
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
121
4.3.4
Serial Media Independent Interface (SMII) and Source Synchronous- 
Serial Media Independent Interface (SS-SMII)
4.3.4.1
SMII Interface
The LXT9785/LXT9785E provides an independent serial interface for each network port, 
complying with the Serial-MII Specification, Revision 1.2. All SMII ports use a common reference 
clock and SYNC signal. The SMII Data Interface exchanges data between the LXT9785/
LXT9785E and multiple Media Access Controllers (MACs). All signals are synchronous to the 
reference clock. One SYNC control stream is sourced by the MAC to the PHY. Both the transmit 
and receive data streams are segmented into boundaries delimited by the SYNC pulses. This 
interface is expected to drive up to 6 inches of trace lengths.
4.3.4.2
Source Synchronous-
Serial Media Independent Interface
The new revision to the SMII interface, SS-SMII, allows for a longer trace length and helps to 
relieve timing constraints, requiring the addition of four new signals, TxCLK, TxSYNC, RxCLK, 
and RxSYNC. The transmit TxCLK and TxSYNC are sourced from the MAC to the PHY and 
referenced to the REFCLK input. The receive RxCLK and RxSYNC are sourced by the PHY to the 
MAC and in reference to the REFCLK.
4.3.5
Configuration Management Interface
The LXT9785/LXT9785E provides an MDIO Management Interface and a Hardware Control 
Interface (via the CFG pins) for device configuration and management. Mode control selection is 
provided via the MDDIS pin as shown in 
Table 9, “Intel LXT9785/LXT9785E MDIO Control 
Interface Signals – PQFP” on page 41
. When sectionalization (2x4) is selected, separate MDIO 
interfaces are enabled (see 
Figure 13 on page 127
). 
4.3.6
MII Isolate
In applications where the MII must be isolated from the bus, the RMII and the SMII/SS-SMII 
configurations can be three-stated using Register 0.10. On each individual port, Register bit 0.10 
controls the isolation of the transmit and receive data signals for that port. Register bit 0.10 on ports 
0 and 4 isolate the RxCLK
n
/TxCLK
n
 and SYNC signals.
When 1x8 sectionalization is selected, TxCLK0, TxSYNC0, RxCLK1, and RxSYNC1 are used for 
the clocking and synchronization interface. Port 4 controls the isolation of RxCLK0, RxCLK1, 
RxSYNC0, and RxSYNC1, and must be used to isolate the receive clock and synchronization 
interface. 
When 2x4 sectionalization is selected, TxCLK0, TxSNC0, RxCLK0, and TxCLK0 are used for 
Port 0 through Port 3 and TxCLK1, TxSYNC1, RxCLK1, and RxSYNC1 are used for Port 4 
through Port 7. Port 0 must be isolated to isolate the receive clock and synchronization interface for 
Port 0 through Port 3. Port 4 must be isolated to isolate Port 4 through Port 7.
4.3.7
MDIO Management Interface
The LXT9785/LXT9785E supports the IEEE 802.3 MII Management Interface, also known as the 
Management Data Input/Output (MDIO) Interface. This interface allows upper-layer devices to 
monitor and control the state of the LXT9785/LXT9785E. The MDIO interface consists of a