ICL3221EM_14 Datasheet, PDF (8/12 Pages) Intersil Corporation – ±15kV ESD Protected, +3V, 1μA, 250kbps, RS-232 Transmitters/Receivers

ISL3221EM
PWR
MGT
LOGIC
FORCEOFF
FORCEON
INVALID
ICL3221EM
CPU
I/O
UART
FIGURE 4. CONNECTIONS FOR MANUAL POWER-DOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
With any of the control schemes, the time required to
exit power-down, and resume transmission is only
100µs. A mouse, or other application, may need more
time to wake up from shutdown. If automatic
power-down is being utilized, the RS-232 device will
reenter power-down if valid receiver levels aren’t
reestablished within 30µs of the ICL3221EM powering
up. Figure 5 illustrates a circuit that keeps the
ICL3221EM from initiating automatic power-down for
100ms after powering up. This gives the slow-to-wake
peripheral circuit time to reestablish valid RS-232 output
levels.
POWER
MANAGEMENT
UNIT
MASTER POWER-DOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ICL3221EM
FIGURE 5. CIRCUIT TO PREVENT AUTO POWER-DOWN
FOR 100ms AFTER FORCED POWER-UP
Automatic Power-down
Even greater power savings is available by using the
devices which feature an automatic power-down
function. When no valid RS-232 voltages (see Figure 6)
are sensed on any receiver input for 30µs, the charge
pump and transmitters power-down, thereby reducing
supply current to 1µA. Invalid receiver levels occur
whenever the driving peripheral’s outputs are shut off
(powered down) or when the RS-232 interface cable is
disconnected. The ICL3221EM powers back up whenever
it detects a valid RS-232 voltage level on any receiver
input. This automatic power-down feature provides
additional system power savings without changes to the
existing operating system.
2.7V
VALID RS-232 LEVEL - ICL3221EM IS ACTIVE
INDETERMINATE - POWER-DOWN MAY OR
MAY NOT OCCUR
0.3V
-0.3V
INVALID LEVEL - POWER-DOWN OCCURS
AFTER 30µs
INDETERMINATE - POWER-DOWN MAY OR
MAY NOT OCCUR
-2.7V
VALID RS-232 LEVEL - ICL3221EM IS ACTIVE
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER
LEVELS
Automatic power-down operates when the FORCEON
input is low, and the FORCEOFF input is high. Tying
FORCEON high disables automatic power-down, but
manual power-down is always available via the
overriding FORCEOFF input. Table 2 summarizes the
automatic power-down functionality.
Devices with the automatic power-down feature include
an INVALID output signal, which switches low to indicate
that invalid levels have persisted on all of the receiver
inputs for more than 30µs (see Figure 7). INVALID
switches high 1µs after detecting a valid RS-232 level on
a receiver input. INVALID operates in all modes (forced
or automatic power-down, or forced on), so it is also
useful for systems employing manual power-down
circuitry. When automatic power-down is utilized,
INVALID = 0 indicates that the ICL3221EM is in
power-down mode.
The time to recover from automatic power-down mode is
typically 100µs.
RECEIVER
INPUTS
INVALID
} _REGION
TRANSMITTER
OUTPUTS
INVALID ^VCC
OUTPUT
0
t_INVL
AUTOPWDN
V+
V_CC
0
V-
t_INVH
PWR UP
FIGURE 7. AUTOMATIC POWER-DOWN AND INVALID
TIMING DIAGRAMS
Receiver ENABLE Control
Several devices also feature an EN input to control the
receiver outputs. Driving EN high disables all the
inverting (standard) receiver outputs placing them in a
8
FN7552.0
December 17, 2009