RS-232 Drivers
The two drivers are identical, and deliver EIA/TIA-232E
and EIA/TIA-562 output voltage levels when V
CC
is
between 1.8V and 4.25V. One transmitter can drive up
to 3kΩ in parallel with 2500pF at up to 120kbps.
Connect unused drivers to either GND or V
CC
. When
FORCEOFF is driven low, or when AutoShutdown circuit-
ry senses invalid voltage levels at all receiver inputs, the
drivers are disabled and their outputs are forced into a
high-impedance state. Driver inputs do not have internal
pull-up resistors.
RS-232 Receivers
The two receivers are identical, and accept both
EIA/TIA-232E and EIA/TIA-562 input signals. The
CMOS receiver outputs are inverting and swing rail-to-
rail. Receivers are disabled only when FORCEON and
FORCEOFF inputs are low. (See Table 2.)
Shutdown
When FORCEOFF is low, power supplies are disabled
and the transmitters are placed in a high-impedance
state. Receiver operation is not affected by taking
FORCEOFF low. Power consumption is dramatically
reduced in shutdown mode. Supply current is minimized
when the receiver inputs are static in any one of three
states: floating (ground), GND, or V
CC
.
AutoShutdown™
A 1µA supply current is achieved with Maxim’s new
AutoShutdown feature, which operates when FORCEON
is low and FORCEOFF is high. When the MAX3218
senses no valid signal level on either receiver input for
typically 30µs, the on-board power supply and drivers
shut down, reducing supply current to 1µA. Internal 5kΩ
resistors pull undriven receiver inputs to ground. This
occurs if the RS-232 cable is disconnected or if the con -
nected peripheral transmitters are turned off. The sys -
tem turns on again when a valid level is applied to either
RS-232 receiver input. As a result, the system saves
power without changes to the existing BIOS or operating
system. When using the AutoShutdown feature, INVALID
is high when the device is on and low when the device is
shut down. The INVALID output indicates the condition
of the receiver inputs.
Table 3 summarizes the MAX3218 operating modes.
FORCEON and FORCEOFF override the automatic cir-
cuitry and force the transceiver into its normal operating
state or into its low-power standby state. When neither
control is asserted, the IC selects between these states
automatically based on receiver input levels. Figure 4
depicts valid and invalid RS-232 receiver levels. The
MAX3218
1µA Supply Current, 1.8V to 4.25V-Powered
RS-232 Transceiver with AutoShutdown™
_______________________________________________________________________________________ 7
Table 3. AutoShutdown Logic
LHNo Normal Operation (Forced On)
LHNo Shutdown (AutoShutdown)
HLYes Shutdown (Forced Off)
LLNo Shutdown (Forced Off)
HHYes Normal Operation
RS-232 SIGNAL
PRESENT AT
RECEIVER INPUT
INVALID OUTPUTFORCEOFF INPUT TRANSCEIVER STATUSFORCEON INPUT
H
L
X
X
X
POWER
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
CPU
I/O
CHIP
WITH
UART
MAX3218
RS-232
FORCEOFF
FORCEON
INVALID
Figure 2. Interface Under Control of PMU
Table 2. Receiver Status
FORCEON FORCEOFF RECEIVER STATUS
Receiver Enabled
Receiver Disabled
XH
LL
HX
