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FreeRTOS is an open-source realtime operating system for microcontrollers. The ATMega644 of ATMEL provides 64KB of flash program memory in a DIY-friendly 40-pin PDIP package. Read on for how to combine both to my new software development platform.

FreeRTOS provides a nice RTOS for embedded systems and is readily available for various platforms. One timer of the hardware is used to implement multitasking features. A library provides routines for creating tasks and synchronization. Unfortunately, FreeRTOS is currently only ported to the ATMega323 – so I created a basic port to the ATMega644 and stripped all unnecessary features.

Typically, the FreeRTOS demo shows how to implement various threads and let them access the hardware. While it is educating to read the code, it is too bloated to serve as a template for new developments. In addition, I prefer Peter Fleury’s UART library for serial outputs. So, I removed all threads and included the library. To make sure that two threads cannot write to the same UART at the same time, I introduced a mutex per UART to synchronize the output on a per-string basis. Please note that there is no Mutex to synchronize read access. The main program starts two tasks which print “foo” and “bar” to the serial line, respectively. I kept the task surveilance mechanism as implemented in the original FreeRTOS demo (this might become handy ;-))

Here is a peek at the main.c file:

[viewcode]src=freertos-atmega644-main.c [/viewcode]

HTML code generated by vim-color-improved v.0.4.0.Download this code: freertos-atmega644-main.c

You can get the whole package (GPL licensed) from the download page. Have fun! Kudos for the CC’ed chip bug picture go to oskay.