
LXT9860/9880
—
Advanced 10/100 Repeater with Integrated Management
70
Datasheet
Document #: 248987
Revision#: 003
Rev Date: 08/07/01
the current, the configuration value is a
‘
0
’
. The LXT98x0 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
–
500 k
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 27
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. 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 70
).
4.3.6.3
Serial LEDs
The LXT98x0 provides a serial interface to support additional LED options. Standard shift
registers, either 74X595s (8-bit Serial-to-Parallel with Output Registers) or 74X164s (8-bit S/P
without registers) can be used to drive these additional LEDs. Collision10/100 and Activity10/100
status indications are provided on multiplexed configuration pins and duplicated on the serial port.
The LED serial interface consists of three outputs: clock (LEDCLK), parallel latch clock
(LEDLAT), and output data (LEDDAT). The parallel latch clock is used only with the 74X595
implementation. Refer to
Figure 28
for an illustration of the LED serial interface circuit.
Potentially, 30 LEDs can be driven by the LED serial interface via 4 S/P devices. The S/P serial
output is connected to the serial input of the first serial input device. To expand the chain, connect
the last serial output to serial input of next serial interface device.
Serial LED data is output in the anticipated priority order, from least likely to most likely to be
used:
Unused
‘
595/
‘
164 parallel outputs
MII Ports - LED1, 2, 3
Figure 27. LED Circuits - Direct Drive & Multiplexed Configuration Inputs
Inside
IC
Outside
IC
Direct Drive
R
V
LED
Inside
IC
Outside
IC
Configuration
=
‘
1
’
Inside
IC
Outside
IC
Configuration
=
‘
0
’
R
V
LED
R
V
LED
= 3.3 to 5 Volts +/- 5%
Multiplexed
Multiplexed
V
cc
= 3.3 Volts +/- 5%
Inside
IC
Outside
IC
Configuration
=
‘
0
’
V
cc
Multiplexed
with Transistor Buffer
R
V
LED
100k
Rb
V
cc