SP3221E_17 Datasheet, PDF (14/22 Pages) Exar Corporation – Intelligent +3.0V to +5.5V RS-232 Transceiver

AUTO ON-LINE^® Circuitry
The SP3221E device has AUTO ON-LINE^®
circuitry on board that saves power in ap-
plications such as laptop computers, PDA's,
and other portable systems.
The SP3221E device incorporates an AUTO
ON-LINE^® circuit that automatically enables
itself when the external transmitter is 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 ac-
tive. Once active, the device is enabled until
there is no activity on the receiver input. The
receiver input typically sees at least ±3V,
which are generated from the transmitter
at the other end of the cable with a ±5V
minimum. When the external transmitter
is disabled or the cable is disconnected,
the receiver input will be pulled down by its
internal 5kΩ resistor to ground. When this
occurs over a period of time, the internal
transmitter will be disabled and the device
goes into a shutdown or standby mode.
When the ONLINE pin 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 14, detects
an inactive input. A logic HIGH is asserted
on R_XINACT if the cable is disconnected or
the external transmitters are disabled. Oth-
erwise, R_XINACT will be at a logic LOW.
The second stage of the AUTO ON-LINE^®
circuitry, shown in Figure 15, processes the
receiver's R_XINACT signal with an accu-
mulated delay that disables the device to
a 1µA typical supply current. The STATUS
pin goes to a logic LOW when the cable
is disconnected. The typical accumulated
delay is around 20µs. When the SP3221E
driver and internal charge pump are disabled,
the supply current is reduced to 1µA typical.
This can commonly occur in handheld or
portable applications where the RS-232
cable is disconnected or the RS-232 driver
of the connected peripheral are truned 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 truth table logic of the SP3221E driver
and receiver outputs can be found in Table 2.
When the SP3221E 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 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 ON-LINE^®
circuitry so this connection acts like a shut-
down input pin
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com
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SP3221E_200_081816