
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
116
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
4.0
Functional Description
4.1
Introduction
The Intel
 LXT9785/LXT9785E is an 8-port Fast Ethernet 10/100 PHY transceiver that supports 
10 Mbps and 100 Mbps networks, complying with all applicable requirements of IEEE 802.3 
standards. The device incorporates a Serial Media Independent Interface (SMII), Source 
Synchronous-Serial Media Independent Interface (SS-SMII), and a Reduced Serial Independent 
Interface (RMII) to enable each individual network port to interface with multiple 10/100 MACs. 
Each port directly drives either a 100BASE-TX line or a 10BASE-T line. The LXT9785/9785E 
also supports 100BASE-FX operation via an LVPECL interface. The device has a 241-ball BGA, a 
208-pin QFP, or a 196-ball BGA package.
The 196-ball BGA package (BGA15) is a reduced feature-set product. The BGA15 package does 
not support the following features:
 RMII
 Fiber
 Sectionalization
 Third LED port (only two LEDs per port)
 Hardware control pins:
— PAUSE
— MDIX
— MDDIS
— PWRDWN
— Lower three PHY address (out of five PHY address bits)
 Extended temperature
Note:
Unless otherwise noted, all information in this document applies to the LXT9785 and LXT9785E.
4.1.1
OSP Architecture
The Intel LXT9785/LXT9785E incorporates high-efficiency Optimal Signal Processing design 
techniques, combining the best properties of digital and analog signal processing to produce a truly 
optimal device. 
The receiver utilizes decision feedback equalization to increase noise and cross-talk immunity by 
as much as 3 dB over an ideal all-analog equalizer. Using OSP mixed-signal processing techniques 
in the receive equalizer avoids the quantization noise and calculation truncation errors found in 
traditional DSP-based receivers (typically complex DSP engines with A/D converters). The result 
is improved receiver noise and cross-talk performance.
The OSP architecture also requires substantially less computational logic than traditional DSP-
based designs. The result is lower power consumption and reduced logic switching noise generated 
by DSP engines clocked at speeds up to 125 MHz. The logic switching noise can be a considerable 
source of EMI when generated from the device’s power supplies.