Embedian, Inc.
68
SMARC-iMX8MM Computer on Module User’s Manual
v. 1.2
2.1.14. Asynchronous Serial Port (UARTs)
The
SMARC
‐
iMX8MM
module
supports
four
UARTs
(
SER0:3
).
UART
SER0
and
SER2
support
flow
control
signals
(
RTS#,
CTS#
).
UART
SER1
and
SER3
do
not
support
flow
control
(
TX,
RX
only).
When
working
with
software,
SER3
is
used
for
SMARC
‐
iMX8MM
debugging
console
port.
The
module
asynchronous
serial
port
signals
have
a
VDDIO
(
1.8V
)
level
signal
swing.
If
the
asynchronous
ports
are
to
interface
with
RS232
level
devices,
then
a
Carrier
RS
‐
232
transceiver
is
required.
The
logic
side
of
the
transceiver
must
be
able
to
run
at
1.8V
levels.
The
selection
of
1.8V
compatible
transceivers
is
a
bit
limited,
although
more
are
appearing
with
time.
Two
such
devices
are
the
Texas
Instruments
TRS3253E,
and
the
Maxim
MAX13235E,
illustrated
in
the
figures
below.
The
TI
part
is
more
cost
effective,
but
has
a
top
speed
of
1
Mbps.
The
MAX
13235E
can
operate
at
maximum
speeds
over
3
Mbps.
The
transceivers
invert
the
polarity
of
the
incoming
and
outgoing
data
and
handshake
lines.
The
other
alternative
is
to
use
a
level
‐
shift
IC
from
1.8V
to
3.3V
when
designing
carrier
board
and
almost
all
transceivers
available
accept
a
3.3V
signal
level:
example
includes
the
Texas
Instruments
MAX3243.
Note
that
RS232
transceivers
invert
the
signal;
a
logic
‘1’
is
a
negative
voltage
(
‐
3.0V
to
‐
15V)
and
a
logic
‘0’
a
positive
voltage
(3.0V
to
15V)
on
the
RS232
line.
