FourThirty mk.1



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.
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.
I could now import the board design into Sketchup and begin to design an enclosure.
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.


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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