ICL3221EM_14 Datasheet, PDF (7/12 Pages) Intersil Corporation – ±15kV ESD Protected, +3V, 1μA, 250kbps, RS-232 Transmitters/Receivers

ISL3221EM
TABLE 2. POWER-DOWN AND ENABLE LOGIC TRUTH TABLE
RS-232 SIGNAL FORCEOFF
PRESENT AT
OR SHDN FORCEON EN TRANSMITTER RECEIVER INVALID
RECEIVER INPUT? INPUT
INPUT INPUT OUTPUTS OUTPUTS OUTPUT
MODE OF OPERATION
ICL3221EM
No
H
H
L
Active
Active
L
Normal Operation
No
H
H
H
Active
High-Z
L
(Auto Power-down Disabled)
Yes
H
L
L
Active
Active
H
Normal Operation
Yes
H
L
H
Active
High-Z
H
(Auto Power-down Enabled)
No
H
L
L
High-Z
Active
L
Power-down Due to Auto
No
H
L
H
High-Z
High-Z
L
Power-down Logic
Yes
L
X
L
High-Z
Active
H
Manual Power-down
Yes
L
X
H
High-Z
High-Z
H
Manual Power-down with
Receiver Disabled
No
L
X
L
High-Z
Active
L
Manual Power-down
No
L
X
H
High-Z
High-Z
L
Manual Power-down with
Receiver Disabled
The ICL3221EM utilizes 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 power-down 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 power-down is desired, the user must strap
FORCEON high to disable the automatic power-down
circuitry.
The INVALID output always indicates whether or not a
valid RS-232 signal 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.
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). Connecting
FORCEOFF and FORCEON together disables the
automatic power-down feature, enabling them to
function as a manual SHUTDOWN input (see Figure 4).
V_CC
V_CC
V_CC
CURRENT
FLOW
V_OUT = V_CC
Rx
POWERED
DOWN
UART
GND
Tx
SHDN = GND
OLD
RS-232 CHIP
FIGURE 2. POWER DRAIN THROUGH POWERED
DOWN PERIPHERAL
V_CC
TRANSITION
DETECTOR
TO
WAKE-UP
LOGIC
V_CC
R2_OUTB
R_X
POWERED
DOWN
UART _TX
V_OUT = HI-Z
R2_OUT
T1_IN
ICL324XE
FORCEOFF = GND
OR SHDN = GND, EN = V_CC
R2_IN
T1_OUT
FIGURE 3. DISABLED RECEIVERS PREVENT POWER
DRAIN
7
FN7552.0
December 17, 2009