Spectravideo SVI 318 Service – Technical Manual page 102

C R T C D E S C R I P T I ON
( F i gu re S TM- 00 1 - I :
c o n s i s t s o f p ro g rammab l e ho r i z on t a l a n d ve r t i c a l t i m i ng gene r a t o r s ,
T h e C RT C
p r o g ramma b l e l i n e a r add re s s re g i s t e r , p ro g rammab l e c u r s o r l o g i c , l i ght pen
ca p t u re r e g i s t e r , and c o n t ro l c i r c u i t r y for in t e r face t o a p r o c e s s o r bus .
A l l t i m in g i s d e r i v e d f rom CLK , us u a l l y the ou tpu t of an e x t e rna l dot ra t e
C RT C
cou n t e r . Co i nc i d ence
the c o n t e n t s o f the p ro g rammab l e r e g i s t e r f i l e , RO-R 1 7 .
g e ne r a t i on , comp a r i sons r e su l t i n : ( 1 ) H o r i z on t a l s y nc p u l s e ( HS ) of a
f re q u e ncy , p o s i t i on , and du r a t i on d e t e rm i ne d by t he reg i s t e r s .
The H or i2on t a l coun t e r p rodu c e s H c l ock wh i c h d r i v e s the S can L i ne Cou n t e r and ,
Ve r t i c a l Con t r o l .
t o t h e Max S c a n L i n e Add re s s Reg i s t e r .
and c l ock s t he Ve r t i c a l C o un t e r .
Compa r i s o n s o f Ve r t i c a l Coun t e r c o n t e n t s and Ve r t i c a l Reg i s t e r s r e s u l t in : ( 1 )
Ve r t i c a l s y nc p u l se ( V S ) o f a f re qu e ncy and p o s i t i on de t e rm i ned by the reg i s t e r
- t h e w id th i s f i xe d a t 1 6 ra s t e r l i ne s i n the ve r t i c a l co n t ro l s e c t i on and i s
n o t p ro g r ammab l e , ( 2 ) Ve r t i c a l D i s p l ay o f a f re quency and po s i t i on d e t e rm ined
b y the re g i s t e r s .
The Ve r t i c a l Co n t r o l Log ic h a s o t h e r func t i ons .
( 1 ) G e n e ra t e row se l e c t s , RAO-RA4 , f rom t he R a s t e r Count f o r t he c o r r e s pond i ng
in t e r l a ce or non- i n t e r l a c e mode s .
( 2 ) E x t e nd the numb e r o f s c a n l i ne s in the ve r t i c a l to t a l by the amoun t p ro­
g r amme d i n t he Ve r t i c a l T o t a l Ad j u s t Reg i s t e r .
T he L i n e a r Add re s s Gene r a t o r i s d r i ve n by CLK and loc a t e s the r e l a t i ve
p o s i t i o n s o f cha r a c t e r s in memory w i th t h e i r p o s i t i o ns on t h e s c r een .
a ,
F ou r t e e n l i ne s , MAO-MA 1 3 , a r e ava i l a b l e f o r a d d re s s i n g up to fou r page s
of 4K c h a r c t e rs
Reg i s t e r , ha rdware s c r o l l i ng th rough 1 6K c ha rac t e r s i s po s s i b l e .
A dd r e s s G e ne ra t o r r e p e a t s t h e s ame se quence o f a dd r e s s e s f o r each s can l ine o f
a c ha ra c t e r row .
T he cu r s o r l o g ic d e t e rm i n e s t h e c u r s o r l o c a t i o n , s i z e , a n d b l ink i ng ra t e o n the
s c re en . A l l a re p rog rammab l e .
T he l i gh t p e n s t robe g o i ng h igh ca u s e s the cu rrent . co n t e n t s o f the Addre s s
C o unt e r t o be la t c he d in t he L i gh t P en Reg i s t e r .
Reg i s t e r a r e s u b s e que n t l y re a d b y the Proce s s o r .
I n t e r na l CRT C reg i s t e r s a re p ro g rammed by t he p r oce s s or th rough t he d a ta b us ,
DO-D 7 , a nd the co n t ro l s i gna l s -
B l ock D ia g r am )
C R T C
c i rc u i t s con t i n u ou s l y compa r e count e r con t e n t s to
( CO )
T he c o n t e n t s of t he R a s t e r Coun t e r are c o n t i nuou s ly comp a r e d
8 pa g e s of 2K chara c t e r s , e tc .
R/ W ,
4 . 69
A co inc i d ence re s e t s t h e Ra s t e r Coun t e r
U s i ng the S t a r t Add r e s s
The con t e n t s of the L i gh t P e n
CS , RS and E .
For ho r i z on t a l t im in g
The L i ne a r