
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
120
Datasheet
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
4.3.2
Internal Loopback
Register bit 0.14 must be set to enable internal loopback operation. Register bits 16.14 and 0.8
must be set for 10 Mbps operation. Intel recommends that auto-negotiation be disabled while
internal loopback is enabled. The normal auto-negotiation process code word exchange cannot be
completed.The following two-step sequence is recommended for the most efficient mode change
when enabling forced 100 Mbps internal loopback mode directly from auto-negotiation mode:
1. Write Register 0 with 0x2100h (forced 100 Mbps), and
2. Write Register 0 with 0x6100h (enable internal loopback with forced 100 Mbps)
This two-step process ensures the 100 Mbps link comes up quickly. If the one-write process of
writing 0x6100h is followed, it may take up to 1.5 seconds before link is established and data is
received on the port. The 1.5 second delay is due to the IEEE auto-negotiation Break Link Timer
(BLT) requirement. The timer must expire before link is established when changing modes directly
from auto-negotiation to internal loopback forced 100 Mbps mode. Use the above two-step process
to eliminate the auto-negotiation BLT timer requirement.
4.3.3
RMII Data Interface
The LXT9785/LXT9785E provides a separate RMII for each network port, each complying with
the RMII Specification, Revision 1.2. The RMII includes both a data interface and an MDIO
management interface. The RMII Data Interface exchanges data between the LXT9785/LXT9785E
and up to eight Media Access Controllers (MACs).
Table 41. Intel
LXT9785/LXT9785E MII Mode Select
ModeSel1
ModeSel0
RMII
1
0
0
SMII
0
1
SS-SMII
1
0
Reserved
1
1
1. Invalid for the BGA15 package.
Figure 8. Intel
LXT9785/LXT9785E Internal Loopback
Loopback
Digital
Block
Analog
Block
RMII/
SMII/
SS-
SMII
inter
face
Fx
Driver
Tx
Driver
LXT9785/9785E