
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
124
Datasheet
Document Number: 249241
Revision Number: 010
Revision Date: 30-May-2006
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
As a matter of good practice, these supplies should be as clean as possible. Typical filtering and
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, “RequiredFor 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 TxDatan
4.4.2.3
TxSYNC Signal (SMII/SS-SMII)
The LXT9785/LXT9785E requires a 12.5 MHz input pulse for SMII synchronization. See
4.4.2.4
RxSYNC Signal (SS-SMII only)
The LXT9785/LXT9785E provides a 12.5 MHz output pulse synchronous with the RxDatan