Hi all,
        I have been off the air for a few days.  First I will respond to
S_GROB's question:

> By the way, what exactly does the TSL213?

        The TSL213 is a 64x1 pixel sensing array.  It comes in an 8-bit
package with a long window exposing the 64 pixels.  When given the SI
(start integration?) signal it ends the previous integration and
transfers the accumulated charge from pixel #1 to the output buffer.  65
successive clocks will step the output through all 64 pixels and get
ready for the next sweep.  The output is analog but when the integration
time and light intensity are selected correctly the output meets digital
1/0 specifications.

        Now for Jaroslaw Lis' question:

>Is it better than standard way? I mean few parallel sensors and Gray coded
strip.

        Bear in mind that I have not used part yet.  I am designing to it
now.  From my understanding the beauty of this approach is that it gives
an absolute position at all times.  There is no position count to keep
track of because the non-repeating code always corresponds to an absolute
position on the strip.  Also the TSL213 costs about $5 where the encoders
I have looked at are in excess of $20 each.  I need 8 of these
sensors per design so ... do the math.

        One thing to keep in mind.  The 64x1 array will be used to
oversample the coded strips.  I figure I can use a 3-bit Binary Chain
Code which gives 8 (2^3) bits in the code.  Precisely the code I will use
is:

                00010111
                ^^^
                [ ] <-- first window

        There are exactly six 3-bit windows on to this pattern:

        000
        001
        010
        101
        011
        111

        It works out that one gets 2^n - n + 1 distinct windows on any n
bit code.

        If I oversample at 16 pixels per bit I can detect the starting
position of the 3-bit code to within one of 16 positions.  That give a total
of 96 (6x16) positions I can resolve.  I only need about 70.  If my edge
detection is a little too noisy I can go to a 4-bit code and get greater
resolution.

        I welcome feedback here.  I would rather spot a colossal boo boo
before I have hardware, but I think this will do what I want it to do.
Absolute position is all I need for this application.

        Thanks for all the discussion of this topic.

Martin Kirk
Arizona State University
mlk@asu.edu
(602) 263-9270