10

FN4878.8

February 27, 2006

Figure 7 illustrates the enhanced powerdown control logic.

Note that once the ICL32XX enters powerdown (manually or

automatically), the 30 second timer remains timed out (set),

keeping the ICL32XX powered down until FORCEON

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/ICL3223/ICL3243) 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 cablerestores 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

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

ICL3225, ICL3245