BTC Optical Mouse Hack
A while back, I happened upon the application note
"Interface to Optical Mouse Sensor" on the
Kronos Robotics website. It
discusses the application of a sensor from an optical mouse for use as a
position sensing coprocessor on a robot. The application note did not go
beyond the proof-of-concept stage. It did not describe how to use all of the
functions of the Agilent mouse sensor, which are available through a direct
interface to the mouse sensor. It turns out that there are many functions
available in the Agilent mouse sensor that cannot be accessed through the PS2
I didn't have easy access to the GE mouse described in the Kronos Robotics
application note. I had to find another optical mouse that employed an Agilent
optical mouse sensor. I also wanted to go beyond the proof-of-concept stage
and acutally use the mouse sensor in a robotics application. Before developing
that application, though, I first had to gain familarity with interfacing
directly to the Agilent optical mouse sensor and accessing its functions from
within my Forth environment. This webpage describes the hack I did to a BTC
optical mouse to enable direct control of the
(now Avago, see below) ADNS-2610 Optical Mouse Sensor and the Forth code
written to control it
Inside the BTC M850 3D Optical Mouse
BTC (Behavior Tech Computer) is a Taiwanese
manufacturer of low-cost computer peripherals and consumer electronics devices.
They have sold a wide range of mice, including standard ball mice, optical
mice, and (apparently) laser optical mice. The M850 is a three-button and
scroll wheel optical mouse. The packaging, illustrated to the right, calls
the mouse as a "3D Optical Mouse", though it really is justs a typical optical
mouse. Perhaps, the "3D" capabilities are derivied from the KeyMaestro
software on a floppy disk that (sometimes) comes with the mouse. I found this
mouse at Fry's Electronics and they usually have plenty in stock (2005 - 2006
time frame). While the Fry's price sticker shows a $6.99 price tag, it is
often on sale for $4.99 (one per customer please!). Note the "Agilent enabled"
label and the M850 model number at the top of the box front, as these are
important to ensuring that you get the correct BTC mouse. Note that Agilent
Technologies has spun off its optical mice components to
Avago Technologies, so if BTC continues
to use the same optical mouse sensors in the future, the packaging may change.
The M850 mouse has a rounded, ergonomic body. The right and left mouse buttons
are formed by a single, wrap-around plastic element that is flush to the body.
The mouse wheel extends out of the center-front of the mouse body. The wheel
is smooth, but easily rotated and pressed. I've seen two mouse color styles:
dark grey body and cord with a white mouse wheel and black body and cord with a
black mouse wheel. The plastic element for the mouse buttons is always silver
in color. The M850 box literature mentions the availability of mice with
either a PS2 connector, a USB connector, or a PS2 connector with a USB adaptor.
I've only seen the version with the PS2 connector sold at Fry's Electronics.
The bottom of the mouse is unremarkable. Just a label describing the product
and its compliance with various governing organization. Access to the guts of
the mouse is accomplished by first removing the two metal screws located on the
the right and left of the bottom rear of the mouse. The mouse button element
must also be released by unclipping it by pressing the hook on the far back end
of the bottom of the mouse. The base of the mouse is then removed by sliding
it back and away from the body shell of the mouse. Be careful that the mouse
button element does to re-clip itself back to the base of the mouse.
Once the mouse body shell is removed the guts of the mouse are clearly visible
and accessible. There is not much to look at. The printed circuit board
holding the mouse electronics is easily remove by unwrapping the mouse cord
from the built-in strain relief in the base of the mouse and lifting the
With the printed circuit board removed, you can see the
(Avago) HDNS-2100 Solid-State Optical Mouse Lens. The lens actually
consists of the focusing lens for the mouse sensor and a light prism that
directs the light from the illuminating LED onto the viewing surface.
The printed circuit board is a simple, single-sided affair. The microswitches
of the mouse buttons and the rotary encoder of the scroll feature are to the
front of the board. The electronics for the optical mouse sensor are to the
rear. The sensor itself figures prominently in the center. The clear plastic
clip is the
Agilent (Avago) HDNS-2200 Solid Statte Optical Mouse LED Assembly Clip. It
holds the LED at the correct position to shine into the light prism of the
HDNS-2200. The part of the clip over the mouse sensor acts as a plastic spring
that keeps the clip and lens assembly aligned. Note that this printed circuit
board supports two BTC products: the M850 and the M860 (4D Optical Mouse).
There are silk screen markings and solder pads for two more microswitches.
The circuit side of the printed circuit board is the typical maze of narrow
traces. One unique feature is the that the mouse controller processor chip is
on board (COB) technology. The chip is hidden under the round black blob
of epoxy at the center of the board. The other (obvious) unique feature is
the bottom of the optical mouse sensor. The small hole at the center of the
round extension of the sensor is the optical aperture. The light from the
illuminating LED reflects off the target surface and is focused by the lens
onto the sensor array through the optical aperture.
Modding the Mouse
With the mouse completely disassembled, it is now time to modify the wiring
to allow direct control of the optical mouse sensor. This is actually very
simple to do. The idea is to cut out the "middle man" (the mouse controller)
and wire the mouse cable's data and clock lines directly to the optical mouse
sensor. Refer to the circuit illustration to the right.
The red dots near the sensor indicate where traces are cut to decouple the
data and clock signals from the mouse controller. The red dots below the mouse
controller indicate the leads of two inductors (labeled L1 and L2 on the
component side of the board) that need to be unsoldered from the circuit. Note
that the other pins of the two inductors are also disconected from the traces
that lead to the cable's data line (white wire) and clock line (green wire).
These lead holes need to be cleared for the insertion of new wires.
The yellow lines indicate the new wiring between the mouse sensor's data and
clock lines and the mouse cable's data and clock lines. The wire attachment
point for the sensor's data signal is an unused lead hole that must be cleared
first. The wire attachment point for the sensor's clock signal is made by
scraping away some of the solder mask off the indicated trace and lap soldering
the wire to it.
The mouse controller is also removed by cutting away the epoxy blob and
removing the chip. This is most easily done by grinding the epoxy off using a
rotary tool. Before performing the modifications, it is strongly advised that
you cover the optical aperture of the mouse sensor to prevent foreign objects
(dust, solder, etc) from getting onto the sensor array or blocking the
aperture. I covered the aperature with a paper dot, created by a hole punch,
followed by tape to hold the paper dot in place.
The two pictures to the right show the finished modifications. I was almost
too agressive with the rotary tool while removing the mouse controller chip. I
succeeded in not only removing the epoxy and the chip, I almost removed the
underlying printed circuit board as well. Fortunately, I didn't need to use
the surrounding circuit traces. The unused microswitches and rotary encoder
were harvested from the mouse for use in other robotics projects. The wiring
was done using 30-gauge wire used for wire-wrapping.
In retrospect, I probably didn't need to remove inductors L1 and L2, though
doing so made it easier to attach the wires. The inductors are probably used
to condition the signals traveling on the mouse cable. The mouse cable was
originally about five feet in length. I shortened it to a bit less than three
feet in length. Losing the inductors didn't appear to have much of an impact
for my testing. YMMV.
I fashioned a new connector at the end of the shortened mouse cable to plug
into my robot. I won't show it here, but what you'll need to do is provide
5 volts power (red wire), ground (yellow wire), and the mouse data and clock
signals. If you want to use the original PS2 connector for this purpose, that
is fine. You may want to keep inductors L1 and L2 in place if you retain the
original cable length. Remember that the data and clock signals on the PS2
connector are now connected directly to the mouse sensor. After this mod, the
signals on the PS2 connector are no longer compliant to the PS2 mouse standard.
The mouse, once reassembled, is now ready for experimentation!
Optical Mouse Sensor Control
My testing of the mouse sensor was performed using a Harris RTXEB single
board computer, which is based on the Harris RTX2001A Real-Time Express
microcontroller. Consequently, the test code presented below is written in
Forth. I believe that the code is sufficiently commented that it could be
easily rewritten in another programming language.
The mouse sensor is accessed through a parallel port placed at address 0x1A
on the G-bus of the RTX2001A microcontroller. Only a few bits of the port are
actually used because of the synchronous serial data interface of the
ADNS-2610 optical mouse sensor. Listed below are the definitions of the bits
of the G-bus data (GD) used by the synchronous serial data interface:
|GD0 (write) - Serial Clock (\SCK)|
|The inverted serial clock used to shift data into and out
of the optical mouse sensor.|
|GD0 (read) - Serial Data Input (SDIO)|
|Input data read from the optical mouse sensor.
|GD1 - Data Direction Control|
|Flow control for data shifted to or from the optical mouse
sensor (0 input, 1 output).|
|GD7 - Serial Data Output (SDIO)|
|Output data written to the optical mouse sensor.
The Forth code is written using the built-in RTXEB Forth interpreter,
EBFORTH. While this interpreter is mostly compliant with the Forth 83
standard, some of the instructions are specific to the RTXEB. These are
described as follows:
G! ( n g -- )
|Writes a value n on the stack into a specified G-bus port
address g and removes the value from the stack.|
G@ ( g -- n )
|Read a value from a specified G-Bus port address g and
pushes it onto the stack.|
H ( -- addr)
|The address of a variable containing the next
available dictionary location for code.|
H-FENCE ( -- )
|The address of a variable containing the beginning address
of the user dictionary (the default is 0x4400).|
HEX ( -- )
|Sets the numeric input-output conversion base
R ( -- )
|The address of a variable containing the next available
memory location for variables.|
R-TOP ( -- )
|The address of a variable containing the maximum memory
address for variables (the default is 0x43FE).|
The Forth code listing defines the
Forth words used to implement the synchronous serial data interface that sends
and receives data from the ADNS-2610. The serial clock on bit GD0 is inverted
because the mouse sensorconsider an idle clock to be held at the high state.
For my implementation of the interface, it was easier to invert the signal to
assume a low level for the clock idle state. Data that is received from the
mouse sensor is delivered most-significant bit first. As sequential bits of
data are read in on bit GD0, they are left-shifted to move them into bits of
greater significance. Data that is sent to the mouse sensor are also delivered
most-significant bit first. Therefore, the data output is aligned to bit GD7.
As sequential bit are written to the mouse sensor, the next least-significant
bit is left-shifted so it is ready to be written to bit GD7 as well. The data
direction control on bit GD1 controls the direction of data written to or read
from the mouse sensor. The synchronous serial data bus is bidirectional.
A number of Forth words were written that encapsulate the operations needed
to exercise the various registers of the ADNS-2610. The words mirror the
register functions described in the ADNS-2610 documentation. With the words
the mouse sensor's operational state can be controlled and monitored, the
motion counts can be read, the image surface can be evaluated, and the
individual pixels of the image can be retrieved.
The animated GIF image to the right shows a sequence of three images taken with
the mouse sensor sitting on top of the "Group 3" lines of a USAF Lens Test
Chart. Please forgive the quality of the test chart, as it was created with a
laser printer. It is not a high-quality photographic film or a metal stencil.
Now that the optical mouse chip can be accessed and controlled directly, it
can be used to experiment with to develop new applications for robotics.
© 2006, 2007 Mac A. Cody
Last updated Friday, March 9, 2007