Tandy 1000 SL Technical Reference Manual page 134

Programmable I n t e r v a l Timer
T h i s s e c t i o n o f t h e KFIT c u s t o m i n t e g r a t e d c i r c u i t is e q u i v a l e n t
t o a n I n t e l 8254-5 a n d i s d e s i g n e d t o u s e w i t h t h e Tandy 1 0 0 0 TX.
I t
i s o r g a n i z e d a s t h r e e i n d e p e n d e n t 1 6 - b i t c o u n t e r s , e a c h w i t h a
c l o c k
o f 1 . 1 9
MHZ.
The 1 . 1 9
MHZ
c l o c k is g e n e r a t e d from l4MHZ
d i v i d e d by 1 2 . All
modes o f o p e r a t i o n a r e s o f t w a r e programmable.
P r
og
r ammab l e I n t e r r u p
t
T h i s s e c t i o n o f t h e KTIF c u s t o m i n t e g r a t e d c i r c u i t is e q u i v a l e n t
t o a n I n t e l 8259A t h a t capable o f h a n d l i n g e i g h t - v e c t o r p r i o r i t y
i n t e r r u p t , i n d i v i d u a l r e q u e s t m a s k and programmable i n t e r r u p t
modes. T h i s c i r c u i t g e n e r a t e s INTR o u t p u t s i g n a l f o r t h e CPU. I n
a d d i t i o n , t h e s h a r i n g i n t e r r u p t l o g i c s a r e implemented i n t h e
d e s i g n f o r
I R Q l
( b e t w e e n k e y b o a r d a n d r e a l t i m e c l o c k i n t e r r u p t )
A d d r e s s D e c o d i n g L o g i c
T h i s s e c t i o n c o n t a i n s 3 t o
8
a d d r e s s d e c o d e t o g e n e r a t e
Programmable I n t e r r u p t C h i p s e l e c t , Programmable I n t e r v a l Timer
c h i p s e l e c t , F l o p p y
D i s k
c h i p s e l e c t (FDCCHP*) and Programmable
P e r i p h e r a l I n t e r f a c e a d d r e s s of t h r e e d e c o d e d a d d r e s s
A,
B a n d C.
(see
I O
s i g n a l d e f i n i t i o n ) . The FDC p o r t i s e n a b l e d by P l a n a r
r e g i s t e r - p o r t 0065hex
b i t
3
when b i t
3
i s
H I G H .
2