Warning! English is not my native language. I am a biologist so don't expect high quality professional results in electronics because it's just my HOBBY!

Welcome to my first attempt to build a flight controller (FC) with ARM chip on board.

Rex Caeli is an absolutely free DIY project mainly for educational purposes. It is not a commercial product and will never be available for sale!

Almost three years ago I started to play with Atmel’s microcontrollers (or Arduino, if you like). It was my first journey into digital electronics from absolute zero. Programming in C was not a big deal because I was familiar with it. Actually, programming was my very first serious hobby.
Six months ago I decided to try something different. Building a flight controller with ARM chip on board for untrained and unqualified person should be a quite challenging task, shouldn't it?
The first question was where to start from? I knew there was an OpenPilot project and their CopterControl (CC) board had an ARM chip. Making a plane copy of CC was not acceptable, because sensors were too expensive and 4-layer PCB board design was expensive too. Nevertheless the open source politics caught my eye.
I downloaded EAGLE software and started to build something similar to CC, but cheaper.
I was really surprised that much powerful ARM chips cost less then Arduino. One of the cheapest solutions was to use STM32F103RBT6 micro-controller unit and experimental LSM330DL sensor (3-axis acc + 3-axis gyro in one chip) from STMicroelectronics. Reading data sheets, getting familiar with the new stuff and changing the board layout was my best free time entertainer for almost four months.
When I was happy with my unprofessional board design,

3D Model of Rex Caeli - ARM based 10-DOF flight controller for multirotors (0 min 13 sec)

I sent Gerber files to one of the cheapest Chinese PCB prototyping services called ITeadStudio.
One month later I saw the result:

After three nights of hand and hot air gun soldering the bear PCB turned into real hardware:

Software adaptation process was the most interesting part. OpenPilot CopterControl firmware is much more complicated then for example MultiWii firmware. I wrote a driver for poorly documented LSM330DL sensor chip and modified other parts of the code to match my hardware. After a few days I was very happy when I saw data coming out from LSM330DL sensor to the Ground Control Station software making lines and curves in response to the board movements.

Rex Caeli + OpenPilot CC. Part-1 - Sensor test. (1 min 40 sec)

Maiden flight was quite terrible. There were a lot of oscillations, but after some firmware tweaks and PI tuning things became much better.

Rex Caeli + OpenPilot CC. Part2 - The First Flight (0 min 45 sec)

The flight is still far from ideal. Quadcopter wobbles a little bit, but HEY it's the very beginning. I am very happy with my first result and I hope that it will get better.

Another adopted firmware is baseflight (MultiWii for STM32) by timecop. I added a very simple SPI driver because my LSM330DL chip is on SPI bus, not on I2C. All sensors are active and working properly.

Rex Caeli + Baseflight. Part 1 - Sensors test. (0 min 49 sec)

if your going to fly FPV I would recommend going X config its allot more stable in my exp as FPV platform and it gets the motors out of the way for the cam

better inertia balance on three bladed props. It is the same in all directions, unlike two bladed props.

vibration is allot better on 3 blade for a 3% loss in efficacy when doing FPV vibration is like kryptonite to superman on plane this means less but on quad it means allot.

One of my recent videos had a slightly lose prop adapter which caused the blade to wobble i bet you can spot it easily that's all it takes to ruin your day with vibration

Make sure all your props are balanced using tape or sanding as well

Blusky1, thank you for the tips and recommendations.
I am not going to fly FPV with this quad. I just wanted to build my own really cheap (less then 40$) flight controller based on ARM chip. I also modified OpenPilot CC firmware to match my board layout and the sensor chip.
This quad flies very nice with other flight controllers. I am sure the wobbling comes from the adopted firmware. Perhaps I missed something to change in the code.
Work is still in progress...

Like I said on Youtube too....
You hardware configuration sound very well but with only one little adjustment! For this kind board you may consider building with a ms5611 pressure sensor.

Good luck!

darockbiz, thank You for suggestion.
I know about ms5611 which is probably better pressure sensor then BMP085, but it is also more expensive. I even don't know how BMP085 works, because I adopted OpenPilot CC firmware which doesn't care about pressure sensor data.

My hardware is very experimental. Now I see some mistakes I did. Sooner or later I will rebuild this board, but now i want to play with different firmware. Baseflight and AeroQuad is the ones I want to adopt too. By the way adaptation of Baseflight is now in progress.

Soon I will post schematics and layout data of the board and explain all mistakes and blunders I did. Adopted firmware source will also be available for download. Everyone who wants to build or modify this experimental board will be able do this.

Give a chance to MultiWii. As far as I know is most stable open source software for quad copters.