Table 2 summarizes the MAX3212 operating modes.

FORCEON and FORCEOFF override the automatic cir-

cuitry and force the transceiver into its normal operat-

ing state or into its low-power standby state. When

neither control is asserted, the IC selects between

these states automatically based on receiver input lev-

els. Figure 4 depicts valid and invalid RS-232 receiver

levels. The MAX3212 shuts down after sensing invalid

RS-232 levels for greater than 30µs, ensuring the

AutoShutdown mode is not enabled for slow-moving

signals (1V/µs).

A mouse or another system with AutoShutdown may

need a period of time to wake up. Figure 5 shows a cir-

cuit that forces the transmitters on for 100ms after start-

up, allowing enough time for the other system to realize

that the MAX3212 system is awake. If the other system

outputs valid RS-232 signals within that time, the RS-

232 ports on both systems remain enabled.

Transition Detector

The MAX3212 also has an on-board transition detector

that monitors activity on the receiver inputs. In systems

with a sleep mode, the transition detector output (TRAN)

can be used to wake up the system when activity at the

receiver inputs is detected. Before putting the system to

sleep, set TRAN low by cycling EN high and then low.

TRAN remains low as long as no activity is detected on

the receiver inputs. When any receiver is toggled, TRAN

latches high. Connect TRAN to a microprocessor inter-

rupt, or if the system wakes up periodically TRAN can

be polled. Transition detection is useful when valid RS-

232 levels are present at the receiver inputs but no data

is being sent. For example, if a printer is attached to the

serial port but is not in use, the microprocessor senses

this and forces the MAX3212 off.

+2.7V to +3.6V-Powered, 1µA Supply Current,

3-Driver/5-Receiver, True RS-232 Transceiver

8

_______________________________________________________________________________________

INVALID IS AN INTERNALLY GENERATED SIGNAL

THAT IS USED BY THE AUTOSHUTDOWN LOGIC

AND APPEARS AS AN OUTPUT OF THE DEVICE.

POWER DOWN IS ONLY AN INTERNAL SIGNAL.

IT CONTROLS THE OPERATIONAL STATUS OF

THE TRANSMITTERS AND THE POWER SUPPLIES.

FORCEOFF

0

0

0

0

1

1

1

1

FORCEON

0

0

1

1

0

0

1

1

INVALID

0

1

0

1

0

1

0

1

POWER DOWN

0

0

0

0

0

1

1

1

POWER DOWN

FORCEOFF

FORCEON

INVALID

+

-

+

-

Rx

RIN_

EN

ROUT_

30µs

DELAY

•

•

•

•

•

•

+0.3V to +3.0V

-0.3V to -3.0V

Figure 3. AutoShutdown Logic