+3.0V to +5.5V, 1µA, up to 250kbps, True
RS-232 Transceiver with AutoShutdown Plus™
MAX3238
POWER
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
V
CC
FORCEOFF
FORCEON
INVALID
PROTECTION
DIODE
MAX3238
T1
V
CC
Rx
UART
I
PREVIOUS
RS-232
5k
Tx
T2
T3
I/O
CHIP
WITH
UART
GND
T4
RS-232
T5
R1
V
CC
R2
TO
µP
R3
LOGIC
TRANSITION
DETECTOR
SHDN = GND
CPU
(a)
OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE
RECEIVER OUTPUT IN SHUTDOWN.
I
PROTECTION
DIODE
MAX3238
Figure 1. Interface Under Control of PMU
V
CC
R1OUTB
AutoShutdown Plus Mode
1µA supply current is achieved with Maxim’s
AutoShutdown Plus feature, which operates when
FORCEOFF
is low and FORCEON is high. When the
MAX3238 senses no valid signal transitions on all
receiver and transmitter inputs for 30sec, the on-board
power supply and drivers are shut off, reducing supply
current to 1µA. This occurs if the RS-232 cable is dis-
connected or if the connected peripheral transmitters
are turned off. The system turns on again when a valid
transition is applied to any RS-232 receiver or transmit-
ter input. As a result, the system saves power without
changes to the existing BIOS or operating system. The
INVALID
output is high when the receivers are active.
Since
INVALID
indicates the receiver inputs’ condition,
it can be used in any mode (Figure 3).
UART
Rx
R1OUT
THREE-STATED
5k
R1IN
Tx
GND
T1IN
FORCEOFF = GND
T1OUT
(b)
NEW MAX3238: IN SHUTDOWN, R1OUTB IS USED TO MONITOR EXTERNAL
DEVICES AND R1OUT IS THREE STATED, ELIMINATING A CURRENT PATH
THROUGH THE UART'S PROTECTION DIODE.
Figure 2. The MAX3238 detects RS-232 activity when the
UART and interface are shut down.
6
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