SP3238 Datasheet, PDF (15/19 Pages) Sipex Corporation – Intelligent +3.0V to +5.5V RS-232 Transceiver

AUTO ON-LINE^® Circuitry
The SP3238 device has a patent pending
AUTO ON-LINE^® circuitry on board that saves
power in applications such as laptop computers,
palmtop (PDA) computers, and other portable
systems.
The SP3238 device incorporates an
AUTO ON-LINE^® circuit that automatically
enables itself when the external transmitters are
enabled and the cable is connected. Conversely,
the AUTO ON-LINE^® 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 is externally controlled by the ONLINE
pin. When this pin is tied to a logic LOW, the
AUTO ON-LINE^® 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 standby mode.
When ONLINE is HIGH, the AUTO ON-
LINE^® mode is disabled.
The AUTO ON-LINE^® circuit has two stages:
1) Inactive Detection
2) Accumulated Delay
The first stage, shown in Figure 17, detects an
inactive input. A logic HIGH is asserted on
R_XINACT if the cable is disconnected or the
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 ON-LINE^®
circuitry, shown in Figure 18, processes all the
receiver's R_XINACT signals with an accumu-
lated delay that disables the device to a 1µA
supply current.
The STATUS pin goes to a logic LOW when the
cable is disconnected or when the external trans-
mitters are disabled.
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 ON-LINE^® mode can be disabled
by the SHUTDOWN pin. If this pin is a logic
LOW, the AUTO ON-LINE^® function will not
operate regardless of the logic state of the
ONLINE pin. Table 2 summarizes the logic of
the AUTO ON-LINE^® operating modes and
the truth table logic of the driver and receiver
outputs.
The STATUS pin outputs a logic LOW signal
when there is no valid RS-232 signal present on
any receiver input. This pin goes to a
logic HIGH when the external transmitters are
enabled and the cable is connected.
When the SP3238 device is shut down, the
charge pump is turned off. V+ charge pump
output decays to V_CC, the V- output decays to
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 200ms.
For easy programming, the STATUS pin can be
used to indicate DTR or a Ring Indicator signal.
Tying ONLINE and SHUTDOWN together
will bypass the AUTO ON-LINE^® circuitry so
this connection acts like a shutdown input pin.
Rev. 6/30/03
SP3238 Intelligent +3.0V to +5.5V RS-232 Transceiver
15
© Copyright 2003 Sipex Corporation