8
FORCEOFF switches high. The rising edge of the Master
Powerdown signal forces the device to power up, and the
ICL3238E returns to enhanced automatic powerdown mode
an RC time constant after this rising edge. The time constant
isn’t critical, because the ICL3238E remains powered up for
30 seconds after the FORCEON falling edge, even if there
are no signal transitions. This gives slow-to-wake systems
(e.g., a mouse) plenty of time to start transmitting, and as
long as it starts transmitting within 30 seconds both systems
remain enabled.
INVALID Output
The INVALID output always indicates (see Table 2) whether
or not 30
µs have elapsed with invalid RS-232 signals (see
Figures 6 and 8) 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).
Enhanced Automatic Powerdown
Even greater power savings is available by using this device
which features an 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
10nA. The ICL3238E 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 7 illustrates the enhanced powerdown control logic.
Note that once the ICL3238E enters powerdown (manually
or automatically), the 30 second timer remains timed out
(set), keeping the ICL3238E 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 60
µs (see Figure 8), but this has no direct effect
on the state of the ICL3238E (see the next sections for
methods of utilizing INVALID
to power down the device).
INVALID
switches high 1µs 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 25
µs.
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
WHEN EXITING FORCED POWERDOWN
FORCEOFF FORCEON
POWER
MASTER POWERDOWN LINE
1MΩ
0.1µF
MANAGEMENT
UNIT
ICL3238E
FIGURE 6. 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 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
30s
TIMER
S
R
FORCEOFF
AUTOSHDN
FORCEON
R_IN
T_IN
EDGE
DETECT
EDGE
DETECT
ICL3238E
