ISL4241E Intersil Corporation, ISL4241E Datasheet - Page 8

• Manufacturer Part Number: ISL4241E
• Description: QFN Packaged, +/-15kV Esd Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps, RS-232 Transmitters/receivers
• Manufacturer: Intersil Corporation

should also be disabled by driving the EN input high (see
next section, and Figures 2 and 3).
The ISL4243E utilize a two pin approach where the
FORCEON and FORCEOFF inputs determine the IC’s
mode. For always enabled operation, FORCEON and
FORCEOFF are both strapped high. To switch between
active and powerdown modes, under logic or software
control, only the FORCEOFF input need be driven. The
FORCEON state isn’t critical, as FORCEOFF dominates
over FORCEON. Nevertheless, if strictly manual control over
powerdown is desired, the user must strap FORCEON high
to disable the automatic powerdown circuitry. ISL4243E
inverting (standard) receiver outputs also disable when the
device is in manual powerdown, thereby eliminating the
possible current path through a shutdown peripheral’s input
protection diode (see Figures 2 and 3).
Connecting FORCEOFF and FORCEON together disables
the automatic powerdown feature, enabling them to function
as a manual SHUTDOWN input (see Figure 4).
With any of the above control schemes, the time required to
exit powerdown, and resume transmission is only 100µs. A
mouse, or other application, may need more time to wake up
from shutdown. If automatic powerdown is being utilized, the
RS-232 device will reenter powerdown if valid receiver levels
aren’t reestablished within 30µs of the ISL4243E powering
up. Figure 5 illustrates a circuit that keeps the ISL4243E
from initiating automatic powerdown for 100ms after
powering up. This gives the slow-to-wake peripheral circuit
time to reestablish valid RS-232 output levels.
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
CPU
LOGIC
PWR
MGT
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
UART
I/O
FORCEOFF
FORCEON
INVALID
8
ISL4243E
ISL4241E, ISL4243E
INVALID Output (ISL4243E Only)
The INVALID output always indicates whether or not a valid
RS-232 signal (see Figure 6) is present at any of the
receiver inputs (see Table 2), 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).
INVALID switches low after invalid levels have persisted on
all of the receiver inputs for more than 30µs (see Figure 7).
INVALID switches back 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. When automatic powerdown is utilized, INVALID = 0
indicates that the ISL4243E is in powerdown mode.
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
FIGURE 5. CIRCUIT TO PREVENT AUTO POWERDOWN FOR
-0.3V
-2.7V
2.7V
0.3V
MANAGEMENT
POWER
UNIT
INVALID LEVEL - POWERDOWN OCCURS AFTER 30µs
VALID RS-232 LEVEL - ISL4243E IS ACTIVE
VALID RS-232 LEVEL - ISL4243E IS ACTIVE
INDETERMINATE - POWERDOWN MAY OR
INDETERMINATE - POWERDOWN MAY OR
100ms AFTER FORCED POWERUP
MASTER POWERDOWN LINE
MAY NOT OCCUR
MAY NOT OCCUR
FORCEOFF
ISL4243E
0.1µF
FORCEON
1MΩ