The ARMinARM board is an STM32 ARM Cortex-M3 microcontroller addon board for the Raspberry Pi Model B+ with a focus on flexibility and hackability, while still being easy to use. The STM32 runs on 72MHz and has 512KB Flash and 64KB SRAM memory.

The Raspberry Pi is used as an "all-in-one" code-editor/compiler/programmer/debugger/communicator for the STM32 Cortex-M3 on the ARMinARM board. All software is open source, and runs on the Raspberry Pi.

All STM32 and Raspberry Pi GPIO pins are available on easy accessible headers, that align on a 0.1" grid. There are 77 GPIO pins in total (the STM32 has 51 and the Raspberry Pi has 26 usable GPIOs).

All GPIO is 3.3 Volt logic, but most of the STM32 GPIO pins are 5 Volt tolerant. That means that generally, you can connect your sensors and chips without having to worry too much about voltage level conversion.

And it's usable as a stand alone board too! Remove it from the Raspberry Pi (keep the standoffs on), move 1 jumper and plug in a PC or laptop with a USB cable.

Having an ARM Cortex microcontroller on your Raspberry Pi adds a lot of extra features.

Hardware is only useful with a decent amount of software to go with it. A set of open source commandline tools is provided to compile, upload and debug your firmware. All software and example source code is customized to run on the ARMinARM board, and most (if not all) of the installation is automated.

The repository with all installation scripts is here: