Sockets and footprints

I spent about 6 hours today racking my brain trying to find parts that will both do what I want and feasibly be used in both the prototype and final version.

My prototype will be on a breadboard, so I need a chip that I can surface mount to a breakout board and later surface mount to an actual custom circuit board.

Here’s what I got from digikey

  • Atmega48A-PU-ND – A 3v AVR that comes in both a tiny surface mount version and a DIP version. I got the DIP version for now, but my code should run on both.
  • DS1337 – RealTimeClock (RTC) unit. This should keep track of my time
  • some resistors for my LEDS
  • 342-1041-1-ND – really expensive compass unit, but it uses I^2C and should just pump out a heading value if I get it to work right.
  • 828-1002-1-ND – Accelerometer. Analog output, I’ll need to use the AVR’s ADC to get this to work.
  • 102-1265-1-ND – Buzzer
  • 32.768kHz crystal
  • Buttons
I read this document: which explains how to use the built in RTC unit in the AVR. I’ll be needing the external RTC anyway to generate 1Hz pulses (actually, I’m not certain that this is true, but they’re cheap parts anyway, so it couldn’t hurt). The AVR module uses about 10 microamperes, but the RTC can get down to 1.5. Besides, using the external RTC means less code for me as it already takes care of date and time counting.

Now for the footprints. The accelerometer is a standard 16-pin LGA footprint, so I had no trouble finding a breakout board for it. The compass on the other hand was a major pain in the ass. It claims to be a “24-pin PLCC”, but as far as I can tell, there is no such thing, so after raking the entire internet with a fine comb, I found that it might just fit into a .8mm pitch 6mm x 6mm 24 pin QFN socket. I’ve got my fingers majorly crossed on this one. This is what I bought from in the way of breakout boards:

  • IPC0015-KIT – 24pin QFN to 24 pin dip PCB and stencil kit. Comes with some solder paste and stencil.
  • PA0102-S – 16 pin LGA stencil
  • PA0102 – 16 pin LGA to 16 pin dip breakout board
To mount these guys, I’ll be needing to do some “solder reflow” or whatever. According to the site, you can accomplish this in a toaster oven. I read through this tutorial and it doesn’t look too impossible.

Well, that’s all I’ve done so far. Goals for when I get the parts in:

  • Get my AVR programmer working again in OSX
  • Configure an I^2C interface with either the RTC or Compass

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