ICL3221EM, ICL3221EF

4. Application Information

FN7552 Rev.2.00

Page 12 of 22

May.29.19

4.5.2 INVALID Output

The INVALID output always indicates whether a valid RS-232 signal is present at any of the receiver inputs (see

Table 5 on page 11), giving you a way to determine when the interface block should power down. If an interface

cable is disconnected and 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 powerdown feature, and enables them to function as a manual SHUTDOWN (SHDN) input

(see Figure 8 on page 13).

Figure 6. Power Drain Through Powered Down Peripheral

Figure 7. Disabled Receivers Prevent Power Drain

Old

VCC

Powered

GND

SHDN = GND

VCC

Rx

Tx

VCC

Current

VOUT = VCC

Flow

RS-232 CHIP

Down

UART

ICL324XE

Transition

RX

TX

R2OUTB

R2OUT

T1IN

FORCEOFF = GND

VCC

VCC

To

R2IN

T1OUT

VOUT = HI-Z

Powered

EN = VCC

Detector

Down

UART

Wake-Up

Logic