Tiny portable AVR projects: Business card breakout boards

Wednesday, March 05 2008 @ 09:30 AM PDT

Contributed by Windell

For all of our different AVR microcontroller based projects, we seem to find ourselves continually wiring up minimalist target boards; little circuit boards that fit both the AVR and a 6-pin header for connecting to your in-system programmer. And, when you find yourself hand-wiring the same circuit over and over again on a protoboard, that's really just life's way of telling you "just lay out a damn printed circuit board already."

Coincidentally, we needed a new business card.

That's a bit of an exaggeration; we didn't need a new card so much as we were enamored with the idea of having a card that did something interesting. We certainly aren't the first to have that itch-- people are already making interesting electronic or circuit business cards, and there was even a contest at Hack a Day to design circuits that would run on their business cards.
(Digression complete.)

Project goals
We've been designing a lot of things based on the ATmega168, and we didn't want to wire up any more by hand. (The ATmega168 is a popular AVR microcontroller; it's the one found in the Arduino.)
Our design goals for this project were (1) to make a printed circuit board version of the minimalist target board for the ATmega168, encompassing a place for the chip and a connection to the 6-pin ISP header, (2) to make a minimal and inexpensive circuit board platform that you could use to deploy a single AVR somewhere with without much fuss, (3) to encompass the capacity of a breakout board, giving extra holes to tap into each pin of the AVR and provide labels for every pin, (4) to fit in some small amount of flexible prototyping space, (5) to make it all fit into a neat business-card form factor, and (6) to release it as an open-source project.

The Design
Here is what we ended up with:

The circuit board is 2" x 3.5" (Standard business card size of course). It is designed so that all of the copper traces are on the bottom side, meaning that it can be built single-sided, either to keep the cost down, or to make it an easy design to fabricate yourself if you like to etch your own boards.
On the top side of the board, dominating the playing field, is the place for the microcontroller, which can sit in a standard DIP socket. While we mainly discuss the ATmega168, it actually works equally well for the ATmega48, ATmega88, and the ATmega328P (if you can find a ATmega328P!), hence the designation ATmegaXX8 Target Board. The board is also compatible with the older ATmega8, although the feature set is slightly different from those in the 48/88/168 genus.
All 28 pins of the microcontroller are labeled with their main functions. Besides the pins that actually connect to the ATmega168, there are three-count-em-three extra access holes connected to every pin of the microcontroller. In the upper left, there are input connections for power (typically 3-5 V) and ground, which are routed to a couple of other locations on the board. In the upper right is a place for the 6-pin ISP header with pin 1 marked. In the lower right is a place to add an optional crystal oscillator with its two capacitors, or a 3-pin ceramic resonator.
On the left and right sides of the chip location are two breadboard-style prototyping areas, each with enough room for a DIP-8 package or the equivalent.
(Not that we could begin to scratch the surface of what cool things can fit in that space, but we might point out common op-amps or optoisolators for input or output signal conditioning, or even a pair of half-bridge drivers for controlling a motor. Or a hexadecimal rotary dip switch. Or, from the obtuse-rather-than-useful department, use the space to add two more tiny AVR chips!)
This board does not have hardwired connections between the power and ground sections and the analog power section (AGND, VREF, AVCC pins), which allows you to substitute in other (or better) references to the analog section if you need them. If you're happy with the standard analog-to-digital converter performance, or if you don't use it at all, it's recommended to jumper the pin AGND to ground and VREF and AVCC to the positive power supply, which are conveniently located nearby, near the upper middle of the chip. (See photo at the top of the page for an example with the jumpers in place.)
Finally, mounting holes are provided in each corner for 6-32 size screws, located with centers 1/8" x 1/8" from each corner.

Using the board

Since this is effectively an implementation of the minimalist target board, you don't need much to run one of these. Ideally, the board, plus a chip, plus the 6-pin ISP header and a programming interface (at least once). To run on its own it will need and external power source as well, and AVRs like this can run (depending on load) from a 3V lithium coin cell or from a more powerful source. We usually run ours on external battery boxes with switches, either with two or three AA cells.

Under the hood
Again, this board is designed for easy single-sided fabrication; the copper layer, which goes on the bottom side of the board, looks like this: ATmegaXX8 Breakout Board: top silkscreen and bottom copper
To get started working with this design on your own, we have a single ZIP file, available for download here, (584 kB .ZIP file) which contains the full schematic diagram (PDF format), a PDF of the PCB layout showing the individual circuit board layers broken out individually with crop marks-- making this layout a cinch even for home-etched circuit boards, and the original PCB layout file. (The circuit board was designed in gEDA PCB-- free, open-source printed circuit board software.) We are releasing the design for this board under open source licenses and under a creative commons license as well.
Or, take the easy way out and get a kit here. :)
Want to talk about it? That's what the forums are for.
Got projects based on this board to show off? We'd love to see your pictures in the Evil Mad Science Auxiliary.