ISL4238E Intersil Corporation, ISL4238E Datasheet - Page 11

• Manufacturer Part Number: ISL4238E
• Description: QFN Packaged, 15kV Esd Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/ 1Mbps, RS-232 Transceivers With Enhanced Automatic Powerdown
• Manufacturer: Intersil Corporation

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 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.
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
ISL4238E/4XE 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.
Figure 7 illustrates the enhanced powerdown control logic.
Note that once the ISL4238E/4XE enters powerdown
(manually or automatically), the 30 second timer remains
timed out (set), keeping the ISL4238E/4XE powered down
until FORCEON transitions high, or until a transition occurs
on a receiver or transmitter input.
The INVALID output signal switches low to indicate that
invalid levels have persisted on all of the receiver inputs for
more than 30µs (see Figure 9), but this has no direct effect
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
-0.3V
-2.7V
2.7V
0.3V
INVALID LEVEL - INVALID = 0
VALID RS-232 LEVEL - INVALID = 1
VALID RS-232 LEVEL - INVALID = 1
INDETERMINATE
INDETERMINATE
11
ISL4238E, ISL4244E, ISL4245E
10nA
. The
on the state of the ISL4238E/4XE (see the next sections for
methods of utilizing INVALID to power down the device).
The time to recover from automatic powerdown mode is
typically 100µs.
Emulating Standard Automatic Powerdown
If enhanced automatic powerdown isn’t desired, the user can
implement the standard automatic powerdown feature (mimics
the function on the ISL4243E) by connecting the INVALID
output to the FORCEON and FORCEOFF inputs, as shown in
Figure 8. After 30µs of invalid receiver levels, INVALID switches
low and drives the ISL4238E/4XE into a forced powerdown
condition. INVALID switches high as soon as a receiver input
senses a valid RS-232 level, forcing the ISL4238E/4XE to
power on. See the “INVALID DRIVING FORCEON AND
FORCEOFF” section of Table 2 for an operational summary.
This operational mode is perfect for handheld devices that
communicate with another computer via a detachable cable.
Detaching the cable allows the internal receiver pull-down
resistors to pull the inputs to GND (an invalid RS-232 level),
causing the 30µs timer to time-out and drive the IC into
powerdown. Reconnecting the cable restores valid levels,
causing the IC to power back up.
Hybrid Automatic Powerdown Options
For devices which communicate only through a detachable
cable, connecting INVALID to FORCEOFF (with
FORCEON = 0) may be a desirable configuration. While the
T_IN
R_IN
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
FIGURE 8. CONNECTIONS FOR AUTOMATIC POWERDOWN
CPU
DETECT
DETECT
EDGE
EDGE
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
FORCEON
UART
I/O
INVALID
FORCEOFF
ISL4238E/4XE
30sec
TIMER
FORCEON
S
R
FORCEOFF
AUTOPWDN