RTOS Training

7/23/2019 RTOS Training

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Real Time Operating Systems

Michael Thomas

Date:

Rev. 1.00Date



2

Objective

Ienti!y embee programming moels

"nerstan basic RTOS e!initions an concepts#oints to consier $hen yo% &'%y or roll yo%r o$n(



)

De!ining application re*%irements

Real time: +omp%ting $ith a ealine , re*%ire level o! service has to be provie in a bo%ne response

time

+onse*%ence o! missing ealines: &-arness( o! theproblemapplication-ar real/time / absol%te ealines m%st be met

irm real/time / lo$ occ%rrence o! missing a ealine can be tolerate

So!t real/time / tolerance in the orer o! h%man reaction time 0ms3



4

'asic concepts an e!initions

Tas5: Sel! containe coe that hanles a sing%lar!%nctionality or semi/inepenent portion o! theapplication that hanles a speci!ic %ty.

Threas: ,ccess to entire memory space

#rocesses: Memory access area limite base on process

creation parameters

#riority




Real-time Scheduling

Hard

Preemptive

Dynamic Static

Preemptive Non-preemptiveNon-preemptive

Soft

Sche%ler Implements the sche%ling policy



6


Determinism

78act time ta5en !or each threa to e8ec%te is a constant

9ernel

Sche%ler Memory management I#+ Synchroni;ation3

primitives

9ernel latency micro5ernel< pico5ernel etc3

9ernel space


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Other In!o

Tas5 ID

#riority

Tas5 Stat%s

Register +ontents

#rogram +o%nter

+onte8t

Typical T+'

+onte8t s$itch time

c%rrent +#" tas5

Save conte8t to T+'1 Save conte8t to T+'2

>oa conte8t from T+'2 >oa conte8t from T+'1

+onte8t S$itch an Tas5 +ontrol 'loc5


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-?S Stac5 !rames %ring a conte8t s$itch

task1 task2

task1 ready

task1 running

task2 ready

Contet S!itchtask2 running

"#C$"

%$R&$'"(S

"#C$"

%$R&$'"(S


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+loc5 Interr%pts occ%r every A10 ms allo$ OS to r%n3OS chec5s to see i! any higher priority process availablein reay *%e%eI! so< s%spen c%rrent process an s$itch to ne$ process

#reemption

# r i o r i t y R)#S Contet-S!itch service

&SR

Time

+onte8t S$itch

task 2

task 1

R*NN&N+

,$&)&N+



7mbee programming moels

System esigne aro%n tas5 sche%ling policy

7nless loop

Simple cyclic e8ec%tive

Time riven cyclic

M%lti/rate cyclic

M%lti/rate cyclic !or perioic tas5s

M%lti/rate cyclic $ith interr%pts bg!g programming3

#riority base pre/emptive



Tas51

Tas52

Tas5

Tas54

Tas5)

Simple cyclic

Tas51

Tas52

Tas5

Tas54

Tas5)

Timer Interr%pt

)ime driven cyclic

Timer Interr%pt

Tas51

Tas51

Tas51

Tas52

Tas5)

ulti-rate cyclic

Tas51

Tas51

Tas51

Tas52

Tas5)

Timer Interr%pt

ulti-r ate cyclic for periodic



Bhere oes an RTOS !it inC

Tas5 management vie$point:


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RTOS Services



Tas5 Management Sche%ling

Tas5 is consiere as a sche%ling %nit $hichimplements a !%nction

OS services that collectively move tas5s !rom one stateto another

#rovie conte8t s$itching services

Tas5 parameters li5e ealine< perio< priority etc nee tobe speci!ie



Tas5 Management

Tas5 states in a typical RTOS:

Dormant

BaitingReay

R%nning

Interr%pte



Tas5 Management

State transitions o! a tas5 in a typical RTOS

Baiting !or

reso%rcesReso%rces

availableInterr%pt

ISR one

Tas5 res%me

Tas5 preempte

tas5

elete


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Sche%ler an Sche%ling #olicy

Sche%ling algorithms generate a !easible e8ec%tionse*%ence base on

tas5 priority

tas5 ealines

reso%rce re*%irements

Ro%n Robin

7arliest ealine irst 7D3

Ma8im%m "rgency irst M"3

#riority +eiling #rotocol Reso%rce base3


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Interr%pt -anling

RTOS Inepenent Interr%ptsISR preempts the RTOS an r%ns

"se !or events that re*%ire acc%rate timing

E RTOS Depenant Interr%pts:

ISR is r%n !ithin the RTOS

E RTOS sho%l allo$ lo$er level ISR to be pre/empte byhigher level ISR RTOS Depenant Interr%pts3

E ISR m%st complete as *%ic5ly as possible.

E 'oth Depenant an Inepenent ISRs can e8ist in onesystem


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+loc5s an Timers

Tic5 timer Decies gran%larity o! system.

System response time limite by tic5 timer

7g: 0 microsec motor control interr%pt cannot be hanle by a 1 millisec

tic5 timer OS.

Timer primitives

,llo$ %ser to speci!y &R%n tas5 every 8 %nit o! time( %ring tas5

creation. eg: scan 5eyboar



Synchroni;ation

Synchroni;ation primitive %se to provie:M%t%al e8cl%sion: +ritical sections not accesse sim%ltaneo%sly

+onitional synchroni;ation: 7ns%re tas5s r%n in a speci!ic orer

E Synchroni;ation constr%cts:

M%te8 'inary semaphore3

Semaphore +o%nting semaphore3



Synchroni;ation an Semaphores

T$o tas5s an single b%!!er

#ro%cer tas5 an cons%mer tas5

+ons%mer tas5 to $ait till pro%cer tas5 is one $ith b%!!er.

Synchroni;ation an ata integrity achieve by %sing a

semaphore to loc5 access to b%!!er.

Iss%es:

Dealoc5s

#riority Inversion: >o$ priority threa obtains a loc5 that bloc5s a

higher priority threa



#riority Inversion

Tas5s > - both nee access to reso%rce F

Tas5 > has loc5e this reso%rce an hence elays -.

Tas5 M preempts > ca%sing !%rther elay to -G this is priority inversion

Reay to r%n

R%nning

time

preemption

#reemption a!ter

reso%rce . release

High priority task H

"o! priority task "

edium priority task

.


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Inter Tas5 +omm%nication

#rimary re*%irementscharacteristics:Hon/bloc5ing comm%nication

'o%ne latency

,synchrono%s comm%nication

Share Memory

Mailbo8Message %e%es



Mailbo8Message %e%e

)ask 1

)ask /

)ask 2

Mailbo8 1



2

Memory Management

Memory Management "nit MM"3-anles Jirt%al memory operations.

Memory protection esp. in process base OS

Dynamic memory allocation

-eapG non/!ragmenting memory allocation techni*%es&#ool( allocation mechanism



26

Dynamic Memory allocation: RTOS heap

/1 02 12 2 11 'uffer si3e

4ree 5uffer list



2=

RTOS classi!ication base on architect%re

So!t< irm an -ar real/time

#re/emptive< Hon pre/emptive

Threa base< #rocess base

Sche%ling policy: RM< 7D< RR< M" etc

Dynamic< Static

+ost str%ct%re



2?

RTOS a$are eb%gging

Stanar so%rce level eb%gging not s%!!icient inm%ltithreaem%ltitas5ing environment.

#otential b%gs:Data corr%ption %e to improper synchroni;ation

Dealoc5Starvation

Stac5 Over!lo$

Memory lea5s

RTOS/a$are eb%ggers provie tools that help etects%ch conitions !or *%ic5er eb%gging.78tra $ino$s sho$ing state o! all tas5s r%nning< $aiting etc3< incl%ing

b%!!ers c%rrently %se by each tas5< semaphores %se etc.



2@

'%y vs Roll yo%r O$n

actors:+ost Str%ct%re.

S%pport an oc%mentation

Scalability

#ortability

Deb%gging tools

Testing



)0


+ost Str%ct%re.Royalty per pro%ct one time payment

Overall evelopment< eb%gging< testing an s%pport reso%rces

!or in/ho%se tool %s%ally !ar o%t$eigh investing in a commercial

RTOS

S%pport an oc%mentation

Ins%!!icient oc%mentation

Small team responsible !or RD an maintenance

+ommercial RTOSes are $ell oc%mente $ith goo s%pport



)1


ScalabilityIn/ho%se tools are %s%ally esigne to meet e8act speci!ications

so it might be a goo !it

+ommercial venors allo$ signi!icant c%stomi;ation o! the 5ernel

in some cases a K"I !or this p%rpose3< so the notion o! too m%ch

overhea coe is %n!o%neSome commercial venors also provie so%rce coe to allay

engineer !ears o! &I cant see the coe so I onLt 5no$ $hatLs

going on(



)2


#ortability#orting an in/ho%se sol%tion to i!!erent har$are $ill re*%ire

signi!icant re/investment o! time an reso%rces

+ommercial venors have ports !or all major har$are targets

an $ill %s%ally port to others as $ell

Deb%gging tools

Deb%gging tools have to be evelopeporte !or in ho%se

RTOSG %s%ally ta5es 48 time neee to evelop the RTOS

+ommercial venors have reaily available eb%ggers an $ill

port over esire !eat%res in most cases



))


TestingIn/ho%se testingG ta5es 108 evelopment time to properly test

RTOS

+ommercial RTOSes are s%!!iciently teste an have also been

%se by many c%stomers so most b%gs $ill have been irone

o%t.



Objectives: Recap

Ienti!y embee programming moels

"nerstan basic RTOS e!initions an concepts

#oints to consier $hen yo% &'%y or roll yo%r o$n(

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