Task Scheduler – cooperative multitasking for Arduino microcontrollers
Version 2.1.0: 2016-02-01

If you find TaskScheduler useful for your Arduino project, please drop me an email: arkhipenko@hotmail.com
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OVERVIEW:
 A lightweight implementation of cooperative multitasking (task scheduling) supporting:
 1. Periodic task execution (with dynamic execution period in milliseconds or microseconds)
 2. Number of iterations (n times)
 3. Execution of tasks in predefined sequence
 4. Dynamic change of task execution parameters (frequency, number of iterations, callback function)
 5. Power saving via entering IDLE sleep mode between tasks are scheduled to run
 6. Task invocation via Status Request object
 7. Task IDs and Control Points for error handling and watchdog timer
 8. Local Task Storage pointer (allowing use of same callback code for multiple tasks)
 9. Layered task prioritization

Scheduling overhead: between 15 and 18 microseconds per scheduling pass (check the banchmark example).

For detailed functionality overview please refer to TaskScheduler documentation in the 'extras' folder.
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Check out what TaskScheduler can do:
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  Houston midi clock project - TaskScheduler with microseconds resolution
    (by chaffneue: My first arduino project. It's a multi-master midi controller with a shared clock and 
     auto count in behaviour. Project files on github: https://github.com/chaffneue/houston
	 youtube: https://www.youtube.com/watch?v=QRof550TtXo)
	  
  Aurora Chaser Cubesat project, University of Sydney (Yay! TaskScheduler goes to space)
    (by Jordan Jolly: http://52.35.220.33/blogs/jordan-jolly-cubesat--aero3670--blog)
	
  Hackabot Nano -  Compact Plug and Play Arduino compatible robotic kit
    (by Funnyvale: http://hackarobot.com/
     also: https://www.kickstarter.com/projects/hackarobot/hackabot-nano-compact-plug-and-play-arduino-robot)
     
  Arduino Nano based Hexbug Scarab Robotic Spider 
    (by arkhipenko: http://www.instructables.com/id/Arduino-Nano-based-Hexbug-Scarab-Robotic-Spider/)
	
  Wave your hand to control OWI Robotic Arm... no strings attached
    (by arkhipenko: http://www.instructables.com/id/Wave-your-hand-to-control-OWI-Robotic-Arm-no-strin/)
	
  APIS - Automated Plant Irrigation System
    (by arkhipenko: http://www.instructables.com/id/APIS-Automated-Plant-Irrigation-System/)
	
  Interactive Halloween Pumpkin
    (by arkhipenko: http://www.instructables.com/id/Interactive-Halloween-Pumpkin/)


Changelog:
=========
v2.1.0:
    2016-02-01 - support for microsecond resolution
    2016-02-02 - added Scheduler baseline start time reset method: startNow()

v2.0.1:
    2016-01-02 - bug fix: issue#11 Xtensa compiler (esp8266): Declaration of constructor does not match implementation

v2.0.0:
    2015-12-22 - _TASK_PRIORITY - support for layered task prioritization

v1.9.2:
    2015-11-28 - _TASK_ROLLOVER_FIX is deprecated (not necessary)
    2015-12-16 - bug fixes: automatic millis rollover support for delay methods
    2015-12-17 - new method for _TASK_TIMECRITICAL option: getStartDelay() 
	
v1.9.0:
    2015-11-24 - packed three byte-long status variables into one byte-long bit array structure data type - saving 2 bytes per each task instance

v1.8.5:
    2015-11-23 - bug fix: incorrect calculation of next task invocation in case callback changed the interval
    2015-11-23 - bug fix: Task::set() method calls setInterval() explicitly, therefore delaying the task in the same manner
	
v1.8.4:
    2015-11-15 - bug fix: Task alignment with millis() for scheduling purposes should be done after OnEnable, not before. Especially since OnEnable method can change the interval

v1.8.3:
    2015-11-05 - support for task activation on a status request with arbitrary interval and number of iterations (0 and 1 are still default values)
    2015-11-05 - implement waitForDelayed() method to allow task activation on the status request completion delayed for one current interval
    2015-11-09 - added callback methods prototypes to all examples for Arduino IDE 1.6.6 compatibility
    2015-11-14 - added several constants to be used as task parameters for readability (e.g, TASK_FOREVER, TASK_SECOND, etc.)
    2015-11-14 - significant optimization of the scheduler's execute loop, including millis() rollover fix option

v1.8.2:
    2015-10-27 - implement Local Task Storage Pointer (allow use of same callback code for different tasks)
    2015-10-27 - bug: currentTask() method returns incorrect Task reference if called within OnEnable and OnDisable methods
    2015-10-27 - protection against infinite loop in OnEnable (if enable() methods are called within OnEnable)
    2015-10-29 - new currentLts() method in the scheduler class returns current task's LTS pointer in one call
	
v1.8.1:
    2015-10-22 - implement Task id and control points to support identification of failure points for watchdog timer logging
	
v1.8.0:
    2015-10-13 - support for status request objects allowing tasks waiting on requests
    2015-10-13 - moved to a single header file to allow compilation control via #defines from the main sketch

v1.7.0:
    2015-10-08 - introduced callback run counter - callback functions can branch on the iteration number. 
    2015-10-11 - enableIfNot() - enable a task only if it is not already enabled. Returns true if was already enabled, false if was disabled. 
    2015-10-11 - disable() returns previous enable state (true if was enabled, false if was already disabled)
    2015-10-11 - introduced callback functions "on enable" and "on disable". On enable runs every time enable is called, on disable runs only if task was enabled
    2015-10-12 - new Task method: forceNextIteration() - makes next iteration happen immediately during the next pass regardless how much time is left

v1.6.0:
    2015-09-22 - revert back to having all tasks disable on last iteration.
    2015-09-22 - deprecated disableOnLastIteration method as a result
    2015-10-01 - made version numbers semver compliant (documentation only)

v1.5.1:
    2015-09-21 - bug fix: incorrect handling of active tasks via set() and setIterations(). 
		 		 Thanks to Hannes Morgenstern for catching this one
				 
v1.5.0:
    2015-09-20 - access to currently executing task (for callback functions)
    2015-09-20 - pass scheduler as a parameter to the task constructor to append the task to the end of the chain
    2015-09-20 - option to create a task already enabled
	
v1.4.1:
    2015-09-15 - more careful placement of AVR-specific includes for sleep functions (compatibility with DUE)
                         sleep on idle run is no longer a default and should be explicitly compiled with _TASK_SLEEP_ON_IDLE_RUN defined
						 
v1.0.0:
    2015-02-24 - Initial release 
    2015-02-28 - added delay() and disableOnLastIteration() functions
    2015-03-25 - changed scheduler execute() function for a more precise delay calculation:
                 1. Do not delay if any of the tasks ran (making request for immediate execution redundant)
                 2. Delay is invoked only if none of the tasks ran 
                 3. Delay is based on the min anticipated wait until next task _AND_ the runtime of execute function itself.
    2015-05-11 - added  restart() and restartDelayed() functions to restart tasks which are on hold after running all iterations
    2015-05-19 - completely removed  delay from the scheduler since there are no power saving there. using 1 ms sleep instead