Spectravideo MKI, Сервисное – Техническое руководство, Страница: 103 / 134

REG ISTER F ILE DESCRIPTION ( S ee Tab le 2 )
N ineteen reg isters in the CRTC can be acce ssed by means of the data bus .
Reg i s t e r a ddress ing and l engths a re shown in Tab le 2 .
Add re ss
The Add re s s Reg i s ter is a 5-b i t write-only reg i s t e r use d as an " indirec t" or
" po inter" reg i s te r . I t s contents a re the a dd ress o f one of the other
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reg i s ters in the f i l e . Whe n RS and CS are low, the Add res s Reg ister i tse l f is
a dd res se d . When RS is high , the Reg i s ter F i l e is acce sse d .
Hor i z ontal RO , Rl , R2 and R3
F igu re 4-7 shows the vi s ib l e display area of a typ i cal CRT moni tor g iving the
point of re f e rence for horizont a l reg i s te rs as the l e f t most d i s p layed
cha racter pos i t ion. Hori zontal registers are p rog ramme d in "charac ter t ime"
un i ts w i th r espect to t he re fe rence .
Hor i zon tal Tot a l Reg i s t e r ( RO)
Hori z ont a l D i sp layed Reg i s t e r
( R l )
Hori z ont al Sync Pos i t i on
Reg i s t e r ( R2 )
Hori zont a l Sync W id th Reg i s t er
( R3 )
Th i s
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b i t wri te-only reg i s t e r determ ine s
the hor i z ontal f re quency of HS . It i s the
tot a l of di splayed plus non-d isp layed
cha racter t ime un its m i nus one .
Th i s
8
b i t wri te-only reg ister dete rm ine s
the number of displayed cha rac ters per
hor i zont a l l ine .
Th is
8
bit write-only reg ister determine s
the hori zontal sync pos i t ion on the
hor i zonta l l ine .
Th i s 4 b i t wr i te-only reg i s ter determines
the width of the HS pu lse . It may not
be
apparent why th is width needs to
be
pro­
g ramme d . Howeve r , cons ider that a l l t iming
widths must
be p rog rammed as· mul t iples of
the charac t e r clock per iod wh ich var ies .
I f HS wid th we re fixed as an interga l
numbe r of cha r�c ter times , i t would vary
w i th charac ter rate and be out of tolerance
for ce rtain mon i tors . The rate pro­
grammab le feature a l lows compensat ing HS
width .
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