Spectravideo SVI 318 Service – Technical Manual page 89

THE PROGRAMMAB L E COMMU N ICATION INTERFACE 8250-B
INS8250-B FUNCTIONAL PIN DESCRIPTION
The fo l l ow i n g d e s c r i b e s t h e func t i on of a l l INS8 2 50 i np u t / ou t p u t p i n s .
t h e s e d e s c r i p t i o n s re f e rence i n t e rn a l c i r c u i t s .
NOTE :
In t h e fo l l ow i ng d e s c r i p t i o n s , a low re p re s e n t s a l o g i c 0 ( 0 vo l t
nom i na l ) and a h i gh re p re s e n t s a l o g i c 1 ( + 2 . 4 vo l t s nom ina l ) .
C h i p S e l e c t ( C SO , C S l , CS 2 ) , P i n s 1 2 - 1 4 :
l ow , t he c h i p i s s e l e c t e d .
s e l ec t s i gna l i·s l a t c h e d w i t h an ac t ive ( l ow) Add re s s S t robe ( ADS ) i npu t .
e n ab l e s comm un i c a t i on b e tween t he I N S8 2 50 and t he
Da t a Input S t r o be ( D I STR , D I STR ) , P i ns 2 2 and 2 1 :
i s l ow wh i l e the c h i p is s e l ec t e d , a l l ow s the CPU to re a d s t a t u s i n f o rma t i on or
d a t a f rom a �� l e c t e d reg i s t e r o f t he INS8 2 50 .
N o t e : O n l y an a c t i ve D I S TR o r biSTR inpu t i s re qu i r e d t o t ra n s f e r d a t a f rom
the INS8250 du r i ng a re a d ope r a t i on .
i n p u t pe rmane n t ly low o r t h e D I STR input p e rmane n t l y h i gh , if n o t u s e d .
Da ta Ou t pu t S t robe ( DOSTR , DOSTR ) , P i ns 1 9 and 1 8 :
i s l ow wh i l e t h e ch ip is s e l ec t e d , a l l ow s t h e CPU t o w r i t e da t a or con t r o l
w or d s i n t o a s e l e c t e d reg i s t e r o f t he INS8 2 50 .
No t e : O n l y an a c t i ve OOSTR o r OOSTR i np u t i s re qu i r e d t o t ra n s f e r da t a t o t he
INS8 2 5 0 du r i sng a wri t e op e r a t i o n .
i n p u t p e rmane n t ly low or t he DOSTR i npu t p e rmane n t l y h i gh , i f n o t u s e d .
A dd r e s s S t robe ( ADS ) , P in 25 :
S e l e c t ( AO , A 1 , A2 ) and C h ip S e l e c t ( CSO , C S 1 , C S 2 ) s i gna l s .
No t e : An a c t i ve ADS inp u t i s re qu i re d when t h e R e g i s t e r S e l e c t ( AO , A l , A2 )
s i g na l s are no t s t ab l e f o r t h e du ra t i o n o f a re a d o r w r i t e ope ra t i on .
I f n o t re q u i re d , t i e t he A DS i n p u t p e rman e n t ly l ow .
Reg i s t e r S e l e c t ( AO , A 1 , A2 ) , P i n s 2 6- 2 8 :
r e a d or wr i t e o pe r a t i on t o s e l e c t an INS 8 2 50 reg i s t e r t o r e a d f rom or w r i te
in t o a s in d i ca t e d in t h e t a b l e be l ow .
Acce s s B i t ( DLAB ) , wh i c h i s t he mo s t s ig n i f i c a n t b i t o f t h e L i ne Con t r o l
Reg i s t e r , a f f ec t s t h e s e l ec t i on o f ce r t a i n INS8 2 50 r e g i s t e r s .
s e t h i gh by t he s y s t em s o f twa re to acce s s t he Baud G e n e ra t or D i v i s o r L a t c he s .
W h en CSO and C S 1 a r e h i gh and C S 2 i s
C h i p s e l e c t i on i s comp l e t e when t he d e c o d e d c h i p
The r e fore , t i e e i t h e r the DOS TR
When low, p ro v i d e s l a tc h ing for t he Reg i s t e r
The s e t h r e e inpu t s a r e u s e d du r in g a
N o t e tha t the s t a t e of t h e D i v i s o r L a t c h
4 . 56
CPU .
When D I STR i s h i gh o r D I STR
The r e fo r e , t ie e i t h e r the D ISTR
When DOSTR i s h i gh o r DOSTR
S ome o f
Th i s
The DLAB mus t be