11
FN4876.10
March 1, 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
s (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
s 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
s.
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
s 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
s 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
RECEIVER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
V+
VCC
0
V-
tINVL
tINVH
FIGURE 8. ENHANCED AUTOMATIC POWERDOWN, INVALID AND READY TIMING DIAGRAMS
READY
OUTPUT
TRANSMITTER
INPUTS
tWU
tAUTOPWDN
tWU
INVALID
REGION
}
FIGURE 9. CONNECTIONS FOR AUTOMATIC POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
F
O
RCEOFF
IN
VAL
ID
CPU
I/O
F
O
RCEON
ICL32XX
UART
ICL3224, ICL3226, ICL3238, ICL3244