readtracks.m

基于串行接口的GPS驱动程序

function tracks = ReadTracks(gps)
% ReadTracks Read all the track data from the GPS

global pid_command_data cmd_transfer_trk pid_xfer_cmplt pid_trk_hdr
global pid_ack_byte pid_nak_byte pid_trk_data

tracks = [];

% Tell the GPS to send the track data to us
WritePacket(gps, pid_command_data, cmd_transfer_trk, 'int16');

% Read standard beginning packet -- the count is the number of packets to
% follow. This stream of packets may consist of multiple tracks.
[id, packets] = ReadPacket(gps, 'int16');

% The latitude and longitude are stored on the GPS as "semicircles", where
% there are 2^31 semicircles in 180 degrees. We want to return the data as
% degrees, so compute a conversion factor.
semi2deg = (180 / (2^31));

% Until the transfer is complete
n = 0;
count = 0;
while (pid_xfer_cmplt ~= id)
    count = count + 1;
    [id, sz] = ReadPacketHeader(gps);
    csum = -1;
    disp(['Reading record ' num2str(count) ' of ' num2str(packets+1) '.']);
    
    if (pid_trk_hdr == id)
        % Resize the previous track, if necessary
        if (n > 0)
            tracks(n).waypoints = tracks(n).waypoints(1:w);
        end
        
        % Start a new track
        n = n + 1;
        w = 0;
        
        % Read the header data, which consists of a numeric identifier and
        % a null-terminated comment 
        tracks(n).display = fread(gps, 1); 
        tracks(n).color = fread(gps, 1);
        term = get(gps, 'Terminator');
        set(gps, 'Terminator', 0);
        tracks(n).comment = fgets(gps);
        set(gps, 'Terminator', term);
        
        % Allocate space for the track packets
         tracks(n).waypoints(packets) = struct('lat', [], 'long', [], 'time', [], ...
                                               'alt', [], 'depth', [], 'newtrk', []);
        
        % Compute the checksum
        csum = ComputeChecksum(id, [tracks(n).display tracks(n).color tracks(n).comment 0]);
        
        disp(['Starting new track. Name: "' tracks(n).comment '"']);
        
    elseif (pid_trk_data == id)
        % Read track log data into the current track
        w = w+1;
        
        % Read the waypoint data
        [ tracks(n).waypoints(w).lat, d ] = ReadData(gps, id, 1, 'int32');
        data = d;
        tracks(n).waypoints(w).lat = tracks(n).waypoints(w).lat * semi2deg;
        
        [ tracks(n).waypoints(w).long, d ] = ReadData(gps, id, 1, 'int32');
        data = [ data d ];
        tracks(n).waypoints(w).long = tracks(n).waypoints(w).long * semi2deg;
        
        [ tracks(n).waypoints(w).time, d ] = ReadData(gps, id, 1, 'uint32');
        data = [ data d ];
        
        [ tracks(n).waypoints(w).alt, d ] = ReadData(gps, id, 1, 'float32');
        data = [ data d ];
        % Convert altitude from meters to feet
        tracks(n).waypoints(w).alt = tracks(n).waypoints(w).alt * 3.28;
        
        [ tracks(n).waypoints(w).depth, d ] = ReadData(gps, id, 1, 'float32');
        data = [ data d ];
        
        [ tracks(n).waypoints(w).newtrk, d ] = ReadData(gps, id, 1, 'uint8');
        data = [ data d ];
       
        % Skip three mysterious bytes
        d = ReadData(gps, id, 3, 'uint8');
        data = [ data d ];

        % Compute the checksum
        csum = ComputeChecksum(id, data);
        
    elseif (pid_xfer_cmplt == id)
        % Resize the current track
        tracks(n).waypoints = tracks(n).waypoints(1:w);
        
        % Read the data in the pid_xfer_cmplt packet
        data = ReadData(gps, id, sz, 'uint8');
        csum = ComputeChecksum(id, data);
    else
        WritePacket(gps, pid_nak_byte, id);
        error(['Unexpected packet id ' num2str(id) ' while reading track log.']);     
    end
    
    cksum = ReadPacketTerminator(gps);
    if (csum ~= cksum)
        WritePacket(gps, pid_nak_byte, id);
        error(['Checksum mismatch in ReadTracks. Expected: ' num2str(csum) ', Received: ' num2str(cksum)]);
    else
        WritePacket(gps, pid_ack_byte, id);
    end
end