fix_type == FIX_3D;
            latitude            = _swapl(&_buffer.msg.latitude)  * 10;
            longitude           = _swapl(&_buffer.msg.longitude) * 10;
            altitude            = _swapl(&_buffer.msg.altitude);
            ground_speed        = _swapl(&_buffer.msg.ground_speed);
            ground_course       = _swapl(&_buffer.msg.ground_course) / 10000;
            num_sats            = _buffer.msg.satellites;

            // time from gps is UTC, but convert here to msToD
            int32_t time_utc    = _swapl(&_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;
        }
    }
    return parsed;
}

/*
  detect a MTK GPS
 */
bool
AP_GPS_MTK::_detect(uint8_t data)
{
	static uint8_t payload_counter;
	static uint8_t step;
	static uint8_t ck_a, ck_b;

	switch(step) {
        case 1:
            if (PREAMBLE2 == data) {
                step++;
                break;
            }
            step = 0;
        case 0:
			ck_b = ck_a = payload_counter = 0;
            if(PREAMBLE1 == data)
                step++;
            break;
        case 2:
            if (MESSAGE_CLASS == data) {
                step++;
                ck_b = ck_a = data;
            } else {
                step = 0;
            }
            break;
        case 3:
            if (MESSAGE_ID == data) {
                step++;
                ck_b += (ck_a += data);
                payload_counter = 0;
            } else {
                step = 0;
            }
            break;
        case 4:
            ck_b += (ck_a += data);
            if (++payload_counter == sizeof(struct diyd_mtk_msg))
                step++;
            break;
        case 5:
            step++;
            if (ck_a != data) {
                step = 0;
            }
            break;
        case 6:
            step = 0;
            if (ck_b == data) {
				return true;
            }
	}
    return false;
}