As seen in this datasheet (and similar ones)
http://www.mcuic.com/bookpic/200811516244620817.pdf
we see a 6MHz clock scaled up to 18MHz and a 6502 core together with some hardware to support USB and tracking the optical flow. Looks like the optical sensor is also integrated onto the same chip.

No mention of any kind of programming sequence? No mention of any program memory for that matter. Intriguing. Besides what it's made for, what could you do with it...

Indeed, unless there are exploitable bugs (as there were in some LCD photo keyrings made with 6502 cores) it's very much a single-use device. I did go so far as to buy a couple of those keyrings, but one had a dud battery and the other didn't work with the exploit code I had, so I returned the pair of them.

If it were mounted on a mobile platform, such as a simple robot, it could track the stationary surface underneath in order to provide navigation data. The flip side of that coin is applications where the sensor is stationary but the adjacent medium is in motion.

Admittedly, it's irksome not to be able to tweak and meddle with the 6502 goodness inside!

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Any info on where I could get hold of one of these?

Here's what I have, but this was 2008/2009 so things will probably have changed, and some of the links may now be dead:

The chip was ST2205U from sitronix
http://www.sitronix.com.tw/sitronix/pro ... enDocument (https://archive.is/4RRyp)

One brand with the right chip at the time was Coby.
"coby keyring project, 6MHz 6502-like processor in a small battery powered LCD display usb-slave device."

A possible other match was sold as
Yuraku Photo 1.5" Digital Photoframe Keyring

"So for a tenner you get a battery powered 6502 computer with colour lcd display and a usb interface, in a nice consumer case"
http://blog.makezine.com/archive/2008/0 ... 6B48984890
http://spritesmods.com/?art=picframe
http://picframe.spritesserver.nl/wiki/i ... /Main_Page

You might be able to find the datasheet if you search for ST2205U_datasheet_v1.0.pdf

A google search for one specific model
http://www.google.co.uk/search?q=Coby+DP-151

Note that models do change. at http://picframe.spritesserver.nl/wiki/i ... ch_chipset we see a note to the effect that the ones with a driver CD are a good bet. One type of wrong kit is based on "AX203 basically a souped up 8051" "suspiciously similar to the CPF5102" and presents itself as a CDROM perhaps containing "DPFMate.exe"

Long comment thread here
"Sprites mods: Use a cheap digital photoframe as a second display for your PC - Result"
http://spritesmods.com/?art=picframe&pa ... owall=true
which contains some drivel and some pointers

The hack, at one time, demands a device bigger than 1M flash

There's a project wiki at
http://picframe.spritesserver.nl/wiki/i ... /Main_Page

The hardware we're looking for should show up as
Bus 002 Device 002: ID 1403:0001 Sitronix Digital Photo Frame
which just might be (as of jan 2009) Yuraku 1.5" digital photo-frame

This one from amazon comes with cdrom and may be the right stuff
http://www.amazon.co.uk/Yuraku-Photo-Di ... =de_a_smtd
because this chap posted the usb id:
https://lists.ubuntu.com/archives/ubunt ... 16250.html

Relevant web pages:
http://picframe.spritesserver.nl/wiki/i ... hotoviewer
http://code.google.com/p/photokeychain/source/checkout
http://www.neophob.com/serendipity/inde ... -Hack.html
http://sitronix-dpf.svn.sourceforge.net ... dpf/trunk/
http://code.google.com/p/photokeychain/ ... id=3&can=1
http://picframe.spritesserver.nl/wiki/i ... _unbranded

Well, it looks like a modular system on a chip and a rather complex one too. Add a second 6502 with a tube interface and you get a real nice machine. Some have computers fit into the keyboard - here you can fit it into the mouse.