Programming Considerations - Spectravideo SVI 318 Service – Technical Manual

PROGRAMM I NG CONSIDERATIONS
I t i a a t i n
n i l i z o
H a r dw a r e Scr o l l i ng
6845 CRTC PIN DESCRIPTION
P r o c e s s o r In t e r face
i n t e r f ac e s t o a p ro c e s s o r
T he C RTC
ng CS , RS , E , and R / W f o r c on t ro l
u s i
( DO-D7 )
D a t a Bus
E na b l e ( E )
S e l e c t ( C S )
C h i p
S
O
Reg i s t e r s R R l
-
p ow
e i s t u r n e d on .
r
l o a ds C RTC re g i s t e r s s e qu e n t i a l l y f rom
t h e
a f i r h
mw a r e
t a b l e .
no t c
a ng e d i n mos t s
r o g ramm e in Tab l e 3 and F i g u r e 1 2 s h o w s a
p
t yp i c a l
CRTC i n i t i A l i z a t i o n .
R e g i s t e r s R 1 2 / R 1 1
l o c a t i t he
n
me m o r y o
G e n e r a t o r
c h a r :1 c t e r o n
L i n e a r Add re s s
b e g i n n i n g c o u n t , t h e d i s
t
h e scre e n m a y b e
c o n
t i nu o u s s t r
i n
1 6K
k o r
l o c
b re f re s h
t h e R 1 2 / R 1 3 p o i n t e r i n t h e m i
ava i l a b l e mem o ry s p a c e , s c r o l l i ng up o r
down i s p o i b
s s l e
cha r a c t e r .
b u s o n t h e b i d i r e c t i o n a l d a t a b u s
s i g n a l s .
The b i d i re c t i o na l d a t a l i n e s
d a t a t r a n s f e r s b e t w e e n t he CRTC
e g r
R i s t e F i l e a n d the
ou tput d r i ve r s a re
rema i n in the h i gh imp e dance s ta t e exc e p t
wh p r o c
en t he e s s o r
ope r . A
a t i o n h igh l eve l
a i c " 1 " .
l o g a l
The b
enab l e s i g na l i s a
p i n pu t wh ich e nab l e s the d a t a
MOS co a t i l e
m
bus in p u t / o u tp u t
a n d
f rom t he CRTC .
d e r i v e d f rom the proc e s s o r c l ock , an d the
h i gh t o l ow t r
a n s i
The CS l i n e i s a h igh impe ndance TTL/MOS
i nput wh i c h s e l e c t s t he CRTC
compa t i b l e
whe n low to re a d or w r i t e the i n t e rna l
R eg i s t e r F i l e .
ac t i ve whe n the re is a va l i d s ta b l e a d d re s s
be ing d e co d e d f rom t he p roce s s o r .
4 . 74
mu s t be
i n i t i a l i z e d
T h e p ro c e s s o r
RO-R l l a re
H e n c e f o r t h ,
y s t e . The 6 8 00
m s
c o n t e n t s
m i
de t e r n e wh i c h
S ( r e e n .
i p t h e
d l a
i s t h e f i r s t s
S i nce
u n t s f ro � t h i s
co
p l y
a e d p o r t i on
a w i n d ow on a n y
w i
t
g o f c h a r a c t e r s
By c e i : i
me m o r y .
d d e
l
o f
by l i ne , pa g e , o r
• . .
( DO-D7 )
( DO-D7 ) a l l ow
i n t e rna l
p ro c e s s o r . D a t a bus
b u f f e r s wh i c h
3 - s t a t e
p e r f o rms a CRTC read
on a data p i n is
h igh impe dance TTL/
and c l ocks
b u f f e r s
s i g n a l
Th i s is u s u a l l y
t i o n t he a c t ive e dge .
is
Th i s s i gna l sh o u l d o n l y be
a f t e r
o a
n l l y
r m
y e d
CRTC
, ) f
;�
i ng
i n
h
e 1 t h e
d a t a t o