ICL3224 Intersil, ICL3224 Datasheet - Page 9

• Manufacturer Part Number: ICL3224
• Description: 1 Microamp/ +3V to +5.5V/ 250kbps/ RS-232 Transceivers with Enhanced Automatic Powerdown
• Manufacturer: Intersil

When using both manual and enhanced automatic powerdown
(FORCEON = 0), the ICL32XX won’t power up from manual
powerdown until both FORCEOFF and FORCEON are driven
high, or until a transition occurs on a receiver or transmitter
input. Figure 5 illustrates a circuit for ensuring that the ICL32XX
powers up as soon as FORCEOFF switches high. The rising
edge of the Master Powerdown signal forces the device to
power up, and the ICL32XX returns to enhanced automatic
powerdown mode an RC time constant after this rising edge.
The time constant isn’t critical, because the ICL32XX 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 30µs 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
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
MANAGEMENT
POWER
CPU
UNIT
LOGIC
PWR
MGT
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
WHEN EXITING FORCED POWERDOWN
UART
MASTER POWERDOWN LINE
I/O
FORCEOFF
FORCEON
INVALID
FORCEOFF
9
ICL32XX
0.1µF
ICL32XX
ICL3224, ICL3226, ICL3238, ICL3244
FORCEON
1MΩ
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 these
devices which feature 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
1µA. The ICL32XX 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 ICL32XX enters powerdown (manually or
automatically), the 30 second timer remains timed out (set),
keeping the ICL32XX powered down until FORCEON
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
T_IN
R_IN
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
-0.3V
-2.7V
2.7V
0.3V
DETECT
DETECT
EDGE
EDGE
INVALID LEVEL - INVALID = 0
VALID RS-232 LEVEL - INVALID = 1
VALID RS-232 LEVEL - INVALID = 1
FORCEON
INDETERMINATE
INDETERMINATE
FORCEOFF
30sec
TIMER
S
R
AUTOSHDN