3V to 5.5V, up to 250kbps True RS-232 Transceiver

with 4µA AutoShutdown Plus and Power-On Reset

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7

reducing supply current to 4µA. This occurs if the RS-232

cable is disconnected or the connected peripheral trans-

mitters turn off. The system turns on again when a valid

transition occurs at any RS-232 receiver or transmitter

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 2).

Tables 1 and 2 and Figure 2 summarize the MAX3320’s

operating modes. FORCEON and FORCEOFF override

the automatic circuitry and force the transceiver into its

normal operating state or into its low-power standby

state. When neither control is asserted, the IC enters

AutoShutdown Plus mode and selects between these

states automatically, based on the last receiver or

transmitter input edge received.

When shut down, the device’s charge pumps turn off,

mitter outputs turn off (high impedance). The time

required to exit shutdown is typically 25µs (Figure 3a).

Software-Controlled Shutdown

If direct software control is desired, use INVALID to indi-

cate DTR or Ring Indicator signal. Connect FORCEOFF

and FORCEON together to bypass the AutoShutdown

Plus feature so the line acts like a SHDN input.

Power-On Reset

In addition to issuing a reset to the microprocessor (µP)

during power-up, power-down, and brownout condi-

tions, the MAX3320 is relatively immune to short-

reset threshold and lasts 20µs or less does not cause a

reset pulse (see

Typical Operating Characteristics).

Additional bypass capacitance mounted as close as

immunity.

Table 2. INVALID Truth Table

+0.3V

-0.3V

INVALID

R_IN

INVALID ASSERTED IF BOTH RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR

AT LEAST 90

µs.

90µs

TIMER

R

Figure 2a. INVALID Functional Diagram, INVALID Low

+2.7V

-2.7V

R_IN

INVALID DEASSERTED IF EITHER RECEIVER INPUT HAS BEEN BETWEEN +2.7V

AND -2.7V FOR LESS THAN 90

µs.

INVALID

90µs

TIMER

R

Figure 2b. INVALID Functional Diagram, INVALID High

AUTOSHDN

R_IN

T_IN

R

S

30sec

TIMER

EDGE

DETECT

EDGE

DETECT

FORCEOFF

FORCEON

Figure 2c. AutoShutdown Plus Logic

POWERDOWN*

AUTOSHDN

FORCEOFF

FORCEON

* POWERDOWN IS ONLY AN INTERNAL SIGNAL.

IT CONTROLS THE OPERATIONAL STATUS OF

THE TRANSMITTERS AND THE POWER SUPPLIES.

Figure 2d. Power-Down Logic

Yes

RS-232 SIGNAL PRESENT AT

RECEIVER INPUT

No

L

H

INVALID

OUTPUT