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SILICON GATE MOS
8251
COMMAND INSTRUCTION DEFINITION
Once
the
functional definition
of
the
8251 has been pro-
grammed by
the
Mode Instruction and
the
Sync
Characters
are loaded (if
in·
Sync
Mode)
then
the
device
is
ready
to
be
used for
data
communication.
The
Command Instruction
controls
the
actual operation
of
the
selected format. Func-
tions such as: Enable Transmit/Receive, Error Reset and
Modem Controls are provided by
the
Command Instruction.
Once
the
Mode Instruction has been written
into
the
8251
and Sync characters inserted, if necessary,
then
all
further
"control
writes"
(C/O =
1)
will load
the
Command
In-
struction. A Reset operation (internal
or
external) will
return
the
8251
to
the
Mode Instruction Format.
STATUS READ DEFINITION
In
data
communication systems it
is
often necessary
to
examine
the
"status"
of
the
active device
to
ascertain if
errors have occurred
or
other
cond
itions
that
require
the
processor's attention.
The
8251 has facilities
that
allow
the
programmer
to
"read"
the
status
of
the
device
at
any
time
during
the
fu nctional operation.
A normal
"read"
command
is
issued
by
the
CPU with
the
C/O input
at
one
to
accomplish this function.
Some
of
the
bits
in
the
Status
Read
Format
have identical
meanings
to
external
output
pins so
that
the
8251
can
be
used
in
a completely Polled environment
or
in
an
interrupt
driven environment.
TRANSMIT ENABLE
1 = enable
0=
disable
SAME DEFINITIONS
AS
I/O PINS
DATA TERMINAL
~
READY
"high"
will
force DTR
output
to
zero
RECEIVE ENABLE
'------~
1 = enable
0=
disable
SEND BREAK
CHARACTER
1
=forces TxD
"Iow"
o=normal operation
ERROR RESET
-----------..
1 =reset all error flags
PE, OE, FE
REQUEST TO SEND
L.--
--..
"high"
will force RTS
output
to
zero
INTERNAL RESET
L.--
.......
"high"
returns 8251
to
Mode Instruction Format
ENTER HUNT MODE
L.--
--..
1 = enable search for Sync
Characters
Command Instruction
Format
PARITY ERROR
The
PE
flag
is
set when a parity
error
is
detected. It
is
reset by
the
ER
bit
of
the
Command
Instruction.
PE
does
not
inhibit
operation
of
the 8251.
OVERRUN ERROR
The OE flag
is
set when
the
CPU
does
not
read a character before
the
next one becomes available.
It
is
reset by the
ER
bit
of
the
Command Instruction. OE does
not
inhibitoperation
of
the
8251;
however,
the
previously overrun
character
is
lost.
FRAMING ERROR (Async only)
The FE flag
is
set when a valid
Stop
bit
is
not
detected
at
the
end
of
every character. It
is
reset
by
the
ER
bit of the Command
Instruction.
FE
does not inhibit
the
operation of the 8251.
Status Read
Format
5-142