A Classification Scheme for Embedded

8/6/2019 A Classification Scheme for Embedded

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A C L A S S I F I C A T I O N SCHEME FOR EMBEDDEDCONTROL SYSTEMS

Jukka Karjalainen " )

Technic al R esearch Centre of Finland , Computer Technology Laboratory,P.O.Box 181, SF-90101 Oulu, Finland+358 81 346 488

Ab s t r a c tT h i s p a p e r p r e s e n t s s ome in f o r ma l d e f in i t io n s o f embe d-ded systems. From th e poi nt of view of a contro l com-p u te r s t r u c tu r e , e mb e d d e d c o n t r o l s y s te ms are d iv id e di n t o s i x d i f f e r e nt classes and some key charac te r is t icsand t yp ic a l examples of each proposed class are g iv e n .T he c h a r a c te r i s t i c s of e mbe dd ed c o n t r o l s y s te ms ar eg ro up e d i n t o e i g h t d i f f e r e n t p r o p e r t i e s an d t h e t y p i -cal numbers o r c a t e g o r i e s o f e ac h c h a r a c t e r i s t i c ar ep r e s e n t e d . Th e c l a s s i f i c a t i o n and c h a r a c t e r i z a t i o n arec ombine d i n o r d e r t o d e f in e a c l a s s i f i c a t i o n sch em e f o re mb ed de d c o n t r o l s y s te ms. T h e c h a r a c te r i s t i c s o f s ixproposed classes a r e sh own in th e f o rm o f e ig h t t a b le s .

KEYWORDS: embedded syst ems , c on tr ol syste ms, r ea l- ti mesys tem s , c ont ro l computer s , microcomputer s i n au toma-t i o n , c l a s s i f i c a t i o n , m o de ll in g

" ) Th e work was done w h i l e t h e a u th o r was a v i s i t i n gs c i e n t i s t a t t h e I n s t i t u t e f o r . E l e c t r o ni c s . t h e Fe d er a lI n s t i t u t e o f Technology, ETH, Z u e r ic h . S wi tz e r l a n d .

I n t r o d u c t io nDur ing the l a s t f i f t e e n y e a r s th e d e v elop men t o f micr o-processor and memory technologies has made i t p o s s i b l et o a p p ly c omp uter t e c h n o lo g ie s in many d iv e r s e a p p l i c a -t ions . Computer s ar e i n t e g r a l p a r t s o f larger s y s te ms ,ranging f rom a s i m p l e e l e c t r o n i c t o y to a l a r g e c o n t r o ls y s te m f o r a nuc lear power p lan t [7 ,9] . These micropro-c e s s o r b a s e d s y s tems are of ten ca l led embedded sys tems .Embedded systems ar e o f t e n h a r d real-time sys tems ,which have time c r i t i c a l e x t e r n al i n t e r f a ce s [12] an dh i gh r e l i a b l i t y . a v a i l a b i l i t y and m a i n t a i n a b i l i t y re-quireme nts . Usuall y al so , components of embedded sys -tems have many phys ica l c ons tra in ts [13]. such aswe ig h t , v olume , h e a t d i s s i p a t io n e t c .The term "embedded computer system" w a s i n t r o d u c e d b yt h e U.S. Department of Defence [5] t o d e n o t e on e t h a ti s l o g i c a l l y i n c o r p o r a t e d i n a l a r g e r s y s te m - e .g . ane le c t r o me c h a n ic a l d e v ic e , a t a c t i c a l s y st e m , a s h i p , ana i r c r a f t , or a communication system - whose primaryf u n c t io n i s no t computation. According to Zave [ l 3 .141. an embedded system i s o n e f o r wh ic h p r o p e r t i e s o fth e physi cal world appear somehow in i t s mandatory re-quirements. The common element among embedded systemsis t h a t t h e i r s u c c e s f u l o p e r a t i o n de pe nd s d i r e c t l y oncognisance o f , a nd a d a p t a t i o n t o , p h y s i ca l p r o p e r t i e so f t h e i r e n v i r o n men ts . Bu e h re r [3 ] defines an embeddedsys tem as a p r o ce s s in g f a c i l i t y h av in g a v a r i e t y o f i n -pu ts f rom th e environment and a v a r i e t y o f o u t p ut s t othe environment .I n t h i s p ap er w e examine an embedded system from thep o i n t of view of i t s computer s t ruc ture . W d e a l w i than embedded contro l computer sys tem, whose def in i t ionw e m od if y ba se d on e a r l i e r d e f i n i t i o n s as follows:

An embedded control computer system is a part ofa larger technical system, whose primary functionis not computat ion nor text processing but v eruclose interaction with an unintelligent environ-ment in absolute time-critical limits. The tasksof the control computers of an embedded controlsystem are to read measurement data from inputlines and/or commands from a user, to processthem according to application specific algorithmsand to send the results t o output control linesand/or to a user. Programming done by a user ofan embedded system is usually restricted to themodifications of activities according to prede-fined tasks designed by the manufacturer of thesys em

Computer a rc h i te c t ur es have been c l as s i f i ed in manyways [ l , 4 . 6. 8 , 111, but these schemes are n o t s u i t -a b l e f o r t h e c l a s s i f i c a t io n o f e mb ed de d c o n t r o l c om-p u t e r s y st e m s. I n some s p e c i a l f i e l d s c l a s s i f i c a t i o nschemas have been d ef ine d , as i n image process i ng ap-p l i c a t i o n s , w he re sys tems have been d iv ided in to gen-e r a l p u rp o s e a r c h i t e c tu r e s a nd f u n c t io n a l ly d e d ic a te da r c h i t e c t u r e s [15]. Zave 1141 in troduces a r equirement-


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l e v e l s y s te m c l a s s i f i c a t io n , whe re c ompu te r s y ste ms ared iv id e d in to th r e e ty p e s : embe dd ed s y ste ms , d a t a p ro -cess ing sys tems and suppor t sys tems . Smith [lo] hasc l a s s i f i e d mic r oc o mp ute rs in to g e n e r a l p u rp o se micr o-computers , d edi cat ed micro process ors and developments y s erns.The purpose of our paper i s t o p r op os e d i f f e r e n tc las ses of embedded cont ro l sys tems and t o sp ec ify t hem ost i m p o r t a n t c h a r a c t e r i s t i c s of each def ined c l a s s .I n s e c t i o n 1 embedded control systems are d i v i d e d i n t os i x d i f f e r e n t g r ou p s an d ty p i c a l p r o d u c t ex a mp le s o feach group are g i ve n . I n s e c t i o n 2 d i f f e r e n t c h a r a c t e r -i s t i c s of embedded control systems are proposed byd e f in in g th e m as e ig h t gr o up s o f p r o p e r t i e s . S e c t io n 3combines sec t i on s one and two and in trodu ces a c las-s i f i c a t i o n s ch eme f o r e mb edd ed c o n t r o l s y s te ms . S e c t io n4 pres en t s the conc luding remarks about our c l a s s i f i c a -t ion scheme.

1. CLASSIFICATION OF EMBEDDED CONTROL SYSTEMS

I n t h i s s e c t i o n , w e c lass ify embedded contro l sys temsi n t o s i x d i f f e re n t classes a c c o rd in g to t h e i r c omp uters t r u c t u r e a nd o t h e r r e l e v a n t p r o p e r t i e s . O ur p o i n t o fview i s based on the contro l computer s o f an e n t i r ec o n t r o l s y s te m. T h e classes are as f o l lo ws :

1 Simple micr ocont ro l le r sys tem2 Microcontro l le r sys tem3 Co n t r o l l e r s y s te m4 Medium s c a l e c o n t r o l s y ste m5 Large contro l sys tem6 S p e c ia l p u r p o s e c o n t r o l s y s te m.

Contro l sys tems are u s u a l l y d i v i d e d h i e r a r c h i c a l l y i n t od i f f e r e n t f u n c ti o n a l l e v e l s , e s p e c i a l l y i n t h e c a se ol a r g e c o n t r o l s y s t e m s , as shown i n f i g u r e 1. Althougth e s y s te m o f f i g u r e 1 i n c l u d es f o u r h i e r a r c h i c ale v e l s , l a r g e s y s te ms c an h av e f ro m on e l e v e l up tseven or even more depending on an appl ica t ion f i e l dUsua l ly , s imple microcontro l le r sys tems and microcont r o l l e r s ys te m s c o n s i s t o f o n e fu n c t i o n a l c o n t r o l le i ,eor t h e y are p a r t s of t h e lo we s t c o n t r o l l e \ e( s e n s o r / a c t o r l e v e l ) of a l a r g e r s y st e m. T he c o n t r o l l esys tem usua l ly inc ludes two contro l l e i e l( s e n s o r / a c to r l e v e l a nd p r o c e s s in g /c o n t r o l l e v e l ) anc o n ta in s s e v e r a l ( s im p le ) mic r o c o n t r o l l e r s y s tems athe lowes t f uc t i on a l lev e l . The medium scale contr -os y s te m, which ty p i c a l l y c o n s i s t s of th r e e f u n c t lo n ale v e l s , c o mp ri s es o f s e v e r a l c o n t r o l l e r s ys tems a nd at h e l o w e s t f u n c t i o n a l l e v e l m i c r o c o n tr o l l e r s y s te m sF ig u r e 1 sho ws t h a t t h e r e e x i s t s b o t h h o r i z o n t a( i n t r a - l e v e l ) a nd v e r t i c a l ( i n t e r - l e v e l ) c om m un ic at iob e twe en f u n c t io n a l l e v e l s . The u s e r in t e r f a c e o f a cont r o l s y s t e m i s u s u a l l y a t t h e h i g h e s t l e v e l o f a s y st e m , bu t i n some app l ica t io ns human/machine in t e r ac t i omay happen a t many func t i ona l l ev e l s , when the&resp ontime r equirements of a u s e r i n t e r f a c e v a r y c o n s i d e r a oa t d i f f e r e n t l e v e l s

Figure 1 Hierarchical levels of a large control sys t em

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The produc ts o f the f i r s t c lass of embedded controlsys tems. s imple mic rocon tro l le r sys tems , are manufac-t u r e d i n l a r g e q u a n t i t i e s a t l ow c o s t . T h e i r p h ys i c als i z e i s usually very small and power consumption tendst o be very l imit ed . They a re based on a one chip micro-computer implementat ion . where the s t ru c t ur e of c ontr o lsof tware i s r a t h e r s t r a i g h t f or w a r d , p o s s i b l y c o n s i s t i n gof a small number of concur ren t p rocesses . Elec tron ict o y s , smart cards, watches and keypad/display-con-t r o l l e r s are some typ ic a l cur ren t examples of s implemic r o c o n t r o l l e r s y ste ms.

Although m icroc ontr o l le r systems are a l s o o f t e n b a s e don a one chi p microcomputer implementation, they havemore features and processing power than simple micro-c o n t r o l l e r s y ste ms. T he y are 8 or 1 6 b i t microcomput-e r s , whose c ontro l sof tw are compr ises of 4 t o 16kbytes. The av ai la bl e memory capa cit y i s l im i t e d a n dap pl ic at io n programming may s t i l l be done a t assemblerl e v e l . T h e h a r d wa r e a r c h i t e c tu r e i s very s imple , bu tresponse time requirements are very t ig h t i n some ap-p l i c a t i o n s . Cameras, domest ic appl iance machines, r ad i-a t i o n me te r s , r a d io t e l e p h o n e s , c a r d r e a d e r s , d a sh b oa rdcomputers , programmable log ic con tro ll er s ( P L C s ) , s l i d ep r o je c to r s , h a rd d i s k c o n t r o l l e r s a nd I /O-c o n t r o l l er sof a larger sys tem be long t o th e class of embeddedmic r o c o n t r o l l e r s y ste ms.

The t h i r d c l a s s , c o n t r o l l e r s ys te m s, c o n s i s t s o f g e n-e r a l purpose 8 o r 16 b i t microcomputers with a l l t h et y p i c a l p a r t s o f a cont ro l computer . The process in te r -face co nta i ns bo th ana log and d ig i ta l l i ne s and commu-n i c a t i o n c a p a b i l i t i e s are needed. In many appl ica ti onsreal- t ime requirements ar e very ti gh t and t he minimumresponse time requirements are i n t h e r an ge of m i l -l iseconds . The contro l sof tware , which i s based on asimple rea l- time ker nel , amounts t o t e n thousands k i lo -bytes and h igh leve l languages are used i n programming.T y p ic a l ex amples i n t h i s c l a s s a re te rmina ls , l aserp r i n te r s , a d v e r t i s in g d i s p la y s y ste ms, e l e c t r o n ic e n-g i n e c o n t r o l l e r s , i n d u s t r i a l f ur n ac e c o n t r o l l e r s . o i lc o n te n t a n a ly z er s . s e r v o c o n t r o l l e r s , t i c k e t a utoma-tons , mobile r ad io te lephones , PID-contro l le r s . AC mo-tor -con tro l le r s and ind us tr ia l we ighing machines.

T h e next c l a s s of embedded con tr ol systems a re mediums c a le c o n t r o l s y s te ms , which u s u a l ly c o n s i s t o f s e v e r a l8 and 1 6 b i t microcomputers . Athough they ar e manufac-t u r ed i n se r i e s production, some customization featuresa re of te n req ui red . The amount of I / O-li nes can be upt o one thous and and minimum respon se times are i n t herange of milli secon ds. Fast communication li n es a r e ?used between subcomputers and al so t o connect them to

a n u pp er c o n t ro l l e v e l . R e l i a b i l i t y and a v a i l a b i l i t ya r e o f t e n imp o r ta n t f a c to r s . T he c o n t r o l s o f twa r eamounts t o h u nd re d s o f k i lo b y te s , i n d ic a t in g th a t a d -vanced design languages and too ls ought t o be used.L i f t c o n t r o l s y ste ms, p a t i e n t mon i tor in g s y ste ms, c a s hte r min a l s y s te ms , f i r e an d b u r g la r d e te c t io n an d alarms ys te m' s. me dic al a n a l y z e r s , p l a n t v e n t i l a t io n c o n t r o lsystems, automatic t e l l e r machines , mine dr i l l ing ma-chin es , cont ro l sys tems fo r weld ing manipula tor s , a i rt r a f f i c c o n t r o l s ys tems an d p r o c e s s s t a t io n s o f a largeautomation system are some examples of medium scalec o n t r o l s y ste ms.

The f i f t h c l a s s of contro l sys tems i s r epresen ted byla rg e con tro l sys tems , pa r t s o f which show a l l t h e f e a -tu r e s o f o th e r c l a s s e s , a n d a lso some ex tr a charac t e r -i s t i c s . A l a rge con tro l sys tem con s is t s o f from tens tohundreds of p rocessors , and i t conta ins a f a s t communi-ca t ion subsys tem, e . g . a c o l l e c t io n o f b u s s e s . Be s id esfa s t numer ica l p rocess ing , symbolic process ing capabi l -i t i e s are n e ed e d, e s p e c ia l l y i n th e f u tu r e . The c o n t r o lsoftware amounts to Mbytes, t ha t means advancedmethods, to ol s and sp ec ia l development environments ar eused. Because the li fe- cy cl e may extend beyond te nyears , main tenance aspec ts and sof tware conf i gura t ionmanagement become very important factors. Telephone ex-change systems, ai rbo rne warning and cont rol systems,proce ss automation systems, c ont rol systems of papermachines , energy d i s t r i bu t i on cont ro l systems and la rg ealarm systems a re typ ica l examples of la rg e contro ls y s ems .The s i x t h c l as s o f embedded c ontr o l sys tems i s c a l l e dsp ec ia l purpose cont rol system, because they have somea p p l i c a t io n s p e c i f i c p r o p e r t i e s , which a r e n o t in c lu d edi n o t h e r c lasses . They usua l ly co ns i s t o f genera l pur -pose computer par ts a s w e l l as s p e c ia l p r o c e s s o r p a r t s .They per form appl i ca t i on sp ec if ic a lgor i thms and haveh i g h p ro c e ss i n g c a p a b i l i t i e s f o r t h o s e s p e c i f i c a p p l i -c a t io n s . P r o c e s s e d d a ta i s huge and i n ad di t i on t o sym-b o l i c p r o c e s sin g c a p a b i l i t i e s , f a s t n umber c r un c h in gc a p a b i l i t i e s are needed. Production volumes are low andcus tomized fea tures are of g r e a t importantance. Imageprocess ing sys tems , advanced robot contro l sys tems , ad-vanced welding control systems, s imulation/emulationenvironments and cont ro l sys tems in space appl i ca t i onsar e examples of spec ia l purpose contr o l sys tems .

2 . CHARACTERIZATION OF EMBEDDED CONTROL SYSTEMS

J n th i s s e c t io n , th e c h a r a c te r i s t i c s of e mbedded con-t r o l c o mp uter s y ste ms a r e d e f in e d. W d iv id e th e s ec h a r a c t e r i s t i c s i n t o e i g h t g ro up s as fo l lows :

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Hardware a rch i tec tureP r oc e ss o r c a p a b i l i t i e sS o ft w ar e c h a r a c t e r i s t i c sP e r f o r ma n c e c h a r a c te r i s t i c sR e l i a b i l i t y a n d s a f e t yEnvironmenta l condi t ionsP h y s i c a l c o n s t r a i n t sO t h e r c h a r a c t e r i s t i c s .

2 . 1 Hardware a rch i tec ture

T he h ar dwa re a r c h i t e c t u r e s p e c i f i e s th e b a s ic c h a r ac -t e r i s t i c s of the system ar ch i t ec tu re of embedded con-trol sys tems . The hardware a rch i t ec t ur e inc lu des th ef o l lo win g p r o p e r t i e s :

1 Number of processorsThe number of processo rs giv es t he number of p r o -gramnable processors of a sys tem. Typica l va lues are 1,5, 10, 50, 100 o r more than 100 p r o c e s s o r s .

2 I n t e r co n n e c t io n s t r u c t u r eT he in te r c o n n e c t io n s t r u c t u r e d e s c r ib e s how d i f f e r e n ti n t e r n a l p r oc e ss i ng p a r t s a t e ac h h i e r a r c h i c a l l e v e lare connected. Typica l means ar e single processor bus,multiprocessor bus, serial busses and special busses.

3 Mass memory si z eThis prope r ty g iv es th e amount of th e secondary s to rag eof a sys tem. Typica l va lues a re 10 MB, 100 MB, 1 GB andmore than 1 GB .

4 I /O- l in e sT h i s p r o p e r ty s t a t e s th e a mo un t o f in p u t /o u tp u t l i n e sof a system. Typ ical numbers of I / O-l ine s a re 8, 16,3 2 , 128, 512, 1024 and more than 1024 lines.

5 Communicat ion a rch i t ec t ur eThe communicat ion a rch i t ec tu re descr ibes how d if fe re n tf u n c t io n a l u n i t s c ommu nica te w i th o th e r u n i t s or e x t e r -n a l s y s t em s . T y p ic a l a l t e r n a t i v e s a r e slow serial lines( speed < 20 k b i t s / s e c ) . tast serial lines (speed > 20kbrts/sec). S L O W LocaL area ne tworks ( L A N s ) (speed iMbits /sec) and fast L A N s (speed > 5 Mb i t s / s e c ) .

6 U s e r i n t e r f a c eT he u s e r i n t e r f a c e o u t l i n e s c h a r a c t e r i s t i c s o f t h e h u-man/machine i nt er fa ce . Usual means are simple key-pad(/display) u nit, keyboard and dispZay terminal , ad-vanced (graphic) interface and special interfaces.

2 .2 P r o c e s s o r c a p a b i l i t i e s

T he p r o c e s s o r c a p a b i l i t i e s e xp r e s s th e p r o c e s s in g p ro p -e r t i e s o f th e mo st p o we rf ul p r o c e s s o r s o f a system. Thec a p a b i l i t i e s a r e a s fo l lo w s :

1' Word lengthT h i s p r o p e r ty i s the number of b i t s i n a word of a p r o -c e s s o r . T y p i c a l va l u e s a r e 4, 8, 16 , 32 or specLa1 num-b e r o f bits.

2 Si ze of phy sic al memoryThe s ize of phy sic al memory gives t he m ou nt of mainmemory in t he processor un i t . Typica l va lues a re 2 kB ,8 kB , 64 kB , 256 k B , 1 M B , 8 MB and more than 8 MB .

3 I n s t r u c t i o n s e tT h i s p r o p e rt y c h a r a c t e r i z e s t h e v e r s a t i l i t y of t h e i ns t r u c t i o n s e t of the processor . I t c an be c h a r a c te r i z eby a szmple, medium or compZex i n s t r u c t i o n s e t . T hs i m p l e i n s t r u c t i o n se t means tha t the amount of d i f f e re n t i n s t r u c t i o n s i s small and a process or has qu i te fewaddressing modes. A processor with th e complex in s t ruct i o n se t conta ins many hundred in s tr uc t i on s and sev erad i f f e r e n t a d d r e s s in g mo de s, a s we l l a s , some h ig h l e v ei n s t r u c t i o n s . A proces sor with a medium in s t ruc t i on s ei s between proce sso rs with t he s imple and complex ins t r u c t i o n s e t .

4 P r o c e s s in g s p e edT h is p r o p e r ty i s a measure of how many in st ru ct io ns proces sor can per form i n one second . Typic a l va lues a r0.1, 0.5, I, 5, 10, 50 and more than 50 M I P S .

5 F lo a t in g p o i n t p r o c e s sin g s pe edT h is p r o p e r ty i s a measure of how many f l o a t i n g po inope ra t ion s a processor can perform i n one second . Typrc a l v a l u e s a r e 10 W L O P S , 100 W L O P S , 1 MFLOPS, 1MFLOPS and more than 1 0 MFLOPS.

6 Symbolic process ing speedT h is p r o p e r ty i s a measure of how many lo gi ca l i n f e rences a proces sor can perform i n one second . Typicav a lu e s a r e 0.1 kLIPS , 1 k L I P S , 10 R L I P S , 100 kLIPS an1 MLIPS.

7 Vi rt ua l memory/protection schemeThis proper ty c har ac t e r i zes the v i r t ua l memory and p r ot e c t i on c a p a b i l i t i e s of a processor . I t ca n be s i m p lo r advanced. The advanced scheme means that a processohas a paged and/or segmented memory with protection cap a b i l l t i e s as w e l l as many p r io r i t y l e v e l s . A process0

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wi th th e s imp le s c h e me h a s o n ly v e r y l imi te d v i r tu a lme mo r y /p r o te c t io n c h a r a c te r i s t i c s .

8 P i p e l i n e / c a c h eT h is p r o p e r ty c h a r a c t e r i z e s p ip e l in e a nd c ac h e f e a tu r e sof a p r o c e s s o r . T h e advanced p i p e l i n e l c a c h e p r o p e r t i e si n c l u d e p i p e l i n e f e a t u r e s , i n s t r u c t i o n a n d d a t a c a ch e s.A p r o c e s s o r w i th th e simple p r o p e r t i e s h a s u s u a l ly on lya small i n s t r u c t i o n c ac he or queue.

2.3 S o f t w a r e c h a r a c t e r i s t i c s

The sof tw are ch ar ac te r i s t ic s o f an embedded systemspe c if y th e amount and the complexi ty of th e con tro ls o f t w a r e as w e l l as th e methods, to ol s and systems usedto d e s ig n t h e c o n t r o l s o f twa r e . T h e s o f twa r e c h a r a c te r -i s t i c s are as f o l lo ws :

1 Amount of c on tr ol sof tw areT h i s p r o p e r ty g i v e s t h e a mo un t o f a b s o l u te c o n t r o lsof tware as l i n e s of source code. Ty pica l va lues are1 000, 10 000, 100 000 and 1 000 000 code lines.

2 Opera t ing sys temT h i s p r o p e r t y d e s c r i b e s t h e v e r s a t i l i t y o f t h e o p e r at -i n g s ys te m a t t h e h i g h e s t f u n c t i o n a l l e v e l o f a system.A simple kernel c o n t a i n s o n ly t h e b a s i c f e a t u r e s o f t h eo p e r a t in g s y ste m a nd i t s s i z e i s a few kbytes. The nextc a te g o r y i s a real-time operating system w i t h t h e s i z eof t ens of kbytes . An advanced operating system ha s a l lt h e f e a tu r e s o f th e mu l t iu s e r o p e r a t in g s ys tems o fminicomputers .

3 Complexi ty of contro l a lgor i thmsThis pro per t y expresses how complica ted the contr o l a l -gor i thms of a system are. Simple c o n t r o l a l g o ri t h m s arebased on simple se r i a l and c o n t r o l s t r a t e g i e s . Mediumcon tro l a lgor i thm s have some fea tur es of an adapt ivec o n t r o l . Advanced a lgor i thms a re the combina t ions ofd i f f e r e n t l i n e a r , n o n - l i n e a r , a d a p t iv e and p a r a l l e l a l -g o r i h ms .4 Languages and toolsT h i s p r o p e r ty d e s c r ib e s how v e r s a t i l e l a n g u a g e s a ndt o o l s are i n d e v e lo p in g an a p p l i c a t i o n s o f t w a re f o r asystem. Assemblers, high level languages ( H L L ) , ad-vanced toots ( s p e c i f i c a t i o n l a n g u ag e s , d e s ig n l a ng u ag e se t c . ) are t y p i c a l e xa mp le s of d i f f e r e n t c a t e g o r i e s .5 Development environmentsThis prop er ty desc r ibe s th e k ind of development envi-ronment used i n deve loping t he Lontrol s o f twa r e . A mod-est devezopment system c o n t a i n s O I ~ L assemblers and

s imp le e mu la to r s . A development system i n c l u d e s assem-b l e r s , H L L ' s . e mu la to r s , a n d o th e r c o n v e n t io n a l to o l s .An advanced development system suppor ts sof tware con-f ig ura t io n management , au tom atic code gen era t ion , elec-t r o n i c m a i l , etc . I n some c a s e s advanced o r special de-velopment environments ar e used .

6 Sof tware conf igura t ion managementThis prop er ty cha rac te r ize s methods and sys tems used insoftware configuration management (SCM). In a modestmethod only s imple too ls ar e a v a i l a b l e . I n an advancedmethod the whole software development and productionsystem i s based on t h e conc ept of SCM.

2 . 4 P e r f o r ma n c e c h a r a c te r i s t i c s

T h e p e r f o r ma n c e c h a r a c te r i s t i c s in d ic a te p e r f o r ma n c emeasures o f a who le c o n t r o l s ys tem. T h e c h a r a c te r i s t i c sar e as f o l lo ws :

1 Co n t r o l c y c le timeT h is p r o p e r ty d e s c r ib e s how lo n g t h e c o n t r o l c y c le i sa l lowed to l a s t by an ex te rna l p rocess /environment .T y p ic a l v a lu e s are 1 s, 100 ms, 10 ms, 1 ms and 0.1 ms.I n a h i e r a r c h i c a l s ys te m t h e r e ar e d i f f e r e n t c y c l etimes a t d i f f e r e n t f u n c t i on a l l e v e l s of a system.

2 Rate o f i n p u t d a t aT h i s p r o p e r ty g iv e s a measure f o r t h e i n p u t d a t a ratet h a t a con tro l sys tem mus t be ab le t o process . Therange i s very ex tens ive : f rom less than 1 b i t / s e c t omore than 100 Mbits /sec .

2.5 R e l i a b i l i t y a nd s a f e t yIn some cases t h e r e l i a b i l i t y and s a f e t y of c o n t r o lsys tems are v e r y imp o r ta n t c h a r a c te r i s t i c s . T h e c h a r a c -t e r i z a t i o n o f t h e se is fo l lowing:

1 R e l i a b i l i t yT h i s p r o p e r t y g i v e s a m ea su re o f t h e l e v e l o f r e l i a b i l -i t y of a c o n t r o l s y s te m. T y p i ca l c a t e g o r i e s are normal,high and very high. The normal categ ory means th att h e r e are n o s p e c i a l r e l i a b i l i t y r e q ui r em e n ts . The highr e l i a b i l i t y l e v e l mea ns th a t t h e do wn -t ime o f a systemc a u s e s i n mo d er a te , r e c o v e r a b le lo s s e s ( t h i s i s t h emodif ic a t ion of the ca teg or i es presen ted by Boehm [Z]).T he ve r y hi g h r e l i a b l i t y l e v e l means t h a t t h e f a i l u r eof a c o n t r o l s y s te m a f f e c t s h i gh f i n a n c i a l l o s s e s .

2 S a f e t yT h i s p r o p e r ty d e s c r i b e s th e imp o r ta n ce o f s a f e ty . T h ec a t e g o r i e s a r e normal, high and very hlgh. The normal

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category t h a t t h e failure f a con t r o l system th a t every de l iv ere d system inc lude s cus tomized fea -c a n no t c a u s e an y r i s k t o human l i f e . I n t h e c a s e O F tures Of Its Own.h ig h c a teg o r y th e f a i lu r e of a contro l sys tem can causesome r i s k to h uman l i f e . wh i l e i n th e v e r y h ig h cate-g o ry t h e r i s k i s very h;gh.

modificationsThis proper t y desc r ib es how of t en hardware and/or so f t -w ar e m o d i f i c a ti o n s a r e c a r r i e d o u t i n t h e l i f e c y c l e o fa d c o n t r o l s y s te m. T he c a te g o ry seldom means tha t theamount of modif ica t ions i s low, while the ca tegory f r e -

The environmental stresses g r e a t l y a f f e c t t h e s t r u c t u r e quently that are made continouslyan d des ig n of a c ont ro l computer sys tem. These charac-

2. 6 Environmenta l condi t ions

during the life yc1e Of a system.te r i s t i c s d e a l w i th c o n t r o l c omp uter s y ste ms u n de r o p-e r a t i o n and a r e a s fo ll ow s: CONTROL SYSTEMS3 . CHARACTERISTICS OF DIFFERENT CLASSES OF EMBEDDED

1 Siz e and weightT h i s p r o p e r ty t e l l s to wha t d e g re e th e s i z e a nd we igh tof a sys tem have an a f f ec t when des ign ing a c on tro lsys tem. A ve r y small ca tegory means th a t spe c i a l compo-n e n t s t e c h n o lo g ie s , e . g . h y b r id or custom design c i r -c u i t t e c h n o lo g ie s , mus t b e u se d . A small ca tegory de-n o t e s t h a t t h e s i z e a nd we ig ht p a r t l y a f f e c t s t h e se -le c t io n of ava i la b le component techn olog ies . The normals i z e a n d we ig h t means t h a t th e y d o n o t l i m i t t h e s e l e c -t i on of component te chnol ogies .

2 Power consumptionT h is p r o p e r ty s p e c i f i e s i f t h e po we r s u p p ly i s l i m i t e d .A v e r y limited catego ry deno tes t he power consumptionof a computer system must be very low (e .g . l e ss th an10 m W ) . A small catego ry means th at power supply i sl i m i t e d t o a c e r t a in s ma l l v a lu e ( e . g . 500 mW). Anormal ca tegory denotes th a t power supply i s n o t re-

I n t h i s s e c t i o n, w e combine the c la sse s and th e charac-t e r i s t i c s of embedded cont ro l sys tems in ord er t o de-f i n e a c l a s s i f i c a t i o n s chem e f o r d i f f e r e n t k i nd o f e m -bedded contro l sys tems .

I n t a b l e 1 t h e ha rd wa re c h a r a c t e r i s t i c s o f t h e d i f f e r -en t c l ass es of embedded con tro l sys tems a re shown. Fore a c h o f th e c l a s s e s th e v a lu e s or t h e p r o p e r t i e s o fn umbe r o f p r o c e s s o r s , i n t e r c o n n e c t io n s t r u c tu r e , massmemory s i ze , I / O- l ine s , communica tion a rc h i te c t ure andu s e r i n t e r f a c e a r e o u t l i n ed .

T he p r o c e s s o r c a p a b i l i t i e s of e a c h c l a s s a r e s umma rize di n t a b l e 2 t h a t g i v e s t y p i c a l v a l u e s or p r o p e r t i e s o fword l e n g th , s i z e o f p h y s lc a l memory, i n s t r u c t io n s e t ,p roce ss ing spe eds , v ir tu a l memory/pro tec t ion scheme andp i p e l i n e / c a c h e c h a r a c t e r i s t i c s .

s r i c t e d . T he s o f twa r e c h a r a c t e r i s t i c s o f a l l c l a s s e s of e mbe dd edc o n t r o l s y s te ms a r e p r e s e n te d in t a b l e 3 . The values orp r o p e r t i e s a r e g i v e n f o r the amount of con tro l sof t -ware, op era t in g sys tem, complexity of cont ro l algo-

2.8 O t h e r c h a r a c t e r i s t i c s

T h i s g ro up o f c h a r a c t e r i s t i c s i n c l u d e s o t h e r r e l e v a n t r i t h m s , languagesnd tools , developmen t environp r o p e r t i e s o f c o n t r o l s y s te ms an d the y a r e a s f o l lo ws : and s of twar e

1 Cos tThe cos t g i ves th e pr i ce r ange of t he computer pa r t s o fa c o n t r o l s y s tem. T y p ic a l v a lu e s a r e $1, $20 , $100,$1000, $10000, $100000, M$ l and PlSlO.

2 Annual production volumeThi s prop ert y des cr i bes how many co nt ro l systems a com-pany manufactures in a y e a r . T y p ic a l v a lu e s a r esingles, lo's, 100's, lOOO's, 10000's and 100000's.

Table 4p r e s e n t s t h e v a l u e s of c o n t r o l c y c l e time andr a t e o f i n p u t d a t a ( pe rf or m an c e c h a r a c t e r i s t i c s ) ofe ac h c l a s s . T a b le 5 g i v e s s a f e t y a nd r e l i a b l i t y p r o p e r-t i e s f o r d i f f e r e n t c l a s s e s . T ab le 6 d e a l s w i th th e e nvironmenta l condi t ion s of embedded contro l sys temsSiz e and weight as w e l l as power consumption con-s t r a i n t s a r e shown i n t a b l e 7. The cos t , annua l p roduc-ti on volumes, customi zation and maintenance modif ica-t io n r e q u i r e me n t s of each embedded control system areg i ll e n in t a b le 8

3 Cus tomiza t ionThis prope rty expresse s how important custom izatio n i s .A medium ca tegory denotes th a t a co ntr o l sys tem pos-se ss es some customized Feature s. A high category means

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As can be seen from the t a b l e s 1 t o 8 t h e r e are noe x a c t b o u n d a r i es b etwe en th e d i f f e r e n t classes ofembedded con tro l sys tem. Some ch ar ac te r i s t ic s o fd i f f e r e n t c l a s s e s o v e r la p t o a c e r t a i n e x t e n t , b u t fort h e mos t p a r t , e a c h class h a s c l e a r l y d e f in e d f i g u r e sa nd p r o p e r t i e s o f i t s own f o r a l l t h e c h a r a c t e r i s t i c s .

CLASS OF EM -IBEDDED CONTROLCOMPUTER SYSTEMSIMPLEMICROCONTROLLERMICRO-CONTROLLER

SYSTEMCONTROLLER

MEDIUM SCALECONTROL SYSTEMLARGE CONTROLSYSTEMSPECIAL PURPOSECONTROL SYSTEM

4. CONCLUSIONS

HARDWARE RCHITECTURENumber of Interconnection Mass me- w- Communication Userprocessors structure mory size lines architecture interface

no external bus ...16 very simple1 single processor bus 10...64 slow serial lines ke ypadldisplay

slow and fast keypadldisplay orbus+ senal bus 20...128 serial keyboard/displaysinglelmul iprocessorL . 1 0bus + ...80MB slow and fast serial ke yb ad di sp la y orserial busses lines + slow LANs simple graphical3 . 3 0

'low and fast seriallines+slow and fast andLANsadvanced graphical10...5oo many multiprocessor .,.loooMB ...10000and serial busses

several general multiprocessor bus + slow and fast serialpurpose+spec ial special busses 10... 00MB lo-..lOOo lines + slow 'Wid

I n t h i s p ap er w e have proposed a c l a s s i f i c a t i o n s c h e m efo r embedded cont ro l computer sys tems . The c l az s i f i ca -tion scheme i s b a se d o n th e c o n t r o l c omp ute r s t r u c t u r eof an embedded con tro l sys tem and t he re lev ant p roper -t i e s of co ntr o l computer s . The spec trum of ap pl i ca t ion s

I

o f c o n t r o l s y ste ms is very ex tens ive and depending onan a p p l i c a t i o n f i e l d t h e r e q u ir e m en t s and c h a r a c t e r i s -t i c s of a c o n t r o l s y s te m c a n b e d iv e r s i f i e d .

W have shown tha t f rom the p o in t of view of a c o n t r o lc o mp u te r s t r u c tu r e it i s p o s s ib l e t o d e f i n e s i x d i f f e r -e n t classes and to s p e c i f y c h a r a c t e r i s t i c s f o r e ac hclass. Al th o ug h so me c h a r a c te r i s t i c s o f d i f f e r e n tclasses may overlap, each class c o n t ai n s q u i t e c l e a r l yd e f in e d p r o p e r t i e s o f i t s own.

Table 1 Characteristicsof hardware architecture.

CLASS OF EM -BEDDED CONTROLCOMPUTER SYSTEFSIMPLEMICROCONTROLLEMICRO-CONTROLLER

InstructionWord length SizeOfphysical memory set418 bits

CONTROLLERSYSTEMMEDIUM SCALE

LARGE CONTROLSYSTEMSPECIAL PURPOSECONTROL SYSTEM

16 bits

32 bits

and special

0.5...16 MB

and special

PROCESSOR CAPABILITIESF'rocessing Floating point Symbo lic pro- Virtual memory/ Pipeline/speed processing speed cessing speed protect. scheme cache

I... .1M I P S0.1 ...0.5 MIPS I I I I I

simple

0.5... MIPS 0 .1 .3 MFLOPS 5...100 kLWS advanced advanced0.5...2 MIPS+ 0.5...10FLOPS 10 ..100kLIPS advanced

Table 2 C haracteristicsof processor capabilities.IECON '88 433


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CLASS OF EM - I

advanced

SOFTWARE CHARACTERISRCS I

advanced deve-HLL's+advanced tools lopment system modest or advanced

advancedmedium

advanced development advancedenvironmentdvanced toolsadvanced+special tools ~ ~ ~ : ~ ~ ~ $ p m e n t modest

CONTROLLERMICRO 5 02 .2sec0 0 m s a n d 5 000 bits/sec norm al normal

SPECIAL PURPOSECONTROL SYSTEM normal highm s . . 1 secand special . 100Mb1t.s/sec

harsh normal or harsharsh

normal and harsh normal and harsh normal and harsh

small normal or limitednormal normal

normal and spccial normal and spccial normal and special normal and special normal a id special

Complexity of Languagescontrol aleorithms and tools Ievelopme nt Software configura-environments tion managemcntOperatingSIMPLEMICROCONTROLLER] 500 ... 000 lines 1 - simple I assemblers I -odest deve-lopment systemMICRO- 1000 .10000CONTROLLER lines Iimple 1 qsemblers , HLL's I development system ICONTROLLER 1 'y;;,:," 1 simple kemtSYSTEM medium 1 assemblers, HLL's I development system I modest 1

Table 3 Software characteristics.

CLASS OF EM -BEDDED CONTROLCOMPUTER SYSTEM

PERFORMANCE C HARACTERISRCSControl cycle time Rate of i npu t data

RELIABILITY AND SAFETY CHARACTERISTICSReliability Safety

SIMPLEMICROCONTROLLER 0. 1 ..0.5 sec ...100bits/sec normal normal--1000...500 000 bitsisecONTROLLER I 1...100 ms an dSYSTEM 0.2..2 sec normal or high normal or highMEDIUM SCALE \ 1...100 ms andCONlXOL SYSTEM 0 .. 5 sec 10000...2 000 000 bitsisec normal to very high normal to high

1...100 ms and1...20 sec ... 0 Mbits/sec I normal to very high normal to very highTable 4 and 5 C3am3enSticsof perfmance , elmbilityand .afery

PHYSICAL CONSTRAINTSSize and weight Power consumption

ENVIRONMENTAL CONDITIONSCLASS OF EM -BEDDED CONTROLCOMPUTER SYSTEM Electncal Climate Mechanicalconditions conditions conditionsSIMPLEMICROCONTROLLER normal normal normal or harsh

~

very small very limitedMICRO-CONTROLLER normal or harsh normal or harsh normal or harsh small limitedCONTROLLERSYSTEME D I U M SCALECONTROL SYSTEML m G E CONTROLSYSTEM normal and harsh normal and harsh nom a1 and harsh 1 normal normalSPECIAL PURPOSECONTROL SYSTEM

Tdblc 6 dnd 7 Ch,uacw ri\iics of cn\ ironrncntal contlitions m l hysicdl constraints

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CLASS O F EM -BEDDED CONTROLCOMPUTER SYSTEM

SIMPLEMICROCONTROLLERMICRO-CONTROLLERCONTROLLERSYSTEMMEDIUM SCALECONTROL SYSTEMLARGE CONTROLSYSTEMSPECIAL PURPOSECONTROL SYSTEM

OTHER CHARACTERISTICSProduction Maintenancevolume Customization modificationsost

$1 .SO >lo0 000

$30 ..1000 10000...500 000

$5 OO... 0 000 1OO0...100 000 medium

k$10 .SO0 500...10 000 medium or high seldom

k$200 ...M U 0 10..1000 high frequently

k$20. ..200 1o... 1000 high

Table 8 Other characteristics.

Acknowledgements

The au tho r wishes t o thank Prof . A lber t Kuendig and D r .Ric h ar d Bu e h re r f o r h e lp f u l a nd e n c o u r ag in g d i s c u s s io n sand comments, which have c rea ted the base fo r t h i s pa-per . Comments and us efu l sugg es t io ns by co l leg ues a tETH Z u e r ic h , a t EPFL Lausanne and a t t h e T e c h n ic a l R e -s e a r c h C e n t r e o f F i n l a n d a s w e l l as b y s e v e r a l p e r s on si n t h e S w i ss e l e c t r o n i c s i n d u s t r y are h ig h ly a p p r e c i -a t e d .

ReferencesAFIPS Taxonomy Committee, Taxonomy of ComputerS c ie n c e & Engineer i ng , L ibra ry of Congress Ca ta-log Card No. 79-57474, Amer ican Federa t ion of In-f o r ma t io n P r o c e s s in g S o c ie t i e s , 1 98 0.B.W. Boehm, S oftw are Eng inee ring Economics.Englewood Cl if f s , N . J . , P r e n t i c e H a l l I n c . , 1981,7 6 7 ~ .R . Buehrer . Dynamic Dataf low Arc hit ect ure and Em-b ed de d S y s te ms , I n te r n a l No te of th e I n s t i t u tfu er Elek tr on ik , ETH, Zuer ich . October 1985.T. Feng , Some Ch ara c t e r i s t i cs o f Assoc i-a t e / P a r a l l e l P r o c e s si n g , P r oc e e d in g s 1972Sagamore Computer Conference, I E E E ComputerS o c i e ty , W a sh ing ton , D . C . , 1972. pp. 5-16.D . A . F is h er , DoD's Common Programming LanguageE f f o r t , I E E E Computer, Vol. 11, No. 3 . March1978 . PP. 24-33.M . J . Flynn. Some Computer Organisations and TheirE f f e c t i v i n e s s , I E E E Tr a n s . on Compu t er s , Vol. 21 .No. 9 . September 1972. pp. 948-960.R . C u e t h . ed. , Computer Systems fo r Process Con-t r o l , P lenum P r e s s . N e w York, 1986.

W . Haendler , The Impact of Cla s s i f i c a t io n S c h e me so n P a r a l l e l Co mp uter Ar c h i t e c tu r e , P r o c . 1977I n t . C o nf . o n P a r a l l e l P r o c e s s in g , p p . 7-15.N . K . Sinha , ed . , Microprocessor -Based Contro lSys tems , D . Rei del Pub lis hin g Company, Dordrecht.Ho l l a n d , 1986.M.F. Smith, A t ax on om i c c l a s s i f i c a t i o n o f d e d i -c a t e d m i c r o p r oc e s so r a p p l i c a t i o n s , J o u r n al ofMicrocomputer Applications, Vol. 9 , No. 1.J a n u a r y 1986 pp . 63-81.P.C. Trel eave n and I . G . L i m a , Future Computers:L o g ic , Data Flow, ..., Contro l F low?, IEEE Com-p u t e r , Vol. 17, No. 3 , March 1984. PP . 47-58.R . Wagreich e t a l . , Methodologies and Environ-ments fo r Embedded Sys tems Li fec yc l e Suppor t , ACMSi gs of t Sof tware Engineer ing Notes , Vol . 10, No.2. A p r i l 1985. pp . 115-117.P. Zave, Rea l Wor ld Pro per t ie s i n the tf feq uire -ments f or Embedded Sys tems , i n Proc . 19 AnnualWashington D . C . ACM Tech. Symp.. pp. 21-26.P. Zave, An Opera tio nal Approach t o RequirementsSp ec if i ca t i on fo r Embedded Sys tems , IEEE Trans .on Sof tware Engineer ing , V o l . SE-8, No. 3 . May1982. pp. 250-269.S . Yalamanchi l i e t a l . , Image Proces s ing Archi-t e c t u r e s : A Taxonomy and Surv ey, i n L . N . Kana1an d A . Ro s en f e ld : P r o g r e s s i n P a t t e r n Re c o g n i t io n2 , North Holland. 1985.

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