FreeRTOS SMP

The SMP (multicore) port of FreeRTOS is included with the core. This allows complex task operations and real preemptive multithreading in your sketches. While the setup1 and loop1 way of multitasking is simplest for most folks, FreeRTOS is much more powerful.

Enabling FreeRTOS

To enable FreeRTOS, simply add

#include <FreeRTOS.h>

to your sketch and select Tools->Operating System->FreeRTOS SMP to enable it.

When using Platform.IO you need to add the following define to your .ini file:

; Enable FreeRTOS Support
build_flags = -DPIO_FRAMEWORK_ARDUINO_ENABLE_FREERTOS

Configuration and Predefined Tasks

FreeRTOS is configured with 8 priority levels (0 through 7) and a process for setup()/loop(), setup1()/loop1(), LWIP, and the USB port will be created. The task quantum is 1 millisecond (i.e. 1,000 switches per second).

setup() and loop() are assigned to only run on core 0, while setup1() and loop1() only run in core 1 in this mode, the same as the default multithreading mode.

There is an LWIP worker thread running on core 0 at priority (configMAX_PRIORITIES - 2) This should allow for LWIP to always make progress even with applications that don’t yield or delay, while leaving the highest priority available for hard real time processes. This setting can be changed by defining LWIP_TASK_PRIORITY in your build process.

You can launch and manage additional processes using the standard FreeRTOS routines.

delay() and yield() free the CPU for other tasks, while delayMicroseconds() does not.

Caveats

While the core now supports FreeRTOS, most (probably all) Arduino libraries were not written to support preemptive multithreading. This means that all calls to a particular library should be made from a single task.

In particular, the LittleFS and SDFS libraries can not be called from different threads. Do all File operations from a single thread or else undefined behavior (aka strange crashes or data corruption) can occur.

More Information

For full FreeRTOS documentation look at FreeRTOS.org and FreeRTOS SMP support.