SP3238EEA datasheet(15/20 Pages) SIPEX

15 / 20 page

SP3238EDS/04

SP3238E +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 1

µA. 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 5k

Ω 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

XINACT if the cable is disconnected or the

external transmitters are disabled. Otherwise,

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

XINACT signals with an accumulated delay

that disables the device to a 1

µA 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 20

µs.

When the drivers or internal charge pump are

disabled, the supply current is reduced to 1

µA.

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

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 200

µs.

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.

SP3238EEA Datasheet(PDF) 15 Page - Sipex Corporation

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