7

FN7552.0

December 17, 2009

The ICL3221EM utilizes a two pin approach where the

FORCEON and FORCEOFF inputs determine the IC’s

mode. For always enabled operation, FORCEON and

FORCEOFF are both strapped high. To switch between

active and power-down modes, under logic or software

control, only the FORCEOFF input need be driven. The

FORCEON state isn’t critical, as FORCEOFF dominates

over FORCEON. Nevertheless, if strictly manual control

over power-down is desired, the user must strap

FORCEON high to disable the automatic power-down

circuitry.

The INVALID output always indicates whether or not a

valid RS-232 signal is present at any of the receiver

inputs (see Table 2), giving the user an easy way to

determine when the interface block should power down.

In the case of a disconnected interface cable where all

the receiver inputs are floating (but pulled to GND by the

internal receiver pull down resistors), the INVALID logic

detects the invalid levels and drives the output low. The

power management logic then uses this indicator to

power down the interface block. Reconnecting the cable

restores valid levels at the receiver inputs, INVALID

switches high, and the power management logic wakes

up the interface block. INVALID can also be used to

indicate the DTR or RING INDICATOR signal, as long as

the other receiver inputs are floating, or driven to GND

(as in the case of a powered down driver). Connecting

FORCEOFF and FORCEON together disables the

automatic power-down feature, enabling them to

function as a manual SHUTDOWN input (see Figure 4).

TABLE 2. POWER-DOWN AND ENABLE LOGIC TRUTH TABLE

RS-232 SIGNAL

PRESENT AT

RECEIVER INPUT?

FORCEOFF

OR SHDN

INPUT

FORCEON

INPUT

EN

INPUT

TRANSMITTER

OUTPUTS

RECEIVER

OUTPUTS

INVALID

OUTPUT

MODE OF OPERATION

ICL3221EM

No

H

H

L

Active

Active

L

Normal Operation

(Auto Power-down Disabled)

No

H

H

H

Active

High-Z

L

Yes

H

L

L

Active

Active

H

Normal Operation

(Auto Power-down Enabled)

Yes

H

L

H

Active

High-Z

H

No

H

L

L

High-Z

Active

L

Power-down Due to Auto

Power-down Logic

No

H

L

H

High-Z

High-Z

L

Yes

L

X

L

High-Z

Active

H

Manual Power-down

Yes

L

X

H

High-Z

High-Z

H

Manual Power-down with

Receiver Disabled

No

L

X

L

High-Z

Active

L

Manual Power-down

No

L

X

H

High-Z

High-Z

L

Manual Power-down with

Receiver Disabled

FIGURE 2. POWER DRAIN THROUGH POWERED

DOWN PERIPHERAL

OLD

POWERED

GND

SHDN = GND

Rx

Tx

CURRENT

FLOW

RS-232 CHIP

DOWN

UART

FIGURE 3. DISABLED RECEIVERS PREVENT POWER

DRAIN

ICL324XE

TRANSITION

FORCEOFF = GND

TO

POWERED

DETECTOR

DOWN

UART

WAKE-UP

LOGIC

ISL3221EM