【正文】 vice s al low a n I/ O p or t to be co nfi gure d as a s ys te m b us to allow o ff c hi p me mo r y a nd I/O e xpa nsi o n. T his fa cili ty is po te nti all y us e ful a s a p r o d uc t r a nge d e v e l op s, s i nc e s uc c e s siv e e nha nc e me nts ma y b ec o me to o bi g for o n c hip me mo r y a nd i t is und esi rab le no t to b uil d o n the e xi s ti ng s o ftw a re b a s e . Serial input/output .S e r i al c o mmuni c a ti o n w i th te r mi na l d e v i c es i s c o mmo n me a ns o f p r o v id ing a l i nk us i ng a s ma l l numb e r o f l i ne s . T hi s s o r t o f c o mmuni c a ti o n c a n a l s o b e e xp l o i te d fo r i nte r fa c ing s p e c ia l func ti o n c hi p s o r l i nki ng s e v e r al mi c r o c o mp ute rs to ge the r . B o th the c o mmo n a s ync hr o no us s ync hr o no us c o mmuni c a ti o n s c he me s r e q ui re p r o to co ls tha t p r o v i de fr a mi ng ( s ta r t a nd s to p ) i nfo r ma ti o n . T hi s c a n b e i mp l e me nte d a s a ha r d w a re fa c i l ity o r U( S ) A RT ( Uni v e rsa l(sync hr o no us) a s ync hr o no us r e c e iv er/tr ans mi tter ) r e l iev ing the p r o c e sso r ( a nd the a p p l ica tio ns p ro gra mme r ) o f this low lev el, ti me co ns umi ng, de tail . t i s me rel y nec ess ar y to s e l e cte d a b a ud r ate a nd p o s s i bly o the r o p ti o ns ( numb e r o f s to p b i ts , p a r i ty, e tc . ) a nd l o a d ( o r r e a d fr o m) the s e r i al tr a ns mi tte r ( o r r e c e iv er) b uffer. Seri ali za tio n o f the da ta i n the ap pr opri a te for ma t is the n ha ndl ed b y the ha r d w a re c i r cui t. Timing/counter facilities. Ma ny a p p l i ca tion o f s i ngl e 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 n g 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 xte 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 al 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。 i t i s unne c e s s a r y to ma ke d i s ti nc tion b e tw e e n R A M a nd p r o c e sso r r e gi s ter a s i s d o ne i n the c a s e o f a mi c r o p ro ce sso r s ys te m s i nc e R A M a nd r e gi s te rs a r e no t us ua l l y p hys i c a ll y s e p a ra ted i n a mi c r o c o mp ute r . Central processing unit (CPU).T he C P U i s muc h l i ke tha t o f a ny mi c r o p r oc ess or. Ma ny a p p l i cati ons o f mi c r o c o mp ute r s a nd mi c r o c o ntro ller s i nv o l v e the ha nd l i ng o f b i na r y c od ed d e c i ma l ( B C D ) d a t a ( fo r nume r i c a l d i s p la ys , fo r e xa mp l e ) , he nc e i t i s c o mmo n to fi nd tha t the C P U i s w e l l a d a p te d to ha nd l i ng thi s typ e o f d a ta . It i s a l s o c o mmo n to fi nd go o d fa c i l itie s fo r te s ti ng, s e tti ng a nd r e s e tting i nd i v i d ual b i ts o f me mo r y o r I/ O s i nc e ma ny c o ntr o l le r a p p l icati ons i nv o