10

FN4876.10

March 1, 2006

With any of the above control schemes, the time required to

exit powerdown, and resume transmission is only 100

µs.

When using both manual and enhanced automatic powerdown

(FORCEON = 0), the ICL32XX won’t power up from manual

powerdown until both FORCEOFF and FORCEON are driven

high, or until a transition occurs on a receiver or transmitter

input. Figure 5 illustrates a circuit for ensuring that the ICL32XX

powers up as soon as FORCEOFF switches high. The rising

edge of the Master Powerdown signal forces the device to

power up, and the ICL32XX returns to enhanced automatic

powerdown mode an RC time constant after this rising edge.

The time constant isn’t critical, because the ICL32XX remains

powered up for 30 seconds after the FORCEON falling edge,

even if there are no signal transitions. This gives slow-to-wake

systems (e.g., a mouse) plenty of time to start transmitting, and

as long as it starts transmitting within 30 seconds both systems

remain enabled.

INVALID Output

The INVALID output always indicates (see Table 2) whether

or not 30

µs have elapsed with invalid RS-232 signals (see

Figures 6 and 8) persisting on all of the receiver inputs, giving

the user an easy way to determine when the interface block

should power down. Invalid receiver levels occur whenever

the driving peripheral’s outputs are shut off (powered down) or

when the RS-232 interface cable is disconnected. 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).

Enhanced Automatic Powerdown

Even greater power savings is available by using these

devices which feature an enhanced automatic powerdown

function. When the enhanced powerdown logic determines

that no transitions have occurred on any of the transmitter

nor receiver inputs for 30 seconds, the charge pump and

transmitters powerdown, thereby reducing supply current to

1

µA. The ICL32XX automatically powers back up whenever

it detects a transition on one of these inputs. This automatic

powerdown feature provides additional system power

savings without changes to the existing operating system.

Enhanced automatic powerdown operates when the

FORCEON input is low, and the FORCEOFF input is high.

Tying FORCEON high disables automatic powerdown, but

manual powerdown is always available via the overriding

FORCEOFF input. Table 2 summarizes the enhanced

automatic powerdown functionality.

FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN

WHEN NO VALID RECEIVER SIGNALS ARE

PRESENT

PWR

FORCEOFF

INVALID

CPU

I/O

FORCEON

ICL32XX

MGT

LOGIC

UART

FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP

WHEN EXITING FORCED POWERDOWN

FORCEOFF

FORCEON

POWER

MASTER POWERDOWN LINE

1M

Ω

0.1

µF

MANAGEMENT

UNIT

ICL32XX

FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS

0.3V

-0.3V

-2.7V

2.7V

INVALID LEVEL - INVALID = 0

VALID RS-232 LEVEL - INVALID = 1

VALID RS-232 LEVEL - INVALID = 1

INDETERMINATE

INDETERMINATE

FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC

30sec

TIMER

S

R

FORCEOFF

AUTOSHDN

FORCEON

R_IN

T_IN

EDGE

DETECT

EDGE

DETECT

ICL3224, ICL3226, ICL3238, ICL3244