8

Figures 4 and 7) 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).

INVALID switches high 1

µs after detecting a valid RS-232

level on a receiver input. INVALID operates in all modes

(forced or automatic powerdown, or forced on), so it is also

useful for systems employing manual powerdown circuitry.

Enhanced Automatic Powerdown

Even greater power savings is available by using the

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

ISL4270E 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 5 illustrates the enhanced powerdown control logic.

Note that once the ISL4270E enters powerdown (manually

or automatically), the 30 second timer remains timed out

(set), keeping the ISL4270E powered down until FORCEON

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

transmitter input.

As stated previously, the INVALID output switches low

whenever invalid levels have persisted on all of the receiver

inputs for more than 30

µs (see Figure 7), but this has no

direct effect on the state of the ISL4270E (see the next

sections for methods of utilizing INVALID to power down the

device).

The time to recover from automatic powerdown mode is

typically 100

µs.

Emulating Standard Automatic Powerdown

If enhanced automatic powerdown isn’t desired, the user can

implement the standard automatic powerdown feature

(mimics the function on the ICL3221E/23E/43E) by

connecting the INVALID output to the FORCEON and

FORCEOFF inputs, as shown in Figure 6. After 30

µs of

invalid receiver levels, INVALID switches low and drives the

ISL4270E into a forced powerdown condition. INVALID

switches high as soon as a receiver input senses a valid

FIGURE 4. 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 5. ENHANCED AUTOMATIC POWERDOWN LOGIC

30s

TIMER

S

R

FORCEOFF

AUTOPWDN

FORCEON

R_IN

T_IN

EDGE

DETECT

EDGE

DETECT

FIGURE 6. CONNECTIONS FOR AUTOMATIC POWERDOWN

WHEN NO VALID RECEIVER SIGNALS ARE

PRESENT

CPU

I/O

ISL4270E

UART

ISL4270E