Flavonoid — 2nd Prototype
Here's a board layout for the second Flavonoid prototype. It's a lot more sophisticated electrically and just conceptually. I've included a 3-axis accelerometer that's rather expensive ($15), a real-time clock (RTC), touch sensor and a bit of flash memory. All these devices run along a four-wire interface (SPI) and are controlled by a microcontroller that's off board. (Using Atmel's, like the ATmega8 and ATmega16 — although I may end up using an ATmega32 as the code seems to be ballooning beyond 16kb.)
The clock is there to design in a way to encourage sustained activity. It'll hold time even without power, so the frame of reference can be on the order of hours or days, rather than just minutes. The touch sensor — well, I have ideas for that, but one pragmatic reason is to help mitigate "cheats." I've tried this sensor and it'll meausre body capacitance through a hunk of wood and fabric. The flash memory is to contain data from the readings. I'll need to use some sort of circular buffer to account for the fact that these EEPROM can withstand 100k read/write cycles and I suspect these I'll need a few orders of magnitude more headroom.
(More information on the boardhouses and a very modest review I wrote is available at http://www.nearfuturelaboratory.com/blog/julian/277.
Here's an assembled prototype using the SPI interface.
The image documentation is here.
2nd Flavonoid Prototype (I2C/TWI)
This is an I2C/TWI prototype. You can see it's smaller, largely because my skills with the PCB layout tools are refining. But, also — I2C requires fewer wires between the parts.
This particular variation is using a dual-axis accelerometer — the MXC6202xJ from Memsic. This prototype taught me a few things, including confirming my opinion that I should really use a tri-axis accelerometer. I also learned a few things about laying out the footprints for small pitch surface mounted parts: you must design the landing pads slightly longer than specified. (I should come up with a percentage oversize as a design rule.) Having a slightly longer pad would make the reflow go easier, it seems, as well as proving a bit more of a visual indicator as to how well the reflow went. Intuition also suggests to me that the device might adhere better with the increase of a bit of alignment due to surface tension of the solder and the pad on the device.
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