function cpdload(fname,devname)
% cpdload(fname,devname)
%
% DOWNLOAD RAW DATA FROM CROSSPAD INTO A FILE
%
% fname is the filename to write out to (if absent, temp name is generated)
% devname is the serial port, defaults to /dev/cua0
%  (note: you must have read AND write permission on this device)


ACK=[6];

if(nargin<2) devname='/dev/cua0'; end
if(nargin<1) fname=[]; end
if(isempty(fname)); fname=[tempname,'.cpd']; end

fid = fopen(fname,'w');    loadassert(fid>2); % check if file open was ok
sid = fopen(devname,'r+'); loadassert(fid>2); % check if port open was ok

fprintf(1,'Select "Upload Ink" on Crosspad now...\n');

[dd,cc] = fread(sid,6,'uchar'); loadassert(cc==6);
loadassert(dd(1)==2);
dd(2:5)
datlen = 2^24*dd(2)+2^16*dd(3)+2^8*dd(4)+dd(5); 
indat=0;
% dd(6) is some kind of checksum

cc=fwrite(sid,ACK,'uchar'); loadassert(cc==1);

[dd,cc] = fread(sid,1,'uchar'); loadassert(cc==1);
loadassert(dd==1);
while(dd==1)
  [dd,cc] = fread(sid,4,'uchar'); loadassert(cc==4);
  % dd(1) and dd(2) are some kind of block number checking
  ll = 256*dd(3)+dd(4); % data length for this block

  [dd,cc] = fread(sid,ll,'uchar'); loadassert(cc==ll); 
  loadassert(length(dd)==ll);
  cc=fwrite(fid,dd,'uchar'); loadassert(cc==ll);
  indat=indat+ll;

  [dd,cc] = fread(sid,1,'uchar'); loadassert(cc==1);
  % dd is some kind of checksum for data

  cc=fwrite(sid,ACK,'uchar'); loadassert(cc==1);
  [dd,cc] = fread(sid,1,'uchar'); loadassert(cc==1);
end

fclose(sid);
fclose(fid);

loadassert(indat==datlen);

fprintf(1,'Download from Crosspad successful!\n');
fprintf(1,'Wrote %d bytes to file %s.\n',indat,fname);

function loadassert(cond)

   if(~cond) error('Error downloading data from crosspad.'); end