RS-232 Drivers

The two drivers are identical, and deliver EIA/TIA-232E

between 1.8V and 4.25V. One transmitter can drive up

to 3k

Ω in parallel with 2500pF at up to 120kbps.

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

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 5k

Ω

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

1µA Supply Current, 1.8V to 4.25V-Powered

RS-232 Transceiver with AutoShutdown™

_______________________________________________________________________________________

7

Table 3. AutoShutdown Logic

L

H

No

Normal Operation (Forced On)

L

H

No

Shutdown (AutoShutdown)

H

L

Yes

Shutdown (Forced Off)

L

L

No

Shutdown (Forced Off)

H

H

Yes

Normal Operation

RS-232 SIGNAL

PRESENT AT

RECEIVER INPUT

INVALID OUTPUT

FORCEOFF INPUT

TRANSCEIVER STATUS

FORCEON 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

X

H

L

L

H

X