We have created a monster. The serial ADXL started out as a simple project that quickly ballooned into all sorts of madness. We have decided to post this project (as well as several others), all completely wide open, in an attempt to redeem ourselves. Read this stuff and take it to heart - it may save you from ripping out your hair on a project of your own.

The following info corresponds to the Serial ADXL breakout board offered on the main SparkFun.com website.

Navigate:

Intro - PCB woes and where stuff goes
Firmware - The current firmware implementation
PCB Revision 3 - SADXL - The Next Generation

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Serial ADXL PCB - revision 1

So what is this serial ADXL?

It was a simple plan - take a tiny PIC (SOIC 12F629), an Analog Devices ADXL202E, and some firmware, and make a little board that reads acceleration in the X and Y axis and outputs that info serially. Everyone loves a good 9600bps signal.

Why didn't anyone tell us?!

Our first PCB for the 202E worked ok, except for one mistake on our part - we forgot to read the data sheet!

First Revision Schematic

What's wrong with this picture? GP3 (Pin 4) is in charge of outputting the Serial signal. But if you read the Microchip datasheet for the 12F629/675 section 3 you should see something like this:

General purpose Input. No output capabilities. So with a little 30awg wire jumper, and some PCB scraping, we fixed AN1 (Pin 6) to be the output man. Ugly, but it worked.

Low cost? Yes please!

Then we stumbled upon the ADXL311. It stuck out on the www.analog.com website as being the "Low-Cost" ADXL. Anything is better than paying $12 each for the ADXL202E. What does low-cost mean? The ADXL311 outputs an analog signal rather than a fancy PWM signal.

An analog signal may sound scary at first, but you forget! The 12F675 is the 12F629 except with a 4-channel A/D converter. Oh boy! So we scurried off and quickly laid out the second revision serial PCB:

Ok, what's wrong with this one?

Again, we are idiots. We failed to read the ADXL311 datasheet thoroughly enough to get to the Pin-out diagram.

Doh!

There seems to be a few 'NC' (no connection) pins on this version. Pin 4 and 5 that were the X and Y PWM output signal pins are now NC? That's because the ADXL311's analog signals come out of Xout and Yout (Pin 7 and 6). So we managed to create another PCB with fouled wiring.

It works - just not like it was supposed to.

Is it broken? Not really. The SIO problem was fixed (moved to Pin 2) so that was ok, and we added C1, a decoupling capacitor for a cleaner power supply. And as long as we continue to use the ADXL202E on this board, everything is peachy. So the Serial ADXL r2 works great with the 202E. We also cleaned up the silkscreen a bit.

Serial ADXL - revision 2

Here is the info you need to know for the 2nd revision. From the silkscreen side, Ground is the first pin, followed by Vdd, and the serial signal. The bias resistor is the loan pads next to the ADXL. The CAPs are located next to each other. And there is a decoupling capacitor (.1uF - nice to have, but not critically required) located in the corner.

On the flip side, Pin 1 of the 12F675 (we are shipping the board with the 12F675 with A/D, but you can also use the 12F629 on your own until the 3rd revision) is located towards the 3-pin header. See how Pin 1 of the PIC is connected to Vdd? That's a good thing.

S1 is a control jumper. This is not really incorporated into the firmware yet, but we like to put it on there for future firmware revisions (PCB revisions, firmware revisions, yea!). In theory, you will simply solder the two pads together effectively pulling that pin to ground. This is read by the firmware as an 'Option' switch and the appropriate routine is run. For example, true 9600 baud output verse bit-banged RS-232 output. But again, there ain't nothin there in the firmware, so just leave these pads alone for now.