Vario Project, looking for guidance on board design

I am putting together a vario project based on a BMP085 sensor, the vario will indicate the rate of altitude rise and fall in meters per second. I am fairly new to designing my own boards and was wondering if anyone could spare a few moments to review my schematic and make any suggestions before I get too far into laying the board out.

The details of the design are.

  • Altitude measurement by Bosch BMP085
  • ATMEGA328 running at 3.3v 8Mhz
  • USB interface with ftdi 232RL ic for programming and setup
  • Battery powered via single cell lipo or USB (auto select)
  • Battery charging over USB with CN3083 lipo charge ic
  • Low voltage drop 3.3v regulator ADP122
  • Charge status and battery voltage read by ATMEGA

I have based the power and charging circuit partly on the seeduino stalker design as I have one and I know it works well with a single cell lipo.

The first revision is just a desktop evaluation so the final version will probably loose the charge and battery status jumpers and the breakout of digital pins.

At the moment I have a header for a serial lcd display but again in the final version this may change to a simple led and sounder setup to output altitude change.

I have thrown a power switch in beside the battery connector but I need to think if this is the best solution, at the moment on battery power it will function as main on/off. The unit will always be on over USB and in this case the switch would then function as a charge switch. Looking for suggestions if anyone can think of a better solution?

I also need suggestions for the type and value of the D1 and D2 diodes, I have been trawling through data sheets but not sure what ratings I should use between the USB 5V and the charging and regulator circuits. Any suggestions?

It is wise to rely on the internal clock on the ATMEGA or should I add a 8Mhz resonator?

Once complete I am looking to share the design and arduino code. Links to the schematic in both jpg and eagle are here.

http://dl.dropbox.com/u/2191955/vario_alt_smd_sch.jpg http://dl.dropbox.com/u/2191955/vario_alt_smd.sch

Thanks, Mark

I have another question about the BMP085 sensor, I read that to improve the error on the sensor it would be best to de-couple the VDDA and VDDD supplys with an inductor, does anyone have any other information on this?

I just built a paragliding vario with the BMP085, works quite well without any hardware filtering at all. I did some experiments with the BMP180 and found out that I get the least noise when I donĀ“t even connect the power supply pins. Apparently voltage from the logic lines somehow gets to Vddd, and Vdda can just be connected to Vddd. I get about 2.7Pa average error from the sensor. What will you use the vario for? My second project is a TEK vario for a manned glider, using two BMP180s

links don't seem to work.

If going surface mount, try a atmega32U4, built in USB, get away from FTDI chip, they are a pain to solder. See the leonardo clone threads here.