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.
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.
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.
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.
Eagle files for the PCB: slimwatch_rev01.zip 109.17K
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.