Posts: 55
Registered: ‎09-11-2012

OTM-02 - Open source Time Machine #2


OTM-02 is an open source watch module designed using KiCad. The project has a strong focus on using readily available off-the-shelf recent or latest technology, from global suppliers. That sentiment seems like it should be quite obvious, however it was only six months ago that the MemoryLCD which this module has in common with the SlimWatch project was even available as samples to most people. For the last few years I have eagerly awaited this period in time when really cool ultra-low power technology is readily available to the masses - so it's only right that a huge thanks goes out to Sharp Microelectronics for developing the MemoryLCD technology and that an equally huge thanks goes to Energy Micro for not only supplying their ultra low power range of MCUs, but for having a level of customer support in place that even a relative novice like me feels they might just about be able to make something quite exciting. Within the Energy Micro team I would like to pay special thanks to Anders, Filip and Adam, without whose heavy-lifting work they've done with code for MemoryLCD technology, my current MemoryLCD code would not be as efficiently integrated with EM's MCUs. Right enough waffle, you're here about a watch...


Only an early development prototype exists - no prototype PCB has been manufactured. A prototype should be made by the end of Feb 2013.


  • Create a watch module that enables the end user to have complete control of its functionality and user interface,

  • Discover the benefits - and any detractions - of (potentially selling) a product developed from the outset in true open source hardware style,

  • Self-education,

  • Discover how much technology can be crammed into a wristwatch designed - and possibly manufactured - 'on the living room table',

  • Design a module that with only a little extra effort, could form the basis of other devices, eg cycle computer, dive computer, data logger, etc.


    The project utilises the following parts and features:

  • EFM32LG332F256 - A USB enabled, Energy Micro ultra low power Leopard Gecko,

  • A 128x128 pixel (23.2 x23.2mm visible area) ultra low power Memory LCD from Sharp Microelectronics,

  • Recharge and programming via USB Micro B connector,

  • Programming via ARM Serial Wire Debug (SWD) protocol also available,

  • 150mAh Li-Po Battery + on-board battery fuel gauge IC,

  • Vibratory Motor,

  • Piezo Electric diaphragm,

  • Five miniature right-angle tact switch buttons, (By MCU pin assignment and feature-set native to the EM MCU, with minor board layout editing, provision exists for alternate UI methods such as capacitive touch buttons.),

  • Provision for a LED based planar light-guide type back light (utilising a 0.4mm high right-angle Avago ChipLED and laser engraved light guide. (N.B. This is highly experimental and yet to be fully developed. Any derivatives of OTM-02 with commercial intent should be aware this technology is heavily patented.)

  • Possible feature additions in modules developed subsequently include: accelerometer, Bluetooth LE, ANT+, GPS

    Module Dimensions

    LCD and primary PCB area: 30.5 x 26.8 mm Including protruding Micro USB connector: 30.5 x 30.75 mm

    Thickness: 6.5 to 7mm, including battery and piezoelectric diaphragm

    Note: PCB is a four layer board and thickness is 1mm. Minimum track thickness is 0.125mm, used in connection with controlled impedance required for USB data lines. Most tracks are 0.15 or wider.


    A non-waterproof, DIY 3D printing optimised case design is at an intermediate stage of development:

    Other cases designed to be 3D printed in metal or other high-resolution materials are also being worked on as time permits.

    The intention is to publish the source CAD data for at least one DIY 3D printable case design before the end of April 2013, earlier if possible.

    Files and Images

    The launch of this project on github is suffering a temporary setback due to internet connection issues. In the meantime, the initial release PCB design files and more images are all available at

    A big thank you to the following people and organisations

    (without whose experience and guidance this would have taken much, much longer.)

    Mark Burton -

    Chris Middlemiss - Würth Elektronik

    Paul Oliver - Würth Elektronik

    Anders, Filip and Adam - Energy Micro

    Avnet Embedded



    And special thanks for putting up with my often ironic obsession with watch making: My family, friends and colleagues.


    CC-BY-SA 3.0

    OTM-02 is Open Source Hardware licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License and, excluding any proprietary files subject to third party copyright, will be open source firmware, probably released under the GNU Lesser General Public License v3.0 You are strongly encouraged to improve upon and hack OTM-02.


    Will be posted here as time permits. Also keep an eye on the project repo at Github:


    05 February 2013 - 01:35 AM: Image of PCB Rev.K replaced with Rev.L to reflect KiCad initial public release design data available on Github.

    Latest edit: Reference to 'JTAG' programming corrected to read 'SWD'.
  • Community Moderator
    Posts: 451
    Registered: ‎05-14-2012

    OTM-02 - Open source Time Machine #2

    Wow!! Now that is cool :-) I have to ask of course, do you have any indication of batterylife/charge cycle for this design? We have of course done a few things with the MemoryLCD ourselves here, but it is interesting to hear how it all adds up with a full watch application in the mix as well.

    And thanks for sharing this with us!
    Community Moderator
    Posts: 457
    Registered: ‎06-01-2012

    OTM-02 - Open source Time Machine #2

    This looks fantastic! You even made room for a vibration motor! If just running a simple watch application on the EM mcu I would guess that the 150mAh could last several years... were it not for self discharge. My ideas for feature creep includes wireless charging, bluetooth LE of course, and capacitive touch. I'm fine with charging my watch every week as long as it is simple, just leave it on a charging pad, no wires, no hassle. :-) Really cool project hairykiwi ! hats off to you!
    Expert Employee
    Posts: 238
    Registered: ‎09-07-2012

    OTM-02 - Open source Time Machine #2

    My views are my own and do not necessarily represent the views of Silicon Labs
    Posts: 42
    Registered: ‎10-26-2012

    OTM-02 - Open source Time Machine #2


    Why you didn't submit it to contest?
    Posts: 55
    Registered: ‎09-11-2012

    OTM-02 - Open source Time Machine #2

    Thank you all for your interest, encouragement and support for this project. And apologies for my slowness to reply - I'm occasionally without a decent internet connection for a few days at a time. And then life's pretty good at getting in the way too.

    Quick update: The initial public release files (Rev.L) are now available on github:

    Just be aware that you'll need a recent version of KiCad to view or modify them. The schematic might load in the KiCad current stable release (kicad-2012-01-19-BZR3256), however the PCB itself will almost certainly not.

    As recommended by AdamSch in another conversation, the schematic will appear on, just as soon as the guys at upverter have modded their conversion script to import the newer KiCad file format.

    uSasha - Thank you :-) - To be honest, back in September I was so deeply engrossed in designing the case you see in my first post - for showing the 3D printing bureaus exhibiting at the TCT Show - that I completely missed the competition. C'est la vie. I only discovered your project a few weeks ago - and much kudos to you! (initially, I just wanted a watch that behaved the way I wanted. I certainly never thought of adding an accelerometer to enable the kinds of bio-analysis your project aims to provide.) So after reading the rules, I decided any submission I made would be too close in its intent to yours. Also, I believe a product is ready when it is - so the competition is a bit of a time distraction. That maybe sounds a bit arrogant, but actually I'm just trying to manage my time the best I can. If you can utilise any aspect of OTM-02 for your own project - please do, I'd be delighted. Equally, I think it's possible that by ditching the LED backlight mosfet on OTM-02, and if a backlight is needed, driving the LED direct (with PWM), just enough PCB area could be freed up on OTM-02 to add one of the accelerometers mentioned on your project page. The physical challenge as far as I can see, would be getting the required SPI lines past the power supply ICs. Signal integrity wise - I'm completely in the dark.

    Anders and AdamSch - I can't thank you enough for all your tech support, suggestions and ideas over the last 18 months of experimenting, learning and developing. The aspect that took the longest with this project was finding the technology and then the companies who sell the technology off the shelf - so I was was really happy to be able to fit a vibratory motor in the design - perfect for an alarm system, either when working in a noisy environment where a piezoelectric buzzer can't be heard - or where a discrete alarm is required. I agree, charge pad technology + Thinergy-type battery technology is going to be really exciting in the years ahead. Also relevant and very interesting are the micro energy harvesting systems you brought to my attention a while back: body temperature powered devices will be very awesome.

    Rasmus - The short answer is no - I don't yet have an accurate indication of battery life/charge cycle for this design. There are too many variations, additional loads and quiescent currents to consider when comparing the dev prototype (below) with the OTM-02 module. But I do have a ballpark expectation of battery life of two to 4 weeks with a 1Hz display refresh. This is based on the STK3300 + 3V coin cell powered dev prototype below, which uses the earlier, but more efficient, 96 x 96 pixel 5V/3V Memory LCD LS013B4DN01. With no correction included for battery self discharge, using the STKs AEM feature, (and with my own inefficient code for driving the Memory LCD) I estimated battery life of 2 years for one display refresh per minute, and 2 months at 1Hz refresh.

    By comparison, OTM-02 uses two sub-uA-quiescent-current LDO linear regulators to drop the (nominal) 4.2V supplied by the Li-Po battery - one is always on to supply 3V VMCU, the other, for backlight and vibratory motor is enabled as required. The battery life calculation is further complicated by changing voltage regulator efficiency, which starts around 70% for a fully charged battery (3V / 4.2V x 100%) and increases as battery voltage drops toward VDO (≅ 25mV).

    So how does this look:

    Assume 70% of battery energy is available (should be worst case, but still excluding self-discharge)

    150mAh x 0.7 = 105mAh

    Power requirement to display static image on 128 x128 Memory LCD = 60uW

    60uW / 3V = 20uA

    ∴ display refreshed every minute ≅ 25uA average (guesstimate, based on prior experience)

    105mAh / 25uA = 4200h

    4200h / 24h/day = 175 days

    1Hz refresh vs 1 min refresh, factor = 12 (based on empirical evidence - and best recollection - could be be better or worse)

    175 days / 12 = 14.58 days

    Now 14.58 days, according to a study Anders or AdamSch shared some time back, is about 7.5 days longer than most people need to loose a charger. Which is another good reason for including a micro USB connector in the watch itself, rather than depending on some kind of proprietary nonsense connector that none of your friends or colleagues ever has in an emergency. Speaking of Micro USB connectors, it will also be quite exciting when this splashproof MicroUSB connector from TE becomes more readily available.

    Perhaps someone can share some good rules of thumb for applying average discharge-cycle battery voltage to the above calcs? In any case I'll do some tests as soon as I've built a prototype.

    Also, I did try finding an ultra-low power buck regulator as they tend to be more efficient. However, the best low power ones I found still had a quiescent current of around 30uA - which would more than double the average current measurements I've seen, and so efficiency would actually drop to 50% or less. More recently, I've wondered if a hybrid buck-linear regulator might be worth experimenting with - where the MCU is used to control the switching mosfet (fully on at reset) connecting battery to linear regulator via inductor - but I'm getting out of my depth now.

    As much as I like the idea of aiming for a watch with long battery life, (24 months) because of what the module was intended to be used for - an instrument rather than jewellery - I don't think battery life is the major issue. However there's no reason why battery life couldn't be extended significantly by sleeping the display when the watch is either not moving, (requires accelerometer) or between certain hours set by the user i.e night time, or by user-determined time-out. Display wakeup could be by shake of the wrist (with accelerometer) or button press.

    - Hamish

    (I'm not a qualified engineer, so please question anything that looks like nonsense - it could well be.)
    Regular Contributor
    Posts: 197
    Registered: ‎09-16-2012

    OTM-02 - Open source Time Machine #2

    hairykiwi wrote:

    Also, I did try finding an ultra-low power buck regulator as they tend to be more efficient. However, the best low power ones I found still had a quiescent current of around 30uA

    Linear Technology LTC3388, 0.72 uA quiescent current

    Linear Technology LTC3104, 2.6 uA quiescent current

    Linear Technology LT3991, 2.8 uA quiescent current
    Posts: 55
    Registered: ‎09-11-2012

    OTM-02 - Open source Time Machine #2

    Thanks very much brouhaha - those nanopower buck regulators look really interesting! On initial look I was very tempted - with the LTC3388 especially - but I do have a few reservations:

  • Ancillary component count would be increased by two or three,

  • The minimum inductor value of 22uH suggested in the datasheet would require a suggested component of size 3.2 x 3.2 x 2 mm which itself is bigger than one of the linear regulator IC currently selected,

  • The net increase in efficiency that might be expected is in the order of 15 - 20% by changing to a buck regulator. resulting in an approximate increased battery life of 2.9 days for the 1Hz refresh example quoted earlier.

    Given the above, I'd possibly need to ditch both linear regulators. So the question is then, 'Is it good practice or even advisable to power the vibe-motor (with typical intermittent draw of 75mA) from the VMCU supply?'

    Also, I've just been discussing my rough Li-Po battery / linear regulator efficiency calcs with a design engineer friend, and he had the following thoughts to share: In one of his his products, he considers his Li-Po battery to be charged at 4.15V and discharged at 3.55V - and the voltage fall to be reasonably linear within in that range.

    So (compared to my earlier worst case assumption of 70% efficiency) approximate efficiency might increase as follows:

    Approximate average voltage during battery discharge is:

    3.55 + ((4.15V - 3.55V) / 2) = 3.85V

    ∴ approximate efficiency is:

    3.0V / 3.85V = 0.78%

    That's nearly +1.5 days run time over the 70% efficiency figures - or half the increase a buck regulator might provide.

    So the buck vs linear regulator issue is looking like it's not really worth worrying about - but If I'm way out somewhere, please say.


  • Community Moderator
    Posts: 457
    Registered: ‎06-01-2012

    OTM-02 - Open source Time Machine #2

    In my opinion a linear regulator is fine, for now at least. I consider the space constraints in a watch application more important than longer battery life.

    What I am curious about is the memory lcd consumption estimates... Have you really measured it to be around 20 uA, or is that a datasheet number? Was this with the lowest possible refresh-rate of the com-inversion pin ( 0,5 - 1 Hz)? What we have found while working on the application note an0048, is that consumption when showing a static image shouldn't be much more than 200-500nA of the display alone. You can actually leave this com-inversion pin toggling at 1 Hz, even if you update the display faster. We just configured the LETIMER to toggle the com-inversion pin of the memoryLCD at 1 Hz, no need to worry about it in software. I think the 60 uW number in the datasheet is for 60Hz toggling of this pin... correct me if I'm wrong!

    Total consumption of the software in an0048, displaying a digital watch face with a few lines updated every second, consumes only 4-5 uA, -> should give batterylife of more than 2 years (given 70% discharge, but not taking self discharge of lipo into account, probably more accurate for non rechargeable lithium cells, than lipo.)
    Posts: 55
    Registered: ‎09-11-2012

    OTM-02 - Open source Time Machine #2

    Thanks Anders - that's fantastic news! All my calcs were based on my previous experience with the earlier Memory LCD, my own lousy (inefficient) code, my bad memory, and a desire to be conservative with expectations. From your experience, it looks like I've been doing the product a disservice.

    I should point out that I haven't yet got my firmware configured for the 128 x 128 MLCD running on my earlier dev board as it was using 'software com-inversion' - it seemed easier / less risky at the time. Right now I have too many other priorities to convert it. In any case, the prototype OTM-02 should be ready soon enough.