ICL3225E_06 Datasheet, PDF (10/20 Pages) Intersil Corporation – ±15kV ESD Protected, +3V to +5.5V, 1Microamp, 1Mbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3225E, ICL3227E, ICL3245E
PWR
MGT
LOGIC
FORCEOFF
FORCEON
INVALID
ICL32XXE
CPU
I/O
UART
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
When using both manual and enhanced automatic
powerdown (FORCEON = 0), the ICL32XXE 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 ICL32XXE powers up as soon as
FORCEOFF switches high. The rising edge of the Master
Powerdown signal forces the device to power up, and the
ICL32XXE returns to enhanced automatic powerdown mode
an RC time constant after this rising edge. The time constant
isn’t critical, because the ICL32XXE 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.
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ICL32XXE
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
WHEN EXITING FORCED POWERDOWN
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 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).
2.7V
VALID RS-232 LEVEL - INVALID = 1
INDETERMINATE
0.3V
-0.3V
INVALID LEVEL - INVALID = 0
INDETERMINATE
-2.7V
VALID RS-232 LEVEL - INVALID = 1
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
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
ICL32XXE 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.
T_IN
EDGE
DETECT
R_IN
EDGE
DETECT
FORCEOFF
S
30s
TIMER
R
AUTOSHDN
FORCEON
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
Figure 7 illustrates the enhanced powerdown control logic.
Note that once the ICL32XXE enters powerdown (manually
or automatically), the 30 second timer remains timed out
10
FN4900.9
February 27, 2006