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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i 53 Revision 1.0 (05-28-09)
DATASHEET
transmit signals an the output of the transformer back to the receiver inputs, and this loopback will
work at both 10 and 100.
5.3.9 Configuration Signals
The hardware configuration signals are sampled during the power-on sequence to determine the
physical address and operating mode.
5.3.9.1 Physical Address Bus - PHYAD[0]
The PHYAD0 bit is driven high or low to give each PHY a unique address. This address is latched into
an internal register at the end of a hardware reset. In a multi-PHY application (such as a repeater),
the controller is able to manage each PHY via the unique address. Each PHY checks each
management data frame for a matching address in the relevant bits. When a match is recognized, the
PHY responds to that particular frame. The PHY address is also used to seed the scrambler. In a multi-
PHY application, this ensures that the scramblers are out of synchronization and disperses the
electromagnetic radiation across the frequency spectrum.
The LAN8720 SMI address may be configured using hardware configuration to either the value 0 or
1. The user can configure the PHY address using Software Configuration if an address greater than 1
is required. The PHY address can be written (after SMI communication at some address is established)
using the 10/100 Special Modes register (bits18.[4:0]).
The PHYAD0 hardware configuration pin is multiplexed with the RXER pin.
The LAN8720 may be configured to disregard the PHY address in SMI access write by setting the
register bit 17.3 (PHYADBP).
5.3.9.2 Mode Bus – MODE[2:0]
The MODE[2:0] bus controls the configuration of the 10/100 digital block. When the nRST pin is
deasserted, the register bit values are loaded according to the MODE[2:0] pins. The 10/100 digital
block is then configured by the register bit values. When a soft reset occurs (bit 0.15) as described in
Table 5.21, the configuration of the 10/100 digital block is controlled by the register bit values, and the
MODE[2:0] pins have no affect.
The LAN8720 mode may be configured using hardware configuration as summarized in Table 5.39.
The user may configure the transceiver mode by writing the SMI registers.
Figure 5.3 Connector Loopback Block Diagram
SMSC
Ethernet Transceiver
10/100
Ethernet
MAC
XFMR
Digital
RXD
TXD
Analog
RX
TX
1
2
3
4
5
6
7
8
RJ45 Loopback Cable.
Created by connecting pin 1 to pin 3
and connecting pin 2 to pin 6.