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FreeRTOS on AVR with external RAM

AVR microcontrollers aren’t the best choice to run FreeRTOS scheduler due to low on-chip RAM. Atmega128 has only 4K of RAM, so this limits the FreeRTOS functionality to very basic. Anyway, this problem can be solved by adding extra RAM which may be connected to an external memory interface. We have already built an external memory block of 8K previously so now we can test it with FreeRTOS applications. Lets continue with our previous code which runs several simple tasks (button state reading, LCD output and LED flash), and  we can add more to it. We are going to set up an external RAM for storing heaps. This will allow to store large data buffers without worrying too much about heap and stack overlaps.

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Running multiple FreeRTOS tasks on AVR

In the previous post, we just run a single task. Running RTOS with a single task has no meaning at all. This can be quickly done with a conventional program. But what if we need to have more separate functions. To execute them at exact timing would require a separate timer or interrupt. But microcontroller cannot guarantee an interruption for every task. This way, it is hard to make code modular, and testing can be painful. Using RTOS solves this kind of problem. It allows programming each task as an endless loop. Kernel scheduler takes care of assuring each task gets its chunk of processing time. Additionally, it does bearing the priority systems – more critical tasks are executed before less important ones. Let us go further with our example code and add more tasks to our FreeRTOS engine. We already have an LED flashing task that toggles LED every second. Additionally, we are going to create another task that checks the button state. Also, we are going to send some information to the LCD. As always, let’s take care of drivers for all of them.

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