Datasheet

MAX3238
+3.0V to +5.5V, 1µA, up to 250kbps, True
RS-232 Transceiver with AutoShutdown Plus
6 _______________________________________________________________________________________
AutoShutdown Plus Mode
1µA supply current is achieved with Maxim’s
AutoShutdown Plus feature, which operates when
FORCEOFF is high and FORCEON is low. 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).
MAX3238
I/O
CHIP
WITH
UART
CPU
RS-232
POWER
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
FORCEOFF
FORCEON
INVALID
T1
T2
T3
T4
T5
R1
R2
R3
Figure 1. Interface Under Control of PMU
MAX3238
T1OUT
R1OUTB
Tx
5kΩ
UART
V
CC
T1IN
THREE-STATED
LOGIC
TRANSITION
DETECTOR
R1IN
PROTECTION
DIODE
R1OUT
FORCEOFF = GND
V
CC
TO
µP
Rx
PREVIOUS
RS-232
Tx
UART
SHDN = GND
V
CC
V
CC
GND
Rx
5kΩ
(a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE
RECEIVER OUTPUT IN SHUTDOWN.
(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.
GND
PROTECTION
DIODE
I
I
Figure 2. The MAX3238 detects RS-232 activity when the
UART and interface are shut down.