
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
125
4.4
Operating Requirements
4.4.1
Power Requirements
The LXT9785/LXT9785E requires four power supply inputs: VCCD, VCCA, VCCPECL and 
VCCIO. The digital and analog circuits require 2.5 V supplies (VCCD, VCCR, and VCCT). These 
inputs may be supplied from a single source although decoupling is required to each respective 
ground. The fiber VCCPECL supply can be connected to either 2.5 V or 3.3 V.
A separate power supply may be used for the MII, JTAG and MDIO (VCCIO) interfaces.   The 
power supply may be either +2.5 V or +3.3 V. VCCIO should be supplied from the same power 
source used to supply the controller on the other side of the interface. Refer to 
Table 53, “Intel 
LXT9785/LXT9785E Digital I/O DC Electrical Characteristics (VCCIO = 2.5 V +/- 5%)” on 
page 174, Table 54, “Intel LXT9785/LXT9785E Digital I/O DC Electrical Characteristics 
(VCCIO = 3.3 V +/- 5%)” on page 175, 
and 
Table 55, “Intel LXT9785/LXT9785E Digital I/O 
DC Electrical Characteristics – SD Pins” on page 175
 for I/O characteristics.
As a matter of good practice, these supplies should be as clean as possible. Typical filtering and 
decoupling are shown in 
Figure 34 on page 168
. The power supplies should be brought up as close 
to the same time as possible. However, there are no specific timing requirements.
4.4.2
Clock/SYNC Requirements
4.4.2.1
Reference Clock
The LXT9785/LXT9785E requires a constant enabled reference clock (REFCLK). REFCLK’s 
frequency must be 50 MHz for RMII or 125 MHz for SMII/SS-SMII. The reference clock is used 
to generate transmit signals and recover receive signals. A crystal-based clock is recommended 
over a derived clock (that is, PLL-based) to minimize transmit jitter. Refer to 
Table 56, “Intel 
LXT9785/LXT9785E Required Clock Characteristics” on page 175
 for clock timing requirements. 
For applications that use a single 8-port sectionalization, REFCLK0 and REFCLK1 must always 
be tied together and to the source. In 2x4 applications, REFCLK0 and REFCLK1 are not tied 
together.
4.4.2.2
TxCLK Signal (SS-SMII only)
The LXT9785/LXT9785E requires a 125 MHz input transmit clock synchronous with TxData
n 
and frequency locked to REFCLK
. 
See
Figure 22 on page 141.
4.4.2.3
TxSYNC Signal (SMII/SS-SMII)
The LXT9785/LXT9785E requires a 12.5 MHz input pulse for SMII synchronization. See 
Figure 22 on page 141.
4.4.2.4
RxSYNC Signal (SS-SMII only)
The LXT9785/LXT9785E provides a 12.5 MHz output pulse synchronous with the RxData
n
outputs. See 
Figure 23 on page 141.