Lightweight and fast preemptive scheduler for ATMega and SAM3X (DUE) Arduino Boards

This library implements a simple, preemptive task scheduler that is executed in parallel to the 1ms timer interrupt used for the Arduino millis() function. It allows to define cyclic tasks or tasks that should be executed in the future in parallel to the normal program execution inside the main loop.

The task scheduler is executed every 1ms. A possibly running task is interrupted by this and only resumed after all succeeding tasks have finished. This means that always the task started last has the highest priority. This effect needs to be kept in mind when programming a software using this library.

More documentation can be found here: https://htmlpreview.github.io/?https://github.com/kcl93/Tasks/blob/master/extras/html/index.html

• Deadlocks can appear when one task waits for another taks which was started before.
• Timing critical tasks may not execute properly when they are interrupted for too long by other tasks. Thus it is recommended to keep task execution as short as possible.
• The Arduino MEGA leaves the interrupts state shortly after starting the task scheduler which makes the scheduler reentrant and allows any other interrupt (timer, UART, etc.) to be triggered.
• The Arduino DUE enables other interrupts by using the lowest possible priority (15) for the task scheduler interrupt.
• For non-static member functions, i.e. class methods, use the adress to a wrapper function created via PTR_NON_STATIC_METHOD() instead of function name. For an example see Class_methods.

The Arduino Due does not support reeantrant interrupts due to HW limitations. This means that unlike the Arduino MEGA the DUE is not able to execute fast 1ms tasks several times before finishing a slower tasks with for example a 10ms timebase. This problem will be especially visible if the sum of the execution time of all tasks is greater than the execution period of the fastest task.

• all boards using the Atmel AVR controllers, e.g. Arduino Uno, Mega, Nano, etc. (only tested with those three)
• all boards using the Atmel SAM3X8E controller, e.g. Arduino Due

• Atmel ATMega328 & ATMega2560: Scheduler uses TIMER0_COMPA interrupt. This maintains millis() and analogWrite() functionality on T0 pins. However, frequent changes of the duty cycle using analogWrite() lead to a jitter in scheduler timing.
• Atmel SAM3X8E: Scheduler uses TC3 interrupt.

Atmel ATMega328 & ATMega2560:

• 5μs without pending tasks
• 12μs + task duration when tasks are executed

Atmel SAM3X8E:

• tbdμs without pending tasks
• tbdμs + task duration when tasks are executed

Have fun!