Make a slim watch with ARM Cortex-M3 EFM32

Hack-a-Gecko project by Anders and Adam 

How to make a very slim watch and keep battery life long? In this Hack-a-Gecko project, they tried to kill two birds with one stone. 


The Idea
We thought it would be cool to utilize the extremely low power EFM32 in combination with an extremely low power display to create a wrist watch demo application. And usually, the smaller and thinner something is, the cooler it is. (Admittedly, wrist watches do not necessarily follow this trend… big watches.)

Anyway, we wanted it slim. The starting point was the memory LCD display from Sharp (link). It is truly a Nano ampere display technology. And it is also thin, only 0.75 mm. A watch also needs a battery, cool new technologies exist such as the Thinergy battery, but the voltage of 4.1 V is a bit awkward. We decided to use a standard 3.0V CR1616 cell as it can power the EFM32 and display directly. Thickness of battery + display is 2.35 mm, is it possible to design the electronics as well within this thickness limit…? Challenge accepted!

We decided to try to fit everything within the width, length and depth constraints of the display + battery stacked on top of each other. Since this was going to be a simple wristwatch, all we needed on the electronics side was an EFM32 + decoupling caps + a low frequency crystal.

These components should be mounted on a printed circuit board, but that would add thickness to the whole thing. So we skipped the standard approach and designed the circuit board to fit the components inside it instead. We could have chosen a thin-film flex-PCB of some sort, but that has been done before. This was all about trying a novel and new approach (fitting components inside a PCB is in itself nothing new: link, but doing it as a hacking project for prototyping isn’t as common).

We started designing the PCB, and after some back and forth, ended up with the following layout design in Eagle:


There are cut-outs for every component; the battery is the big round cutout in the corner. Display cable and connector fits within the other big cut-out at the lower side of the PCB.
The QFN24 packaged EFM32TG110 device fits within the square cutout with drill-hits in the corners. Capacitors and crystal has some smaller and shared cutouts scattered around. There was even room for two capacitive touch buttons.

There are plated drill-hits that act as PCB-pads for the capacitors and crystal. The plan was at first to have the same plated drill-hits at each pad of the connector and EFM32, so the solder could stay inside the plated hole and thereby reduce the amount of solder on top of the PCB. After some research, it turned out that this would drastically increase the price of the PCB, so we decided to put the solder tin on top of the PCB; we reasoned that it wouldn’t increase the thickness too much.

It all comes together
Here is a picture of the PCB before soldering.


Here is a microscope picture of the efm32, fitted inside and soldered.


Admittedly not the prettiest soldering job… Notice that we had to take the ground connection through an unused GPIO-pad on the package because the EFM32 QFN packages only have ground on the big center pad.

And here is a picture of the PCB with components and display.

The software is more or less straight from the Energy Friendly Display application note, AN0048. Since the tiny gecko devices have limited memory, we had to remove the full Segger graphics library to reduce memory footprint.

Here is a small video demonstrating the watch.

Improvement Ideas
The watch obviously needs a shell and a wrist band to be a fully functional wrist watch. The way the components are mounted on the PCB would also introduce big problems if it were put into mass production. It is more a conceptual way of mounting components that is suitable for one-off hacking projects. It would be interesting to implement a rechargeable watch using the thinergy battery, see how well the size fits with the display:


If we do a 2.0 version of this watch, it will probably use an EFR device for wireless capabilities like Bluetooth LE.

Link to a slide based presentation of the slimwatch project: here. Credits for this project also go to AdamSch.

Eagle files for the PCB:   109.17K

HaG logo

This Hack a Gecko project is a result of a “fun hacking session” and are provided as is, free of charge with no guarantees or support from Energy Micro, to partially or fully show and demonstrate EFM32 Gecko microcontroller capabilities. Get inspired, use at own risk, and build some awesome and cool applications.”


For technical questions about this project, please use our support forum. The Hack-a-Gecko team will answer your questions directly.

19 thoughts on “Make a slim watch with ARM Cortex-M3 EFM32

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  5. I’m afraid you can’t catch birds with a single stone. The phase is “kill two birds with one stone”. If you had a lot of stones however you could build an imprisonment around them.

  6. It could be so very cool if you got this to market so we all could buy it
    It’s even compact and small enough to use as a badge, could be very nice with the wireless, hope you do version 2.0

    • Yes, I’m looking forward to doing the 2.0 version with radio! If you want to play around with the memory LCD itself, I would recommend that you make the STK3700 plugin board, then you can use the software project in application note an0048 directly. The tiny gecko is on the small side, memory wise. I updated the slimwatch post in the forum with gerbers and eagle project files for the memory lcd plugin board:
      Feel free to continue the discussion in the forum!

    • I believe we got our samples from Avnet in europe. Took almost 3 months though. I believe the 5V version of the memory lcd is much easier to get a hold of than the 3.3V version that we used. But IMHO, the 5 V version is useless for low power applications, almost all mcu’s in this segment, including the EFM32 stops at ~3.6 V- 3.8 V.

    • The LCD model number is: LS013B7DH03. That is the 3.3V, 128*128 pixel device. I should probably have mentioned that in the forum-post as well.

  7. Perhaps a ring of very thin copper wires around the lcd or incorporated into the bezel of the final project for inductive charging of the rechargeable battery?

    • That is a possibility yes, I was thinking of embedding the charging coil in the PCB. Much simpler and cheaper to produce. But I’m not sure if enough loops can be embedded around the edge of the PCB.

      • with thebutton cell removed for the lipo battery and the components centralized on the board would there not be enough room for enough coils? Also could you do a multiple layer pcb to add more coils or would that be out of your capabilities?

  8. Well, the capabilities lies with the PCB manufacturing industry, anything is possible, but everything comes at a price. Remember that for us, the slimwatch is just a hacking project done over a couple of days. It would require much more engineering effort to make an actual product out of it. I haven’t looked into how much room such coils might need, or what would be the simplest solution for a one-off design vs. what might be manufacturable. I was just guessing that pcb-coils might be cheaper and easier in a mass production setting. Would be cool if someone picked up the idea and did some more work on it!

  9. Would having a multi layer pcb give you enough coils for the inductive charging, assuming you move all the components towards the middle of the pcb?

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