Spectravideo MKI, Сервисное – Техническое руководство

Страница: 107 / 134

PROGRAMMING CONSIDERATIONS
I
n
i
t
i
a
l i
z
at
i
o
n
H a r dware Scro l l i ng
6845
CRTC PIN DESCRIPTION
Proce s s o r In t e r face
Reg i s t e r s R
O
-
R l S
mus t be
i n i t ia l i z e d
a f te r
p
o
w e
r
i s t u
r
n e
d on . The p roc e s s o r
n
o
r m
a
l l
y
l o a ds t h e C RTC re g i s t e r s s e qu e n t i a l l y f rom
a
f i r mw a r e t a b l e . H e n c e f o r t h ,
RO-R l l
a re
no t
c
h a ng e d i n mos t s
y
s t e m s
.
The
6800
p
r o g ramm e in Tab l e
3
and F i g u r e 1 2 shows a
t yp i ca l
CRTC i n i t i A l i z a t i o n .
Reg i s t e r s
R 1 2 / R 1 1 c on t e n t s
d e t e
r
m
i n e
wh i c h
me m o r y l
o
c a t i
o
n
i s t h e f i r s t
d
i
s
p
l a
y e d
cha r :1c t e r
o n t he
S ( r e e n .
S i nce
t he
CRTC
L i n e a r Add re s s Gene r a t o r co
u n
t
s
f ro �
t h i s
b
e g i n n i
n
g c o u n t , t h e d i s
p
l
a
y
e
d p o r t i on ,)f
t
h e scre e n m a y b e a w i n d ow on
an
y
c
o
n
t i n u o u s s t r
i n
g
o f c h a r a c t e r s
w
i t
h
i n
;�
1 6K
b
l
o
c k
o r
re f re s h me m o r y . By
c e i : i
e 1
i ng
the
R 1 2 / R 1 3
p
o
i
n
t e
r
i n
t h e m
i
dd
l
e
o f t he
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
s s
ib
l e
• . .
by l i ne , pag e , or
cha ract e r .
T he C RTC
inter fac e s t o a p roce s sor
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
( DO-D7 )
u s i
ng
CS , RS ,
E , and
R /W
for con t ro l
s ig n a l s .
Dat a Bus
( DO-D7 )
E na b l e ( E )
C h i p
S e lec t
( C S )
The b i d i re c t i o na l d a t a l i n e s
( DO-D7 ) a l l ow
d a ta t ra ns f e r s b e t w e e n t he CRTC
i n t e rna l
R
e g
i s t e
r
F i l e a n d the
p roce s s o r . Data bus
ou tput d r i ve r s
a re 3 - s t a t e
b u f f e rs wh ich
rema i n in the
h i gh
impedance s ta te exc e p t
wh
en t he
p r
o
c
e s s o r
pe r f o rms a CRTC read
ope
r a t i o n
. A
h igh l eve l
on a data p in is
a
l
o
g
i
c a l
" 1 " .
The
enab l e s i gna l is a
h igh impe dance
TTL/
MOS
c
o m
p
a t i
b
l e
i npu t wh ich enab l e s the dat a
bus input
/
o u tp u t
bu f f e r s
and c l ocks
d a t a
t o
and
f rom t he CRTC . Th i s
s ignal
is u s u a l l y
derived f rom the proc e s s or c l ock , an d the
h i gh
t o l ow
t r
a
n
s i
t i
o
n
is
the act ive e dge .
The
CS
l in e i s a h igh impe ndance
TTL/MOS
compa t i b l e
i nput wh ich se l e c t s t he
CRTC
whe n low to re ad or w r i t e the int e rna l
Reg i s t e r F i l e .
Th i s s igna l should only be
ac t ive when the re is a va l i d s ta b l e addre s s
be ing decoded f rom the p roce s s or .
4 . 74