10

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

transmitter input.

The INVALID output signal switches low to indicate that

invalid levels have persisted on all of the receiver inputs for

more than 30

µs (see Figure 8), but this has no direct effect

on the state of the ICL32XX (see the next sections for

methods of utilizing INVALID to power down the device).

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.

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 ICL3221/23/43) by connecting

the INVALID output to the FORCEON and FORCEOFF

inputs, as shown in Figure 9. After 30

µs of invalid receiver

levels, INVALID switches low and drives the ICL32XX into a

forced powerdown condition. INVALID switches high as

soon as a receiver input senses a valid RS-232 level, forcing

the ICL32XX to power on. See the “INVALID DRIVING

FORCEON AND FORCEOFF” section of Table 2 for an

operational summary. This operational mode is perfect for

handheld devices that communicate with another computer

via a detachable cable. Detaching the cable allows the

internal receiver pull-down resistors to pull the inputs to GND

(an invalid RS-232 level), causing the 30

µs timer to time-out

and drive the IC into powerdown. Reconnecting the cable

restores valid levels, causing the IC to power back up.

Hybrid Automatic Powerdown Options

For devices which communicate only through a detachable

cable, connecting INVALID to FORCEOFF (with FORCEON

= 0) may be a desirable configuration. While the cable is

attached INVALID and FORCEOFF remain high, so the

enhanced automatic powerdown logic powers down the RS-

232 device whenever there is 30 seconds of inactivity on the

receiver and transmitter inputs. Detaching the cable allows

the receiver inputs to drop to an invalid level (GND), so

INVALID switches low and forces the RS-232 device to

power down. The ICL32XX remains powered down until the

cable is reconnected (INVALID = FORCEOFF = 1) and a

transition occurs on a receiver or transmitter input (see

Figure 7). For immediate power up when the cable is

RECEIVER

INPUTS

TRANSMITTER

OUTPUTS

INVALID

OUTPUT

V+

0

V-

FIGURE 8. ENHANCED AUTOMATIC POWERDOWN, INVALID AND READY TIMING DIAGRAMS

READY

OUTPUT

TRANSMITTER

INPUTS

INVALID

REGION

}

FIGURE 9. CONNECTIONS FOR AUTOMATIC POWERDOWN

WHEN NO VALID RECEIVER SIGNALS ARE

PRESENT

CPU

I/O

ICL32XX

UART

ICL3224, ICL3226, ICL3238, ICL3244