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Advanced 10/100 Unmanaged Repeater — LXT9883/9863

Datasheet

Document #: 249115

Revision #: 003

Rev. Date: 08/07/01

4.2.9

LED Circuits

4.2.9.1

Direct Drive LEDs

Each Direct Drive LED has a corresponding open-drain pin. The LEDs are connected, via a current

limiting resistor, to a positive voltage rail. The LEDs are turned on when the output pin drives Low.

The open-drain LED pins are 5V tolerant, allowing use of either a 3.3V or 5V rail. A 5V rail eases

LED component selection by allowing more common, high forward voltage LEDs to be used.

Refer to Figure 13 for a circuit illustration.

4.2.9.2

LED Pins Multiplexed with Configuration Inputs

Some static configuration inputs are multiplexed with LED pins to reduce the LXT98x3 pin count.

These LED pins are configured by current sinking (open- drain output) and sourcing (open-source

output). If the LED pin sinks the LED current, the configuration value is ‘1’. If LED pin sources

the current, the configuration value is a ‘0’. The LXT98x3 detects the configuration value

following reset and then selects the appropriate output drive circuit (open drain or source). If the

LED function of a multiplexed configuration pin is not used, tie the pin to Ground or Vcc via a

100–500k

Ω resistor to set the configuration value. Multiple LED configuration pins can be tied off

with a single resistor to set them all to the same value. Refer to Figure 13 for a circuit illustration.

For configuration values of ‘1’, a 3.3V or a 5V rail can be used to drive the LEDs (to ease LED

selection as with Direct Drive LEDs).

For configuration values of ‘0’, external buffering is used when 5V LED driving is desired. (This

buffering could be as simple as a single transistor.) As an alternative, use the copies of the

multiplexed LED data found on the LED serial interface. 5V LED driving is achieved. Also, if a

5V tolerant serial-to-parallel device is used for the LED serial interface, 5V LED driving is

achieved (see "Serial LEDs” on page 40.).

Figure 13. LED Circuits - Direct Drive &

Multiplexed Configuration Inputs

Inside

Outside

Direct Drive

VLED

Inside

Outside

Configuration

= ‘1’

Inside

Outside

Configuration

= ‘0’

VLED

VLED = 3.3 to 5 Volts +/- 5%

Multiplexed

Vcc = 3.3 Volts +/- 5%

Inside

Outside

Configuration

= ‘0’

Vcc

Multiplexed

with Transistor Buffer

VLED

100k

Ω

Vcc

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