Electronics

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V1 Design files

• Block diagram
• Schematics PDF version
• Layout PDF version
• PCB production files ZIP file

You can find these files in the Files section of the Freewatch project.
You can also find them in the Repository (hw/ folder).

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This design is Copyright Julian Lewis 2014.
This documentation describes Open Hardware and is licensed under the CERN OHL v.1.2.
You may redistribute and modify this documentation under the terms of the
CERN OHL v.1.2. (https://www.ohwr.org/cernohl). This documentation is distributed
WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY,
SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE

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Design tools

This electronics design has been made with the KiCad EDA Software Suite.
KiCad is a Free/Libre Open Source Software Tool which runs on GNU/Linux, Windows and Apple OS X. It is licensed under the GPL license.

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Component selection

• Component selection

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Design notes

GPS passive antenna

• http://www.smokingresistor.com/2013/11/16/gps-ground-plane/

GPS RF design guidelines

• http://www.kitarm.com/project/ppsi/wikis/News#ppsi-v-1301-ppsi-for-wrpc_swtgz-released

Low power design

• http://www.digikey.com/en-US/articles/techzone/2011/jun/ultra-low-power-mcus-enable-energy-harvesting-designs

• http://www.silabs.com/Support%20Documents/TechnicalDocs/implementing-low-power-graphical-displays.pdf

• http://www.digikey.com/en/articles/techzone/2014/jan/selecting-the-right-ultra-low-quiescent-current-ldo-regulator

• http://www.ecnmag.com/articles/2012/07/specifying-right-ultra-low-iq-ldo

Backlight

Use 0.5 or 1mm plexiglas sheet behind the LCD with four white LEDs.
Here is a cheap plexiglas sheet supplier:
http://www.plexiglas-shop.com/CH/fr/plexiglas-folie-5m9m5apyxic/plexiglas-folie-blanc-99532-gt-1-0-mm-iqq6a1wurpd~p.html

> Note: After some preliminary tests, it turn out that the plexiglas idea doesn't work well.

Learn more on backlight:

• Tutorial video from 3M (1)
• Tutorial video from 3M (2)

As optical films are hard to get for private customers, using backlight from a broken cellphone display might be an alternative.

Battery life calculation

• http://www.oregonembedded.com/batterycalc.htm
• http://www.csgnetwork.com/batterylifecalc.html
• http://www.ajdesigner.com/phpbatterylife/battery_life_peukert_number_time.php\#ajscroll

Bootloader

To enter the bootloader, DBG_SWCLK (PF0) must be pulled high and the EFM32 must be reset.
See application note AN0042

> Note: We should find a way to run the bootloader from the main application (without having to open the case!).

The original bootloader has been modified to allow entering it from the application (reset menu + watch button pressed).

Power profiling

Foresee sensors, test points on the PCB in next revision?

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