9

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, ICL3243E)

by connecting the INVALID output to the FORCEON and

invalid receiver levels, INVALID switches low and drives the

ICL3238E into a forced powerdown condition. INVALID

switches high as soon as a receiver input senses a valid RS-

232 level, forcing the ICL3238E 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

60

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 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 ICL3238E 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

reattached, connect FORCEON to FORCEOFF through a

network similar to that shown in Figure 5.

Capacitor Selection

3.3V (5% tolerance) operation. For other supply voltages

refer to Table 3 for capacitor values. Do not use values

smaller than those listed in Table 3. Increasing the capacitor

values (by a factor of 2) reduces ripple on the transmitter

capacitors).

RECEIVER

INPUTS

TRANSMITTER

OUTPUTS

INVALID

OUTPUT

V+

0

V-

FIGURE 8. ENHANCED AUTOMATIC POWERDOWN, AND INVALID TIMING DIAGRAMS

TRANSMITTER

INPUTS

INVALID

REGION

}

FIGURE 9. CONNECTIONS FOR AUTOMATIC POWERDOWN

WHEN NO VALID RECEIVER SIGNALS ARE

PRESENT

CPU

I/O

ICL3238E

UART

TABLE 3. REQUIRED CAPACITOR VALUES

3.0 to 3.6 (3.3V

±10%)

0.22

0.22

3.15 to 3.6 (3.3V

±5%)

0.1

0.1

4.5 to 5.5

0.047

0.33

3.0 to 5.5

0.22

1

ICL3238E