Spectravideo SVI 318 Service – Technical Manual page 84

THE PROGRAMMAB L E PERIPHERAL INTERFACE.
The 8 2 5 5A PP I i s u s e d to s t robe the ke yboa r d l i n e s , t o int e r f a c e the j oy s t i c k
f i re b u t t on sw i t c h , the p a dd l e and to cont ro l t h e ca s s e t t e tap e s y s t e m .
p o r t s a r e p ro g r amme d as f o l low :
Prog ramme d as i n p u t por t .
Po r t A
b i t s 4 and 5 a r e u s e d f o r the j o y s t i ck f i re sw i t c h i np u t and b i t s
7
6 and
P ro g rammed as ou t p u t p o r t and i s u s e d for keyboa r d r e a d d a t a .
P o r t B
P ro g ramme d a s o u t p u t p o rt .
P o r t C
s c a nn i ng .
7 i s u s e d to m i x PSG s o und da ta .
The func t i ona l co n f i g u ra t i o n of the 8 2 5 5 A i s p ro g ramme d by the s y s t e m s o f twa re
so t h a t norma l ly no e x t e rn a l log i c i s ne c e s s a ry to i n t e r f a ce p e r iphe ra l d e v i c e s
o r s t r uc t u r e s
•
Da t a Bus Bu f f e r
Th i s 3 - s t a t e b i d i re c t i o na l 8-b i t bu f f e r i s us e d t o in t e r f ace the 8 2 5 5 A t o t he
s y s t em d a t a b us . D a t a i s t ra n sm i t te d o r r e c e i ve d by t he b u f f e r u pon e x ecu t ion
o f i n p u t o r o u t p u t i n s t r uc t i ons by the CPU .
ma t i on a re a l s o t ra ns f e r re d t h rough t he d a t a bus bu f f e r .
R ea d /W r i t e and Cont r o l Log ic
The f u nc t i o n · o f th i s b l ock is to ma nage a l l o f the int e rn a l and e x t er n a l
t ra n s f e rs of b o th Da ta and Con t r o l or S t a t us w o rd s .
CPU Add re s s and Cont ro l bus s e s in t u rn , i s s u e s comma n d s t o bo t h o f the Con t r o l
G r oup s .
( C S )
"
C h i p S e l e c t . A low
8 2 5 5A and t he CPU .
a re u s e d f o r s igna l s f rom t he c a s s e t t e .
B i t s 4 - 6 ou t p u t s con t r o l s i gna l s f o r t h e ca s s e t t e .
"
on th i s input p i n enab l e s the commun i c a t i on be tween the
4 . 51
8255A
0-3
B i t a r e u s e d f o r the pa dd l e i np u t ,
0-3
B i t s ou t p u t s BCD d a t a f or k e y b o a r d
Con t ro l wo r d s and s t a t us i n f o r­
I t a c c e p t s i np u t s f rom the
The
B i t