Introduction: Control an Arduino With a Wristwatch (TI EZ430 Chronos)

About: So many things to learn and make, so little time! I like things that are cool, useful, efficient, well crafted.

The watch
The Texas Instruments eZ430 Chronos is a cool gadget. It has bidirectional radio communication, and a whole bunch of sensors: temperature, pressure, acceleration.

In fact, it is a development kit for the MSP 430 (a low-power microcontroller), that TI packaged as a watch (pure genius!), and delivered with plenty of tools. Its firmware can be modified to implement new features into the watch.

Even without developing anything on the watch, it is a terrific toy:
- the watch alone is pretty cool --and has the TI logo ;-),
- the default features allow for a wide range of cool experimentations and developments around it.

The RF communication has a quite short range, and quite low consumption. It is not Bluetooth, but SimpliciTI[TM]  or BlueRobin[TM].

Pretty much documentation about the watch (sample code, specifications, schematics) can be found on the Web. Google is your friend.

Watch to PC
The watch comes along with an access point (i.e. the peer device the watch can talk to) in the form of a dongle, incorporating a TI chip that can, on one side, talk to the watch via RF, and on the other side, talk to the PC via an USB slave interface.

When the access point is plugged into a PC, a PC application provided by TI can exploit it to communicate with the watch: graph sensors, set the watch time, calibrate sensors, send fitness data. The nice thing is that TI wrote it in a scripting language (venerable Tcl/Tk) to run on multiple OS-es.

It is very easy to write your own PC application, as shown by some examples written in Python. There are numerous such examples that can be found on the web.

Watch to Arduino
What about using an Arduino instead of a PC? You could make completely pocket-sized projects!

Surprisingly, I could not find any clue about it on the web.

I evaluated several USB host libraries, and finally found the right application source code to get started (ACM on CDC, for modem, by Circuits@Home,; Oleg, thanks for the library, examples, and for the help!).

It finally works, it's very simple, and that's what I'm covering with the present post.

Just basic Arduino knowledge is required. However, this post is not an Arduino tutorial.

Enough said, let's get to the point!

Step 1: Needed Stuff

The watch
You can purchase the TI eZ430 Chronos watch from several resellers (e.g. Mouser, Farnell, or Amazon). It will come along with:
  • software and doc on a CDROM,
  • an USB dongle that is an RF access point,
  • an USB dongle to reprogram the watch, (we won't use it here),
  • a screwdriver to open the watch (we won't use it either * )
(*) but hey, when did you last get a screwdriver with a watch, and a manual explaining how --and encouraging to-- open it?

The Arduino
Any more or less standard Arduino will do, and the most important thing is to have USB host capability.

For this, the following configurations are among the possible ones:
  • Arduino (mine is Duemilanove) and an USB host shield (mine is from Circuits@Home)
        - or -
  • Arduino Mega ADK (important: must be ADK variant! it has USB host on board)

Step 2: The Arduino Sketch

Have USB host library, v2.0
Install this library into your Arduino system:
  • Important: use this 2.0 version of the library
  • On Linux, the library files will have to reside in
Customize the USB host library for your board
  • Edit the library file avrpins.h
    (on Linux: /usr/share/arduino/libraries/USB_Host_Shield_2_0/avrpins.h)
  • and uncomment one of the lines #define BOARD_MEGA_ADK or #define BOARD_TEENSY if your board is one of these. For say Uno or Duemilanove, leave them commented out.
This is pretty annoying, especially if you have some of each boards, because you must remember to check/tweak the library after changing of board and before compiling... I wish the #define could reside in the sketch file.

Create the Arduino sketch
Create an Arduino sketch from the file attached in this step. Compile, upload and run.

How it works
The initialization is done by instantiating these objects:
  USB Usb;                   // instantiate USB
  ACMAsyncOper AsyncOper;    // Create CDC
  ACM Acm(&Usb, &AsyncOper); // Create ACM and bind CDC to USB

Then, in the loop(), one shall call this to let the USB stack run:

To exchange data with the USB device:
  Acm.SndData(frame_len, frame_to_send);
  Acm.RcvData(&len, receive_buffer); // init len w/ buffer size, will be updated with len received

We need to send two different frames, one to turn the dongle's RF on, and one to request data from the watch. Then we will need to receive and decode one frame, containing the watch response (or absence thereof).

Step 3: Using This Demo

This demo application initializes the RF access point in SimpliciTI[TM] mode, then keeps on sending accelerometer queries to the watch, gets the response from the watch, filters out, interprets and displays the result.

The access point returns responses by such frames:
  0xFF 0x06 0x07 KK XX YY ZZ

  KK = 0xFF when there is no response (no watch in range or sending, or frame lost)
  KK = 0x01 when no buttons is pressed,
  KK = other values depending on buttons
  XX, YY, ZZ = when KK is not 0xFF: value of X resp. Y and Z acceleration, as 8 bit signed int

The Arduino uses its slave interface connected to a PC to get its power from the PC, and send result messages to a terminal.

On the picture, you can see the output while I moved the watch and pressed on the #, * and up buttons. Note that the watch does not debounce the buttons, so one press can generate several events. If this is an issue, debouncing has to be implemented on the Arduino side.

Step 4: Next Steps

Very next steps
It is easy to modify the sketch in order to control LEDs, servos, relays, etc. by using the ACC or PPt modes of the watch. Arduino-based home automation and robotics can now get controlled by the watch.

In the PPt mode, the X, Y and Z acceleration values are all zero, and KK = 0x12, 0x32, 0x22 upon button presses.

Further steps
There are other features that are easy to use in SimpliciTI[TM] mode (getting sensor, time and date data, calibrating sensors), by slightly modifying this sketch, and setting the watch in the Sync mode.

Also, in the BlueRobin[TM] mode, one can send fitness measurement data to the watch: the Arduino can become a sport sensor.

Even Further steps
Then, by modifying the watch firmware itself, one can create brand new modes, displays and functions on the watch. I did not explore this area yet, but examples can be found on the web. It requires slightly more advanced knowledge (C, MSP430, and IAR toolchain).

I hope this post was useful.
Now, get a watch, an Arduino with USB host, and make and post an awesome project!

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