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The Goldilocks 1284p story is here. http://feilipu.me/2013/03/08/goldilocks-1284p-arduino-uno-clone/
If this repository is too complicated to get started, a minimum AVR freeRTOS configured using the Watchdog Timer and Heap 3 is also also available. https://github.com/feilipu/miniAVRfreeRTOS
For the Arduino platform there is a freeRTOS Library available in the Arduino IDE Library manager. https://github.com/feilipu/Arduino_FreeRTOS_Library