(完整版)3-电气工程及其自动化专业_外文文献_英文文献_外文翻译_plc 方面
economical manu f a c t u r e o f t h e d e v i c e s r e q u i r e s t h a t the contents
of the program m e m o r y b e c o m m i t t e d p e r m a n e n t l y d u r i ng the
manufacture of c h i p s . C l e a r l y , t h i s i m p l i e s a r i g o r o u s approach to
ROM code develo p m e n t s i n c e c h a n g e s c a n n o t b e m a d e a fter
manufacture .Th i s d e v e l o p m e n t p r o c e s s m a y i n v o l v e e mulation using
a sophisticated d e v e l o p m e n t s y s t e m w i t h a h a r d w a r e emulation
capability as w e l l a s t h e u s e o f p o w e r f u l s o f t w a r e tools.
Some manufacturer s p r o v i d e a d d i t i o n a l R O M o p t i o n s b y
including in th e i r r a n g e d e v i c e s w i t h ( o r i n t e n d e d f o r use with) user
programmable me m o r y . T h e s i m p l e s t o f t h e s e i s u s u a l ly device which
can operate in a m i c r o p r o c e s s o r m o d e b y u s i n g s o m e of the
input/output li n e s a s a n a d d r e s s a n d d a t a b u s f o r a ccessing
external memory . T h i s t y p e o f d e v i c e c a n b e h a v e f u n ctionally as
the single chip m i c r o c o m p u t e r f r o m w h i c h i t i s d e r i ved albeit with
restricted I/O a n d a m o d i f i e d e x t e r n a l c i r c u i t 。 The use of these d e v i c e s
is common even i n p r o d u c t i o n c i r c u i t s w h e r e t h e v o l ume does not
justify the dev e l o p m e n t c o s t s o f c u s t o m o n — c h i p R O M [ 2] ;there can
still be a sign i f i c a n t s a v i n g i n I / O a n d o t h e r c h i p s compared to a
conventional mi c r o p r o c e s s o r b a s e d c i r c u i t . M o r e e x a ct replacement
for ROM devices c a n b e o b t a i n e d i n t h e f o r m o f v a r i ants with
'piggy—back’ EPROM ( E r a s a b l e p r o g r a m m a b l e R O M ) s o c k e t s o r devices
with EPROM inst e a d o f R O M . T h e s e d e v i c e s a r e n a t u r a lly more
expensive than e q u i v a l e n t R O M d e v i c e , b u t d o p r o v i d e c omplete circuit
equivalents. EP R O M b a s e d d e v i c e s a r e a l s o e x t r e m e l y attractive for
low—volume appl i c a t i o n s w h e r e t h e y p r o v i d e t h e a d v a ntages of a