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Baud
Rate Generator
Directly
above
the
UART
in
Figure
4
is
a
block
labeled
BRG
(Baud
Rate
Generator). This
section
of
the interface
outputs
two
clock
signals essential
to the
proper
operation of the
UART.
The two
clock
signals
(TRANSMIT
FREQ &
RECEIVE FREQ)
determine
the
transmit
and
receive
baud
rates
of the
serial
channel.
A
5.0688
MHz
crystal
is
used
as
a
timing
reference for the
BRG.
The
BRG
can be
programmed
to divide
this
reference
frequency
down
to the
frequen-
cies
needed by
the
UART.
For both
transmit
and
receive
operations,
the
UART
requires
a
clock
at
16 times the
baud
rate desired.
The
BRG
transmit
and
receive
frequency outputs can be independently
programmed by
loading the appropriate constants
into
its
internal
registers.
This
feature
of
the
BRG
makes
it
possible to transmit
and
receive
data
at
different
baud
rates.
Sense
Switches
Directly
below
the
UART
in
Figure
4
is
a
block
labeled
SENSE
SWITCHES.
This section
is
made
up
of
eight single-pole
single-throw
switches (S1-S8)
which you
use to
select
the
baud
rate, bits
per
word,
parity
(odd
or even), stop
bits
(one, one-and-one-half, or
two),
and
enable or
disable parity generation.
If
you
want,
you
can
ignore these switches
and under
software
control, directly
configure
the Interface
for
non-standard
baud
rates,
different receive
and
transmit frequencies,
etc.
Table
3
summarizes
the operation of the
sense switches.
Handshake
Latch
The
Radio Shack RS-232-C
Interface
can control (with proper
soft-
ware)
the
logic state
of
two
of
the control
signals
on
the Interface
(Request-To-Send and Data Terminal Ready).
This
is
accomplished
by
loading the
Handshake
Latch with
the appropriate
bit
pattern.
Four
of
the interface
signals
can be sensed
by
the
CPU
(Clear-To-
Send.
Data
Set
Ready,
Carrier Detect,
and Ring
Indicator)
by
reading
the
Modem
Status
Register,
shown
in
Figure
4.
The
Handshake
Latch and
Modem
Status Register,
with
the appropriate
software, allows
a
handshake
dialog to
occur
between
the
CPU
and
the
equipment connected
to the
RS-232-C
Interface.
12
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