SP3238EEY-L-TR Datasheet, PDF (12/21 Pages) Exar Corporation – Intelligent +3.0V to +5.5V RS-232 Transceiver

AUTO ONLINE CIRCUITRY
The SP3238E 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 SP3238E 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 inter-
nal 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^® func-
tion 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 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 3 summarizes the logic of the AUTO ON-
LINE^® operating modes.
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 shutdown, the
charge pumps are 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 can
be used to indicate DTR or a Ring Indica-
tor signal. Tying ONLINE and SHUTDOWN
together will bypass the AUTO ON-LINE^® cir-
cuitry so this connection acts like a shutdown
input pin
The second stage of the AUTO ON-LINE^®
circuitry, shown in Figure 18, 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 SP3238E drivers or inter-
nal charge pump are disabled, the supply current
is reduced to 1µA. This can commonly occur in
handheld or portable applications where
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
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