【正文】 c hip mi c roc o mp ute rs r eq uire ac c ur a te ev al ua tio n o f el aps ed re al ti me . T hi s c a n be d e te r mi ne d b y c a r e ful a s s e ssme nt o f the e xe c uti o n ti me o f e a c h b r a nc h i n a p r o gr a m b ut thi s r a p i dly b e c o me s i ne ffi c i ent fo r a l l b ut s i mp l e st p r o gr a ms . T he p r e fe rre d a p p r o ac h i s to us e ti me r c i r c ui t tha t c a n i nd e p e nd e ntly c o unt p r e c ise ti me i nc r e me nts a nd ge ne r a te a n i nte r r up t a fte r a p r e s e t ti me ha s e l a p se d . T hi s typ e o f ti me r i s us ua l l y a r r a nge d to b e r e l o ad ab le w i th the r e q ui r ed c o unt . T he ti me r the n d e c r e me nts thi s v a l ue p r o d uc i ng a n i nte r r up t o r s e tti ng a fl a g w he n the c o unte r r e a c he s ze r o . B e tte r ti me r s the n ha v e the a b i l ity to a uto ma ti c a l ly r e l o a d the i ni ti a l c o unt v a l ue . T hi s r e l i ev es the p r o gr a mme r o f the r e s p o ns ib ility o f r e l o ad ing the c o unte r a nd a s s e ss ing e l a p s ed ti me b e fo r e the ti me r r e s ta r ted , w hi ch o the r w i se w o und b e ne c e s s a ry i f c o nti nuo us p r e c i sely ti me d i nte r r up ts w e re r e q ui r ed ( a s i n a c l o c k , fo r e xa mp l e ) . S ome ti me s a s s o ciate d w i th ti me r i s a n e ve nt c o unter. Wi th this facili ty the re is us uall y a sp ecia l i np ut pi n , tha t c a n d r i v e the c o unte r d i r e c tly. Timing ponents. T he c l o c k c i r c ui try o f mo s t mi c r o c o mp ute r s r e q ui r es o nl y s i mp l e ti mi ng c o mp o ne nts . If ma xi mum p e r fo r ma nc e i s r e q ui r ed ,a c r ys ta l mus t b e us e d to e ns ur e the ma xi mum c l o c k fr e q ue nc y i s a p p r o ac he d b ut no t e xc e e d e d. Ma ny c l o c k c i r c ui ts a l s o w o r k w i th a r e s i sto r a nd c a p a c itor a s l o w co st ti mi ng c o mp o ne nts o r c a n b e d r i v en fr o m a n e xte r na l s o ur c e . T hi s l a tte r a r r ange me nt i s us e ful i s e xt e r na l s ync hr o ni za ti o n o f the mi c r o c o mp ute r i s r e q ui r ed . WORDS AND TERMS culmination spilt volatile n. 易變的 mit albeit custom variant piggyback socket n. 插座 B:PLC[1] P LC s ( p r o gr amma b l e l o gi c a l c o ntr o l ler) fa c e e v e r mo r e c o mp l e x c hall e nges these da ys . Wher e o nc e the y q uie tl y re pla ce d rel a ys a nd ga ve a n o cc asio nal r ep or t to a co rp ora te ma i nfra me , the y are now gr o upe d i nto ce lls, gi v e n ne w j o b a nd ne w l a ngua ge s , a nd a r e fo r c e d to c o mp e te a ga i ns t a gr o w i ng a r r a y o f c o ntr o l p r o d uc ts. F o r thi s ye a r 39。 T he s e d e v i ce s a r e na tur a l l y mo r e e xp e ns i v e tha n e q ui v a le nt R O M d e v i c e, b ut d o p r o v i d e c o mp l e te c i r cui t e q ui v a lents . E P R O M b a s e d d e v i ce s a r e a l s o e xtr e me l y a ttr a c ti ve fo r l o w v olume a p p l ic atio ns w he r e the y p r o v i de the a d v a nta ge s o f a s i ngl e chip d e v i ce , i n te r ms o f o n c hi p I/ O , e tc . , w i th the c o nv e ni e nc e o f fl e xi b le us e r p r o gr a mma b i lity. Random access memory (RAM).R A M i s fo r the s to r a ge o f w o r ki ng v a r i ab les a nd d a ta us e d d ur i ng p r o gr a m e xe c uti o n. T he s i ze o f thi s me mo r y v a r i es w i th d e v i c e typ e b ut i t ha s the s a me c ha r a c teris tic w i d th ( 4 , 8 , 1 6 b i ts e tc . ) a s the p r o c es sor , S p e ci al func ti o n r e gi s ters , s uc h a s s ta c k p o i nte r o r ti me r r e gi s te r a r e o fte n l o gi c a lly i nc o r p o rate d i nto the R A M a r e a . It i s a l s o c o mmo n i n H a r a r d typ e mi c r o c o mp ute r s to tr e a t the R A M a r e a a s a c o l l ec tion o f r e gi s te r。 p i ggy b a c k39。 Output unit Reset Interrupts Power Fig35A3. Principal features of a microputer Read only memory (ROM).R O M i s us ua l l y fo r the p e r ma ne nt, no n v o l a tile s to r age o f a n a p p l i ca tions p r o gr a m . Ma ny mi c r o c o mp ute rs a nd mi c r o c o ntr olle rs a r e i nte nd e d fo r hi gh v o l ume a p p l ic atio ns a nd he nc e the e c o no mi c a l ma nufa c tur e o f the d e v i c es r e q ui res tha t the c o nte n ts o f the p ro gra m me mo r y b e c o mmi tte d per ma ne ntl y d ur i ng the ma nufa c tur e o f c hi ps . C l e a r ly, thi s i mp l i e s a r i go r o us a p p r oa ch to R O M c o d e d e v e l op me nt s i nc e c ha nge s c a nno t b e ma d e a fte r ma nufa c tur e . T hi s d e v e l o pme nt p r o c es s ma y i nv olve e mul a ti o n usi ng a so p his tica ted de vel op me nt s ys te m w i th a har dw are e mul a ti o n c a p a b ility a s w e l l a s the us e o f p o w e rful s o ftw a re to o l s . S o me ma nufa c tur e r s p r o v i d e a d d i tiona l R O M o p ti o ns b y i nc l ud i ng i n the i r r a nge d e v i c es w i th ( o r i nte nd e d fo r us e w i th) us e r p r o gr a mma b l e me mo r y. T he s i mp l e s t o f the s e i s us ua l l y d e v i c e w hi c h c a n o p e r a te i n a mi c r o p r oc ess or mo d e b y us i ng s o me o f the i np ut/ o utp ut l i ne s a s a n a d d r e ss a nd d a ta b us fo r a c c e ss ing e xte r na l me mo r y. T hi s typ e o f d e v i c e c a n b e