ardupilot/libraries/AP_GPS/AP_GPS_MTK.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_MTK.h"
#include <stdint.h>

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

// Public Methods //////////////////////////////////////////////////////////////
void 
AP_GPS_MTK::init(void)
{	
	_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;
	
	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;
	int		numc;
	bool	parsed = false;

	numc = _port->available();
	for (int 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
			long time_utc	= _swapl(&_buffer.msg.utc_time);
			long 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;
}