ICL3225 Datasheet, PDF (9/18 Pages) Intersil Corporation – 1 Microamp, +3V to +5.5V, 1Mbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3225, ICL3245
PWR
MGT
LOGIC
FORCEOFF
FORCEON
INVALID
ICL32XX
CPU
I/O
UART
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
With any of the above control schemes, the time required to
exit powerdown, and resume transmission is only 100µs.
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.
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ICL32XX
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 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.
T_IN
EDGE
DETECT
R_IN
EDGE
DETECT
FORCEOFF
S
30s
TIMER
R
AUTOSHDN
FORCEON
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
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