ardupilot/libraries/AP_GPS/AP_GPS_UBLOX.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: t -*-
//
//  u-blox UBX 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.
//

#include "AP_GPS_UBLOX.h"
#include <stdint.h>

// Constructors ////////////////////////////////////////////////////////////////

AP_GPS_UBLOX::AP_GPS_UBLOX(Stream *s) : GPS(s)
{
}

// Public Methods //////////////////////////////////////////////////////////////

void
AP_GPS_UBLOX::init(void)
{
	// XXX it might make sense to send some CFG_MSG,CFG_NMEA messages to get the 
	// right reporting configuration.

	_port->flush();
	
	_epoch = TIME_OF_WEEK;
}

// Process bytes available from the stream
//
// The stream is assumed to contain only messages we recognise.  If it
// contains other messages, and those messages contain the preamble
// bytes, it is possible for this code to fail to synchronise to the
// stream immediately.  Without buffering the entire message and
// re-processing it from the top, this is unavoidable. The parser
// attempts to avoid this when possible.
//
bool
AP_GPS_UBLOX::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();

		switch(_step){

			// Message preamble 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 1:
			if (PREAMBLE2 == data) {
				_step++;
				break;
			}
			_step = 0;
			// FALLTHROUGH
		case 0:
			if(PREAMBLE1 == data)
				_step++;
			break;

			// Message header processing
			//
			// We sniff the class and message ID to decide whether we
			// are going to gather the message bytes or just discard
			// them.
			//
			// We always collect the length so that we can avoid being
			// fooled by preamble bytes in messages.
			//
		case 2:
			_step++;
			if (CLASS_NAV == data) {
				_gather = true;					// class is interesting, maybe gather
				_ck_b = _ck_a = data;			// reset the checksum accumulators
			} else {
				_gather = false;				// class is not interesting, discard
			}
			break;
		case 3:
			_step++;
			_ck_b += (_ck_a += data);			// checksum byte
			_msg_id = data;
			if (_gather) {						// if class was interesting
				switch(data) {
				case MSG_POSLLH:				// message is interesting
					_expect = sizeof(ubx_nav_posllh);
					break;
				case MSG_STATUS:
					_expect = sizeof(ubx_nav_status);
					break;
				case MSG_SOL:
					_expect = sizeof(ubx_nav_solution);
					break;
				case MSG_VELNED:
					_expect = sizeof(ubx_nav_velned);
					break;
				default:
					_gather = false;			// message is not interesting
				}
			}
			break;
		case 4:
			_step++;
			_ck_b += (_ck_a += data);			// checksum byte
			_payload_length = data;				// payload length low byte
			break;
		case 5:
			_step++;
			_ck_b += (_ck_a += data);			// checksum byte
			_payload_length += (uint16_t)data;	// payload length high byte
			_payload_counter = 0;				// prepare to receive payload
			if (_payload_length != _expect)
				_gather = false;
			break;

			// Receive message data
			//
		case 6:
			_ck_b += (_ck_a += data);			// checksum byte
			if (_gather)						// gather data if requested
				_buffer.bytes[_payload_counter] = data;
			if (++_payload_counter == _payload_length)
				_step++;
			break;

			// Checksum and message processing
			//
		case 7:
			_step++;
			if (_ck_a != data)
				_step = 0;						// bad checksum
			break;
		case 8:
			_step = 0;
			if (_ck_b != data)
				break;							// bad checksum

			if (_gather) {
				parsed = _parse_gps();			// Parse the new GPS packet
			}
		}
	} 
	return parsed;
}

// Private Methods /////////////////////////////////////////////////////////////

bool
AP_GPS_UBLOX::_parse_gps(void)
{
	switch (_msg_id) {
	case MSG_POSLLH:
		time		= _buffer.posllh.time;
		longitude	= _buffer.posllh.longitude;
		latitude	= _buffer.posllh.latitude;
		altitude	= _buffer.posllh.altitude_msl / 10;
		break;
	case MSG_STATUS:
		fix			= (_buffer.status.fix_status & NAV_STATUS_FIX_VALID) && (_buffer.status.fix_type == FIX_3D);
		break;
	case MSG_SOL:
		fix			= (_buffer.solution.fix_status & NAV_STATUS_FIX_VALID) && (_buffer.solution.fix_type == FIX_3D);
		num_sats	= _buffer.solution.satellites;
		break;
	case MSG_VELNED:
		speed_3d	= _buffer.velned.speed_3d;				// cm/s
		ground_speed = _buffer.velned.speed_2d;				// cm/s
		ground_course = _buffer.velned.heading_2d / 1000;	// Heading 2D deg * 100000 rescaled to deg * 100
		break;
	default:
		return false;
	}
	return true;
}