FourThirty mk.1

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I’ve been wearing my MSP430 watch pretty regularly. It proved to be a practical timepiece, so I figured I’d have a go at a new version with some upgraded components.

I will use the same OLED display from my previous watch design. While the power consumption of this display is rather high when active (up to 7ma @ 3v), the contrast and readability of the display is unmatched by any alternative. I will use the MSP430FR5738 in this design. This is a fairly new addition to the MSP430 series of 16-bit ultra low power microcontrollers from TI. The chip consumes as little as 1.5uA while maintaining a real-time clock and monitoring interrupts. The Ferroelectric RAM in this chip is also capable of zero-power data retention.  This allows for a neat trick that saves the clock variables across a reset or battery change; You don’t need to re-set the time.  The watch will also get an accelerometer. LIS3DH seems like a good choice. I’m interested in it’s ability to autonomously detect ‘tap’ and ‘double tap’ events and fire an interrupt to wake the main processor. It even has an internal high-pass filter to reject gravitational acceleration. It should be able to share a SPI bus with the display. I’ll include a nice LDO and under-voltage protection for use with an unprotected li-po, but design primarily for a CR2016 or CR2025 coin cell. First step is to design symbols and footprints for EagleCAD and 3D models in Sketchup.
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Next, I put together the schematic and board layout. You may notice the huge ground plane. This was necessary to allow a CR2025 to fit in a holder designed for CR2016. By doing this, I will get a longer run time without compromising height. It also made routing rather difficult without resorting to a four layer board. You may notice some shortcuts I took to overcome the limitation.
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I could now import the board design into Sketchup and begin to design an enclosure.
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Orders were placed to Shapeways, OSHPark, and DigiKey. Most of the components were soldered in my reflow (toaster) oven.  The display, battery holder, buttons, and programming header were hand soldered.
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w22

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Coding for most of the major functions is complete. I wear the thing most days. Some cool tricks with the accelerometer are possible, but coding is slow. I’m building from the ground up here, and I’m not a great coder. Future updates will be posted to this page.

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