FUNCTIONAL DESCRIPTION
I I I H . S C R A T C H P A D R E G I S T E R
T h i s r e g i s t e r i s n o t u s e d b y t h e 1 6 4 5 0 / 1 6 5 5 0 . I t
may be used by the programmer for data storage.
I V . F I F O I N T E R R U P T M O D E O P E R A T I O N *
1 . T h e r e c e i v e d a t a i n t e r r u p t i s i s s u e d w h e n t h e
FIFO reaches the trigger level. The interrupt is
c l e a r e d a s s o o n a s t h e F I F O f a l l s b e l o w t h e
trigger level.
2 . T h e i n t e r r u p t i d e n t i f i c a t i o n r e g i s t e r ' s r e c e i v e
data available indicator is set and cleared along
with the receive data interrupt above.
3 . T h e d a t a r e a d y i n d i c a t o r i s s e t a s s o o n a s a
c h a r a c t e r i s t r a n s f e r r e d i n t o t h e r e c e i v e r F I F O
and is cleared when the FIFO is empty.
V . B A U D R A T E S E L E C T I O N
The 16450 ACE determines the baud rate of the serial
o u t p u t a n d u s e s a c o m b i n a t i o n o f t h e c l o c k i n p u t
f r e q u e n c y a n d t h e v a l u e w r i t t e n t o t h e d i v i s o r l a t c h e s .
Standard PC, PC/XT, PC/AT, and PS/2 serial interfaces use
a n i n p u t c l o c k o f 1 . 8 4 3 2 M H z . T o i n c r e a s e v e r s a t i l i t y ,
t h e Q S - 2 0 0 M / Q S - 3 0 0 M u s e s a n 1 8 . 4 3 2 M H z c r y s t a l a n d a
f r e q u e n c y d i v i d e r c i r c u i t t o p r o d u c e t h e s t a n d a r d c l o c k
frequency.
Jumper block J1 is used to set the frequency input
t o t h e 1 6 4 5 0 . I t m a y b e c o n n e c t e d t o d i v i d e t h e c l o c k
i n p u t b y 1 , 2 , 5 , o r 1 0 . F o r c o m p a t i b i l i t y a s s t a t e d
a b o v e , J 1 s h o u l d b e c o n f i g u r e d t o d i v i d e b y 1 0 a s s h o w n
i n f i g u r e 1 4 ( d ) . A t a b l e o f b a u d r a t e s a v a i l a b l e u s i n g
the 1.8432 MHz input is given in figure 14.
* For optional 16550 only.
