FreeRTOS on STM32

High density line of STM32 microcontrollers have quite a bunch on features that can be used in user programs. The more features you add to source the more complicated program becomes and this way it starts to be difficult to keep up with all things. Using only main loop and interrupts becomes time consuming task to manage. If you don’t want to struggle in tuning things up manually you can use one of many real time operating systems (RTOS). They are great when you need lots of separate functions to run in parallel so no task would be missed. RTOS scheduler takes care of giving each task a decent time to perform. There are lots of great RTOS systems around. Many of them are free and opensource. It happens so that I love using FreeRTOS which has quite long history and is flexible enough to fit multiple types of hardware.…

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FreeRTOS on AVR with external RAM

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

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Running multiple FreeRTOS tasks on AVR

In previous post we just run a single task. Running RTOS with single task has no meaning at all. This can be easily done with conventional program. But what if we need to have more separate functions. To execute them at exact timing would require separate timer or interrupt. But microcontroller cannot guarantee an interrupt for every tasks. 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 endless loop. Kernel scheduler takes care of assuring each task gets it’s chunk of processing time. Additionally it does bearing the priority systems – more important tasks are executed prior to less important ones. Let us go further with our example code and add more tasks to our FreeRTOS engine. We already have LED flashing task that toggles LED every second. Additionally we are going…

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Using FreeRTOS kernel in AVR projects

FreeRTOS is known as Real Time Operating System. Probably it would be too dare call it real-time-os, rather a real time scheduler where applications can be split in to independent tasks that share full processor resources by switching them rapidly it looks like all tasks are executed in parallel. This feature is called multitasking. There are lots of debates on using RTOS on AVR microcontrollers as they are arguable too small for running scheduler. The main limitation is small amount of ram and increased power usage. If you are gonna use lots tasks in application, probably you will run out of ram that is used for saving context when switching between tasks. Consider FreeRTOS only if you use larger scale AVRs like Atmega128 or Atmega256. Surely you can find smaller schedulers that are specially designed for smaller microcontrollers even tiny series. In other hand if you master FreeRTOS it can…

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