Spectravideo SVI 318 Service – Technical Manual page 86

The func t i on a l conf i g ura t i on o f e a c h po r t i s p ro g ramme d by t h e s y s t ems s o f t ­
wa re . I n e s s e nc e , t he CPU " ou t p u t s " a c o n t r o l word to the 8 2 5 5A .
wor d c o n t a i n s i n f o rma t i on s u c h a s "mo d e " , " b i t s e t " , " b i t re s e t " , e t c . , tha t
i n i t i a l i z e s t he f unc t i ona l c o n f i g u r a t i on o f t he
E a c h o f t h e Co n t r o l b l o c k s ( G roup A a n d G roup B ) a c c e p t s "comma n d s " f rom t h e
R e a d / W r i t e Con t r o l Log ic , r e c e i v e s " c o n t ro l wo r d s " f rom t he i n t e r na l d a t a b u s
a n d i s s u e s t h e p ro p e r commands to i t s a s s o c i a t e d po r t s .
Con t r o l G r o up A - P o r t
Co n t r o l Group B - P o r t B and Por t C l owe r ( C3 - CO )
T he Co n t ro l Wo r d R eg i s t e r can on l y be wr i t t e n in t o .
C on t ro l W o r d R eg i s t e r i s a l lowe d .
P o rt A , B and C
The 8 2 5 5A cont a i n s t h r e e 8-b i t p o r t s ( A , B a n d C ) .
w id e v a r i e t y o f f un c t i on a l c h a r a c t e r i s t i c s by t he s y s t em s o f t w a re b u t e ac h h a s
i t s o w n s p e c ia l f e a t u re s o r "pe r s ona l i t y " t o fu r t he r enhance the powe r and
f l e x i b i l i t y of t he 8 2 5 5 A .
Por t A . One 8 -b i t d a t a ou t p u t l a t c h / b uf f e r and o n e 8-b i t da t a inp u t l a t c h .
One 8-b i t da t a inpu t / ou t put l a t c h / b u f f e r and one 8-b i t d a t a i npu t
P o r t B .
buf f e r .
Por t C . One 8-b i t da t a o u t p u t l a t c h / b u f f e r and one 8-b i t d a t a inp u t b u f f e r ( no
la t c h f o r input ) .
con t ro l . E ac h 4-b i t po r t cont a i n s a 4-b i t l a t c h an d i t can be us e d for the
c on t r o l s i g n a l o u t p u t s and s t a t us s i gna l i np u t s i n conj unc t i on wi th p o r t s
B .
Mode S e l e c t i on
T he r e a r e t h r e e ba s i c modes of op e ra t ion t h a t can be se l ec t e d by the s y s t e m
s of twa re :
Mode 0 - Ba s i c Inpu t / Ou t p u t
Mode 1 S t robe d Input / Ou t pu t
-
Mode 2 - B i -D i re c t i o na l Bus
and P o r t C u p p e r ( C 7 - C 4 )
A
T h i s po r t c an be d i v i d e d i n t o two 4-b i t p o r t und e r t h e mode
4 . 53
8 2 5 5 A .
N o R e a d ope ra t i on o f t h e
A l l can be conf i g u r e d i n a
The c o n t r o l
and
A