SP3223H Datasheet, PDF (17/23 Pages) Sipex Corporation – High Speed Intelligent +3.0V to +5.5V RS-232 Transceivers

Auto-Online Circuitry
The SP3223H and SP3243H devices have 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 SP3223H and SP3243H devices
incorporate 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 23, 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-Online circuitry,
shown in Figure 24, 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 SP3223H and SP3243H 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
SP3223H and SP3243H 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 SP3223H and SP3243H devices
are shut down, 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-Online circuitry so this
connection acts like a shutdown input pin.
Rev. 6/30/03
SP3223H +3.0V to +5.5V RS-232 Transceivers
17
© Copyright 2003 Sipex Corporation