lan8720 Standard Microsystems Corp., lan8720 Datasheet - Page 29

• Manufacturer Part Number: lan8720
• Description: Small Footprint Rmii 10/100 Ethernet Transceiver With Hp Auto-mdix Support
• Manufacturer: Standard Microsystems Corp.

Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i
4.8
accepting 50MHz
REF_CLK
Capable of
MAC
clock
example, only a 25MHz clock can be used (clock cannot be 50MHz). Similar to the 25MHz crystal
mode, the nINT function is disabled.
The RMII REF_CLK is a continuous clock that provides the timing reference for CRS_DV, RXD[1:0],
TXEN, TXD[1:0] and RXER. The LAN8720 uses REF_CLK as the network clock such that no buffering
is required on the transmit data path. However, on the receive data path, the receiver recovers the
clock from the incoming data stream, and the LAN8720 uses elasticity buffering to accommodate for
differences between the recovered clock and the local REF_CLK.
The purpose of the Auto-negotiation function is to automatically configure the transceiver to the
optimum link parameters based on the capabilities of its link partner. Auto-negotiation is a mechanism
for exchanging configuration information between two link-partners and automatically selecting the
highest performance mode of operation supported by both sides. Auto-negotiation is fully defined in
clause 28 of the IEEE 802.3 specification.
Once auto-negotiation has completed, information about the resolved link can be passed back to the
controller via the Serial Management Interface (SMI). The results of the negotiation process are
reflected in the Speed Indication bits in register 31, as well as the Link Partner Ability Register
(Register 5).
The auto-negotiation protocol is a purely physical layer activity and proceeds independently of the MAC
controller.
The advertised capabilities of the transceiver are stored in register 4 of the SMI registers. The default
advertised by the transceiver is determined by user-defined on-chip signal options.
The following blocks are activated during an Auto-negotiation session:
Auto-negotiation
nINT not available in this
configuration
Auto-negotiation (digital)
Interface
RMII
Figure 4.6 LAN8720 Sources REF_CLK from External 25MHz Source
2
2
2
MDIO
MDC
TXD[1:0]
TXEN
RXD[1:0]
CRS_DV
RXER
REFCLKO
LED[2:1]
nRST
DATASHEET
10/100 PHY
LAN8720
24-QFN
29
RMII
XTAL1/CLKIN
XTAL2
RXN
RXP
TXP
TXN
Mag
25MHz
Clock
Revision 1.0 (04-15-09)
RJ45