
12
FN4910.18
March 8, 2005
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 powerdown feature, enabling them to function as 
a manual SHUTDOWN input (see Figure 4).
With any of the above control schemes, the time required to 
exit powerdown, and resume transmission is only 100μs. A 
mouse, or other application, may need more time to wake up 
from shutdown. If automatic powerdown is being utilized, the 
RS-232 device will reenter powerdown if valid receiver levels 
aren’t reestablished within 30μs of the ICL32XXE powering 
up. Figure 5 illustrates a circuit that keeps the ICL32XXE 
from initiating automatic powerdown for 100ms after 
powering up. This gives the slow-to-wake peripheral circuit 
time to reestablish valid RS-232 output levels.
Automatic Powerdown 
(ICL3221E, ICL3223E, ICL3243E Only)
Even greater power savings is available by using the 
devices which feature an 
automatic
 powerdown function. 
When no valid RS-232 voltages (see Figure 6) are sensed 
on any receiver input for 30μs, the charge pump and 
transmitters powerdown, thereby reducing supply current to 
1μA. Invalid receiver levels occur whenever the driving 
peripheral’s outputs are shut off (powered down) or when the 
RS-232 interface cable is disconnected. The ICL32XXE 
powers back up whenever it detects a valid RS-232 voltage 
level on any receiver input. This automatic powerdown 
FIGURE 2. POWER DRAIN THROUGH POWERED DOWN 
PERIPHERAL 
OLD
V
CC
POWERED
DOWN
UART
GND
SHDN = GND
V
CC
Rx
Tx
V
CC
CURRENT
FLOW
V
OUT 
=
V
CC
RS-232 CHIP
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
ICL324XE
TRANSITION
DETECTOR
R
X
T
X
R2
OUTB
R2
OUT
T1
IN
FORCEOFF = GND
OR SHDN = GND, EN = V
CC
V
CC
V
CC
TO
R2
IN
T1
OUT
V
OUT 
=
HI-Z
POWERED
DOWN
UART
WAKE-UP
LOGIC
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN 
WHEN NO VALID RECEIVER SIGNALS ARE 
PRESENT
PWR
MGT
LOGIC
FORCEOFF
INVALID
CPU
I/O
FORCEON
ICL3221E,
ICL3223E,
UART
ICL3243E
FIGURE 5. CIRCUIT TO PREVENT AUTO POWERDOWN FOR 
100ms AFTER FORCED POWERUP
ICL3221E, ICL3223E, ICL3243E
FORCEOFF
FORCEON
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
1M
0.1μF
ICL3221E, ICL3222E, ICL3223E, ICL3232E, ICL3241E, ICL3243E