The crew at Element 14 has announced the release of a combo R-Pi/STM32 interface board compatible with Arduino shields.
The Embedded Pi is based on the STMicroelectronics STM32F103 MCU and is open-source (see Technical Documents for schematic, layout and BOM). It can operate as a bridge between Raspberry Pi and Arduino shields and in standalone mode as a Cortex-M3 evaluation board.
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Depending on the jumper placement on the Embedded Pi, you can select each of the three modes of operation:
STM32/Standalone Mode: The Embedded Pi works as a base platform, the STM32 controls the Arduino shields directly without the use of Raspberry Pi.
ST-Adapter Mode: The Embedded Pi STM32 controls the Arduino shields, and the Raspberry Pi works as the GUI or command line console to send commands/data to and receive data from Embedded Pi.
Raspberry Pi Mode: The Embedded Pi works as a hardware connection bridge between Raspberry Pi and Arduino shields, allowing the Raspberry Pi to interface directly with existing Arduino shields.
The board contains an ARM Cortex-M3 MCU STM32F103RBT6. For more information visit the Embedded Pi webpage at Element 14. The 48 page PDF datasheet can be downloaded from Farnell. The Embedded Pi lists for $34.