I have found it interesting to develop an open source realtime kernel for the Arduino platform - but is also portable to other platforms

• latest version 2001 *

changes about clip of semaphores so Liy & Layland RMA is detectable (when it fails deadlines)

• SEE SOME NOTES BELOW ABOUT TIMERS AND PINS
• Now doxygen docu at html directory :-)
• See krnl.h for further comments
• • timers
• • 8/16 MHz setting
• • etc

See github.com/jdn-aau or http://es.aau.dk/staff/jdn/edu/doc/arduino/krnl

See some warnings in the bottom !!!

• open source (beer license)

• well suited for teaching

• easy to read and understand source

• priority to straight code instead insane optimization(which will make it nearly unreadable)

• well suited for serious duties - but with no warranty what so ever - only use it at own risk !!!

• easy to use

• just import library krnl and you are ready

• automatic recognition of Arduino architeture

• supports all atmega variants I have had available (168,328,1280,2560 - uno, duemillanove, mega 1280 and 2560)

• Priority ceiling supported with k_prio_wait and k_prio_signal where you supply with task priorities

• Mmeory usage As of vrs 2001 -- static usage around 70B -- a semaphore/task occpy 18B in krnl data structure -- a msg-Q occupy 17B plus 18B(an internal semaphore) -- the two above allocated from heap

So k_init(2,3,4) gives 71B global (static) and 18*((2+1)+(3+1)+4) + 4*17 = 266B which

the +1 on task for a descriptor for main/dummy the +1 on semaphore for the timer/sleep semaphore

And then you have to add up arrays for stack for each task. Less than 50B is not a good idea. You can runtime check how deep the stack has been used by k_unused_stak

• preemptive scheduling
• Basic heart beat at 1 kHz. SNOT can have heeartbeat in quants of milli seconds
• static preemptivepriority scheme
• support task, semaphores, message queues
• All elements shall be allocated prior to start of KRNL
• support user ISRs and external interrupts

The Arduino has 3 or 6 timers (Mega has 6 the rest has 3)

• Timer0 and 2 is a 8bit timer.
• In the Arduino world Timer0 is been used for the timer functions, like delay(), millis() and micros().
• If you change Timer0 registers, this may influence the Arduino timer function.
• So you should know what you are doing.

• Timer1 is a 16bit timer.
• In the Arduino world the Servo library uses Timer1 on Arduino Uno (Timer5 on Arduino Mega).

• Timer2 is a 8bit timer like Timer0.
• In the Arduino work the tone() function uses Timer2.

• timer 3,4,5 are only available on Arduino Mega boards.

• These timers are all 16bit timers.

• krnl can be configures to use tmr 1,2 and for mega also 3,4,5 for running krnl tick

• For timer 0 you should take care of millis and it will require some modifications in arduino lib

• see krnl.h for implications

Accuracy

• 8 bit timers (0,2) 1 millisecond is 15.625 countdown on timer

  • example 10 msec 156 instead of 156.25 so an error of 0.25/156.25 ~= 0.2%

• 16 bit timers count down is 1 millisecond for 62.5 count

• • example 10 msec ~ 625 countdown == precise :-)

• timers default

• all except MEGA use timer 1 ( 8 bit)

• MEGAs (1280/2560) use timer 5

• you can change it in krnl.h

k_start accepts 1..10 and 20,30,40,...10000 milliseconds timer quants So you can not run krnl with an internal timer at 16 msec You can change it in k_start but be aware of implications of 8/16 bit timer usage

The user can provide two functions which will be called when a semaphore/msgQ are overflooding

... from http://arduino-info.wikispaces.com/Timers-Arduino

• Servo Library uses Timer1.

• • You can’t use PWM on Pin 9, 10 when you use the Servo Library on an Arduino.
• • For Arduino Mega it is a bit more difficult. The timer needed depends on the number of servos.
• • Each timer can handle 12 servos.
• • For the first 12 servos timer 5 will be used (losing PWM on Pin 44,45,46).
• • For 24 Servos timer 5 and 1 will be used (losing PWM on Pin 11,12,44,45,46)..
• • For 36 servos timer 5, 1 and 3 will be used (losing PWM on Pin 2,3,5,11,12,44,45,46)..
• • For 48 servos all 16bit timers 5,1,3 and 4 will be used (losing all PWM pins).

• Pin 11 has shared functionality PWM and MOSI.

• • MOSI is needed for the SPI interface, You can’t use PWM on Pin 11 and the SPI interface at the same time on Arduino.
• • On the Arduino Mega the SPI pins are on different pins.

• tone() function uses at least timer2.

• • You can’t use PWM on Pin 3,11 when you use the tone() function an Arduino and Pin 9,10 on Arduino Mega.

See in krnl.h for information like ...

... from http://blog.oscarliang.net/arduino-timer-and-interrupt-tutorial/

Install from github:

1. cd whatever/sketchbook/libraries - see Preferences for path to sketchbook
2. git clone https://github.com/jdn-aau/krnl.git

You have from Arduino inherited many critical regions which you have to protect - like

• Serial channels - only on thread at time must have access
• digital and analog IO (digitalRead, AnalogRead,...)
• and in general all libraries - so take care

This is NOT an Ardunio problem but a standard feature i multithreaded systems :-)

(c)

• "THE BEER-WARE LICENSE" (frit efter PHK) *
• jdn@es.aau.dk wrote this file. As long as you *
• retain this notice you can do whatever you want *
• with this stuff. If we meet some day, and you think*
• this stuff is worth it ... *
• you can buy me a beer in return :-) *
• or if you are real happy then ... *
• single malt will be well received :-) *

•                                                *
  

• Use it at your own risk - no warranty

Happy hacking

See also http://es.aau.dk/staff/jdn/edu/doc/arduino/krnl

/Jens

and https://github.com/greiman/FreeRTOS-Arduino has freertos for Arduino and Arduino Due