Spectravideo MKI, Сервисное – Техническое руководство

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6845
CRT CONTROLLER
SYSTEM BLOCK
DIAGRAM D ESCRIPT ION
As shown in STM-00 2 - I , the p r i ma r y func t i on of the CRTC is to gene rate re f r e s h
a dd re s se s
( MAO-MA 1 3 ) ,
row se l e c t s
( RAO--RA4 ) ,
and video mon i t or t i m ing ( HSYNC ,
VSYN C
)
and D isp l a y E na b l e . Oth e r func t i o n s inc lude an int e rna l cursor reg i s t e r
wh ich ge nera t e s a Cu r s o r outpu t when i t s con t e n t s compa re to the curre nt
Ref re sh Add re s s . A l i gh t-pen s t robe inpu t s i gna l a l l ows cap t u re o f Re fresh
Add re s s in an i n t e rn a l l i ght pen reg i s t e r .
A l l t im ing in t he
CRTC
i s d e r i ve d f rom t he CLK input . In a lphanume r ic
te rm ina l s , t h i s s i gna l i s the charac t e r ra te . Cha ra c t e r ra t e i s d iv i d e d down
f rom v ideo ra te by ex t e rnal H i gh S peed T im ing when t he video f re qu ency i s
gre a t e r than
3
MH z . S h i f t Reg i s t e r , La t c h , and MUX Con t ro l s i gna l s are a l s o
p rov ided b y ext e rnal H i gh Speed T im ing .
The p roce s s or commun i c a t e s with the CRTC th rough a buf f e r e d 8-b i t Dat a Bus 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 of 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 tween the P roce s s o r and CRTC . Data
appea r s on a S e c ondary Bus wh ich i s b u f f e re d f rom t he p roce s so r Pr imary Bus . A
numb e r of approa che s are poss i b l e for s o l v ing
�
onten t i on s for the Re f re s h
Memory .
1 . Proce s sor a l ways ge t s p r i or i t y
•
. ,
2 . Proce s sor g e t s p r i o r i t y acce s s any t i me , b u t can be synchroni z e d by an
i n t e rrupt to p e r f o rm acce s s e s on ly du r ing hor i z on t a l and ver t ical r e t race
t i me s .
3 .
Synchron i z e p roce s s o r by memor y wa i t cyc l e s .
4 . Synch ronize p roce s s or to chara c t e r r a t e ( S e e F i gu re 4-6 ) . The 6800 MPU
fam i l y l e n d s i t se l f to th i s conf igura t i on because i t ha s con s tant cyc l e
l engt hs . Th i s me thod p rov ides z e ro burden on t he p roce s sor because there
i s ne ver a cont e n t i o n for memory . A l l acce s se s are " t ransparent" .
The s ec ondary d a t a bus concept in no wa y p rec l udes us ing the Re f re s h RAM for
� t he r pu rpo s e s . It looks
1
ike any othe r RAM to th e P roce s s o r . For examp l e ,
us ing App roach 4 , a 64K byte RAM Re f re sh Memory could p e r f o rm re f re sh and
p rog ramme s t orage func t i ons transparent ly .
4 . 68