Spectravideo SVI 318 Service – Technical Manual page 101

6845 CRT CONTROLLER
SYSTEM BLOCK
D I AGRAM D E S C R I P T ION
As shown in STM-00 2 - I , t h e p r i ma r y func t i o n of the CRTC is to gene r a t e r e f r e s h
( MAO-MA 1 3 ) ,
a dd re s se s
)
VSYN C and D i s p l a y E na b l e .
wh i c h ge n e ra t e s a Cu r s o r o u t p u t whe n i t s con t e n t s compa r e t o the curre n t
Ref re sh Add re s s .
A dd re s s i n an i n t e rn a l l i ght p e n reg i s t e r .
A l l t im ing in t he
te rm ina l s , t h i s s i g na l i s t h e charac t e r ra t e .
f rom v i d e o r a t e by ex t e rn a l H i gh S pe e d T im ing when t he v i d e o f re qu ency i s
3
g re a t e r than MH z .
p rov i d e d b y e x t e rn a l H i gh S p e e d T im ing .
The p roce s s or c ommun i c a t e s w i t h t h e CRTC t h rough a b uf f e r e d 8-b i t Da t a B u s by
r ea d ing/w r i t ing i n t o t he 1 8 -reg i s t e r f i l e o f t he CRTC .
The Re f re sh Memory add re s s i s mu l t i p l e xe d be t we e n the P roc e s s o r and CRTC .
app e a r s o n a S e c ondary Bus wh i c h i s b u f f e re d f rom t he p ro c e s s o r P r imary Bus .
numb e r of approa che s a r e p o s s i b l e for s o l v i n g
Memo r y .
1 . Proce s so r a l ways ge t s p r i or i t y
Proce s s or g e t s p r i o r i t y a c c e s s an y t i me , b u t can be synchro n i z e d by an
2 .
i n t e rr u p t t o p e r f o rm a c c e s s e s on l y du r ing ho r i z on t a l and ve r t i c a l r e t race
t i me s .
3 .
S y nc h ron i z e p ro c e s s o r by memor y wa i t c y c l e s .
4 . Synch ro n i z e p ro c e s s o r t o c h a ra c t e r r a t e ( S e e F i gu re 4-6 ) .
fam i l y l e n d s i t s e l f to th i s c o n f i g u ra t i on be c a u s e i t ha s con s tant cyc l e
l engt hs . Th i s me thod p rov i d e s z e ro b u r d e n on t he p ro c e s sor because t h e r e
i s ne ver a con t e n t i o n f o r memo r y .
T h e s ec o ndary d a t a b u s conce p t in no wa y p r e c l ud e s u s ing the Re f r e s h RAM f o r
� t he r pu rpo s e s .
I t l o oks
us ing App roach 4 , a 64K b y t e RAM Re f re sh Memory cou l d p e r f o rm re f re sh and
p ro g ramme s t orage func t i ons t r a n s p a r e n t l y .
( RAO--RA4 ) ,
row se l e c t s
O th e r fun c t i o n s inc l u d e an i n t e r na l cursor r e g i s t e r
A l i gh t-pen s t robe inpu t s i g na l a l l ows cap t u re o f Re f r e s h
i s d e r i ve d f rom t he CLK i np u t .
C R T C
S h i f t Reg i s t e r , La t c h , a n d MUX Con t ro l s i g na l s a r e a l s o
•
. ,
1
ike any o t h e r RAM to th e P roce s s o r .
4 . 68
and v i d eo mon i t o r t i m ing
In a l p h anume r ic
Cha ra c t e r ra t e i s d iv i d e d down
�
o n t e n t i on s for the Re f re s h
A l l ac c e s s e s a re " t ra n s p a r e n t " .
( HSYNC ,
D a t a
A
The 6800 MPU
Fo r examp l e ,