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.