ardupilot/libraries/AP_GPS/AP_GPS_SIRF.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: t -*-
//
//  SiRF Binary GPS driver for ArduPilot and ArduPilotMega.
//	Code by Michael Smith.
//
//	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_SIRF.h"
#include "WProgram.h"

// Initialisation messages
//
// Turn off all messages except for 0x29.
//
// XXX the bytes show up on the wire, but at least my test unit (EM-411) seems to ignore them.
//
static uint8_t	init_messages[] = {
	0xa0, 0xa2, 0x00, 0x08, 0xa6, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xa8, 0xb0, 0xb3,
	0xa0, 0xa2, 0x00, 0x08, 0xa6, 0x00, 0x29, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0xd0, 0xb0, 0xb3
};

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

// Public Methods //////////////////////////////////////////////////////////////
void AP_GPS_SIRF::init(void)
{	
	_port->flush();

	// For modules that default to something other than SiRF binary,
	// the module-specific subclass should take care of switching to binary mode
	// before calling us.

	// send SiRF binary setup messages
	_port->write(init_messages, sizeof(init_messages));
}

// 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.
//
void AP_GPS_SIRF::update(void)
{
	byte data;
	int numc;

	numc = _port->available();
	while(numc--) {

		// 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 length
			//
			// We always collect the length so that we can avoid being
			// fooled by preamble bytes in messages.
			//
		case 2:
			_step++;
			_payload_length = (uint16_t)data << 8;
			break;
		case 3:
			_step++;
			_payload_length |= data;
			_payload_counter = 0;
			_checksum = 0;
			break;

			// Message header processing
			//
			// We sniff the message ID to determine whether we are going
			// to gather the message bytes or just discard them.
			//
		case 4:
			_step++;
			_accumulate(data);
			_payload_length--;
			_gather = false;
			switch(data) {
			case MSG_GEONAV:
				if (_payload_length == sizeof(sirf_geonav)) {
					_gather = true;
					_msg_id = data;
				}
				break;
			}
			break;

			// Receive message data
			//
			// Note that we are effectively guaranteed by the protocol
			// that the checksum and postamble cannot be mistaken for
			// the preamble, so if we are discarding bytes in this
			// message when the payload is done we return directly
			// to the preamble detector rather than bothering with
			// the checksum logic.
			//
		case 5:
			if (_gather) {						// gather data if requested
				_accumulate(data);
				_buffer.bytes[_payload_counter] = data;
				if (++_payload_counter == _payload_length)
					_step++;
			} else {
				if (++_payload_counter == _payload_length)
					_step = 0;
			}
			break;

			// Checksum and message processing
			//
		case 6:
			_step++;
			if ((_checksum >> 8) != data) {
				_error("GPS_SIRF: checksum error\n");
				_step = 0;
			}
			break;
		case 7:
			_step = 0;
			if ((_checksum & 0xff) != data) {
				_error("GPS_SIRF: checksum error\n");
				break;
			}
			if (_gather) {
				_parse_gps();					 // Parse the new GPS packet
			}
		}
	} 
}

void
AP_GPS_SIRF::_parse_gps(void)
{
	switch(_msg_id) {
	case MSG_GEONAV:
		time			= _swapl(&_buffer.nav.time);
		//fix				= (0 == _buffer.nav.fix_invalid) && (FIX_3D == (_buffer.nav.fix_type & FIX_MASK));
		fix				= (0 == _buffer.nav.fix_invalid);
		latitude		= _swapl(&_buffer.nav.latitude);
		longitude		= _swapl(&_buffer.nav.longitude);
		altitude		= _swapl(&_buffer.nav.altitude_msl);
		ground_speed	= _swapi(&_buffer.nav.ground_speed);
		// at low speeds, ground course wanders wildly; suppress changes if we are not moving
		if (ground_speed > 50)
			ground_course	= _swapi(&_buffer.nav.ground_course);
		num_sats		= _buffer.nav.satellites;
		_setTime();
		valid_read = 1;
		break;
	}
	new_data = true;
}

void
AP_GPS_SIRF::_accumulate(uint8_t val)
{
	_checksum = (_checksum + val) & 0x7fff;
}