Table of Contents
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P o r t
C6
R/W
P o r t
C7
R/W
A m p l i t u d e / f r e q u e n c y
MSN
f o r
DAC
sound c h a n n e l .
The
a m p l i t u d e
w i l l
b e
programmable
i n
7
l e v e l s ,
w i t h
a p p r o x i m a t e l y
3
dB p e r
l e v e l .
The maximum
l e v e l ('111') w i l l
c l o s e l y a p p r o x i m a t e
t h a t i n t h e e x i s t i n g sound c h i p .
A
v a l u e of
' 0 0 0 ' w i l l
r e s u l t i n no o u t p u t .
T h i s l e v e l c o n t r o l a l s o a p p l i e s
t o t h e raw
DAC
o u t p u t when o u t p u t t i n g d i g i t i z e d sound.
The ramp
w i l l c o u n t
up t h e f i v e
M S B ' s
of t h e
DAC.
The t r i a n g l e
w i l l c o u n t up t h e f o u r
M S B ' s
of t h e
DAC
f o r t h e f i r s t h a l f of t h e
wave, t h e n
c o u n t them
back down
f o r
t h e
s e c o n d
h a l f .
The
f r e q u e n c y r a n g e
of t h e
DAC
a s a sound c h a n n e l w i l l h a v e t h e same
u p p e r l i m i t
and a
l o w e r l i m i t
of o n e o c t a v e l o w e r t h a n t h e new
f r e q u e n c y r a n g e of t h e sound c h i p (down t o 27.3 Hz.).
O b v i o u s l y ,
t h e b i t
programming o r d e r
o f t h e
f r e q u e n c y is
d i f f e r e n t .
The
a c t u a l f r e q u e n c y
w i l l
b e
111.86
KHz
d i v i d e d
by
t h e
number
programmed i n t o t h e sound f r e q u e n d y r e g i s t e r
( s )
.
P o r t
2 0 0
-
207
WR
--
C l e a r J o y s t i c k
DAC
c o u n t e r
A
w r i t e
t o p o r t 20X, where
X =
0 t o 7 , w i l l c l e a r a f r e e - r u n n i n g
c o u n t e r , and
l o a d a
v a l u e o f
1 6 i n t o
t h e 1 2 - b i t d i v i d e r .
The
e i g h t b i t
f r e e - r u n n i n g c o u n t e r
w i l l b e
c l o c k e d by t h e 3.58 M H z
s i g n a l d i v i d e d
by 2 4 , o r 1 4 9 . 1 KHz.
The o u t p u t of t h e e i g h t b i t
c o u n t e r w i l l
d r i v e t h e
DAC
t o
p r o d u c e a
s t a i r s t e p wave, which
s i m u l a t e s a
ramp f o r
u s e by t h e j o y s t i c k c o m p a r a t o r s .
When t h e
c o u n t e r r e a c h e s
a c o u n t
of 255,
i t w i l l s t o p u n t i l p o r t 20X i s
w r i t t e n t o a g a i n .
The e l a p s e d
time f o r t h e c o m p l e t e ramp
w i l l
be
a p p r o x i m a t e l y 1 . 7
m i l l i s e c o n d s , c l o s e l y
a p p r o x i m a t i n g
t h e
e l a p s e d
time
of
t h e
c u r r e n t Tandy 1 0 0 0 J o y s t i c k c i r c u i t r y .
1 0
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