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8251
A/S2657
FEATURES
AND
ENHANCEMENTS
8251A
is
anadval'lced
designon"-e
industry stan,
dard USAR:r, the Intel® 8251. The 8251A oper-
ates with an extended range
of
I ntel micropro-
cessors
that
includes the new
8085
CPU
and main-
tains compatibility with
the
8251. Familiarization
time
is
minimal because
of
compatibility and
involves only knowing
the
additional features and
enhancements, and reviewing
the
AC
and
DC
·speci-
fications
of
the
8251A.
The 8251A incorporates
all
the
key features
of
the
8251 and has the following additional features
and enhancements:
• 8251A has double-buffered data paths with
separate
I/O registers for control, status,
Data
In,
and Data Out, which considerably
simplifies control programming and mini-
mizes
CPU
overhead.
•
In
asynchronous operations,
the
Receiver
detects and
handles
"break"
automatically,
relieving
the
CPU
of
this task.
•
A refined
Rx
initialization prevents
the
Receiver from starting when
in
"break"
state, preventing unwanted interrupts from
a disconnected
USART.
•
At
the conclusion
of
a transmission, TxD
line will always return to
the
marking state
unless SBRK
is
programmed.
9-2
• Tx Enable logic enhancement
pre~ents
a
Tx Disable command from halting trans-
mission until all data previously· written has
been transmitted. The
logic also prevents
the
transmitter from turning off in
the
middle
of
a word.
• When External Sync Detect
is
programmed,
Internal Sync Detect
is
disabled, and an
Ex-
ternal Sync Detect status
is
provided
via
a
flip-flop which clears itself upon a status read.
• Possibility
of
false sync detect
is
minimized
by ensuring
that
if double character sync
is
programmed, the characters be contiguously
detected and
also by clearing the Rx register
to
all
ones whenever Enter Hunt command
is
issued
in
Sync mode.
• As iong
as
the
825iA
is
not seiecteu, the
RD
and
WR
do
not
affect
the
internal opera-
tion
of
the device.
• The 8251A Status can
be
read
at
any time
but
the
status update will
be
inhibited during
status read.
• The 8251A
is
free from extraneous glitches
and has enhanced
ACand
DC
characteristics,
providing higher speed and better operating
margins.
• Synchronous Baud rate from
DC
to
64K.
• Fully compatible with Intel's new industry
standard,
the
MCS-85.
00216A