KRNL - a small preemptive kernel for small systems

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 !!!

Some highlights

• 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

Some characteristics:

• 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

Timers

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

timer0

• 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

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

timer2

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

timer3, timer4, timer5

• 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

timer quants (heartbeat)

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

Overflow detection

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

warning / info

... 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

Warning

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