SP3238E Datasheet, PDF (15/20 Pages) Sipex Corporation – Intelligent +3.0V to +5.5V RS-232 Transceivers

Auto-Online Circuitry
The SP3238E device has a patent pending
Auto-Online circuitry on board that saves power
in applications such as laptop computers, palmtop
(PDA) computers, and other portable systems.
The SP3238E device incorporates an
Auto-Online circuit that automatically
enables itself when the external transmitters are
enabled and the cable is connected. Conversely,
the Auto-Online circuit also disables most of the
internal circuitry when the device is not being
used and goes into a standby mode where the
device typically draws 1µA. This function can
also be externally controlled by the ONLINE
pin. When this pin is tied to a logic LOW, the
Auto-Online function is active. Once active, the
device is enabled until there is no activity on the
receiver inputs. The receiver input typically sees
at least +3V, which are generated from the
transmitters at the other end of the cable with a
+5V minimum. When the external transmitters
are disabled or the cable is disconnected, the
receiver inputs will be pulled down by their
internal 5kΩ resistors to ground. When this
occurs over a period of time, the internal
transmitters will be disabled and the device goes
into a shutdown or standy mode. When ONLINE
is HIGH, the Auto-Online mode is disabled.
The Auto-Online circuit has two stages:
1) Inactive Detection
2) Accumulated Delay
The first stage, shown in Figure 18, detects an
inactive input. A logic HIGH is asserted on
R INACT if the cable is disconnected or the
X
external transmitters are disabled. Otherwise,
R_XINACT will be at a logic LOW. This circuit is
duplicated for each of the other receivers.
The second stage of the Auto-Online circuitry,
shown in Figure 19, processes all the receiver's
R_XINACT signals with an accumulated delay
that disables the device to a 1µA supply current.
The STATUS pin goes to a logic LOW when the
cable is disconnected, the external transmitters
are disabled, or the SHUTDOWN pin is
invoked. The typical accumulated delay is
around 20µs.
When the drivers or internal charge pump are
disabled, the supply current is reduced to 1µA.
This can commonly occur in hand-held or
portable applications where the RS-232 cable is
disconnected or the RS-232 drivers of the
connected peripheral are turned off.
The Auto-Online mode can be disabled by the
SHUTDOWN pin. If this pin is a logic LOW,
the Auto-Online function will not operate
regardless of the logic state of the ONLINE pin.
Table 3 summarizes the logic of the Auto-Online
operating modes. The truth table logic of the
driver and receiver outputs can be found in
Table 2.
The STATUS pin outputs a logic LOW signal
if the device is shutdown. This pin goes to a
logic HIGH when the external transmitters are
enabled and the cable is connected.
When the SP3238E device is shut down, the
charge pump is turned off. V+ charge pump
output decays to V , the V- output decays to
CC
GND. The decay time will depend on the size of
capacitors used for the charge pump. Once in
shutdown, the time required to exit the shut
down state and have valid V+ and V- levels is
typically 200µs.
For easy programming, the STATUS can be
used to indicate DTR or a Ring Indicator signal.
Tying ONLINE and SHUTDOWN together
will bypass the Auto-Online circuitry so this
connection acts like a shutdown input pin.
SP3238EDS/04
SP3238E +3.0V to +5.5V RS-232 Transceivers
15
© Copyright 1999 Sipex Corporation