LAN8720AI datasheet(34/79 Pages) SMSC

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

Revision 1.4 (08-23-12)

SMSC LAN8720A/LAN8720Ai

DATASHEET

3.7.4.1

REF_CLK In Mode

In REF_CLK In Mode, the 50MHz REF_CLK is driven on the XTAL1/CLKIN pin. A 50MHz source for

REF_CLK must be available external to the device when using this mode. The clock is driven to both

the MAC and PHY as shown in Figure 3.7.

3.7.4.2

REF_CLK Out Mode

To reduce BOM cost, the device includes a feature to generate the RMII REF_CLK signal from a low-

cost, 25MHz fundamental crystal. This type of crystal is inexpensive in comparison to 3rd overtone

crystals that would normally be required for 50MHz. The MAC must be capable of operating with an

external clock to take advantage of this feature as shown in Figure 3.8.

In order to optimize package size and cost, the REFCLKO pin is multiplexed with the nINT pin. In

REF_CLK Out mode, the nINT functionality is disabled to accommodate usage of REFCLKO as a

50MHz clock to the MAC.

Note: The REF_CLK Out Mode is not part of the RMII Specification. Timing in this mode is not

compliant with the RMII specification. To ensure proper system operation, a timing analysis of

the MAC and LAN8720 must be performed.

Figure 3.7 External 50MHz clock sources the REF_CLK

LAN8720A/LAN8720Ai

10/100 PHY

24-QFN

RMII

TXP

TXN

Mag

RJ45

RXP

RXN

XTAL1/CLKIN

XTAL2

TXD[1:0]

RXD[1:0]

CRS_DV

RMII

LED[2:1]

Interface

MDIO

MDC

nINT

nRST

TXEN

MAC

Accepts external

50MHz clock

50MHz

Reference

Clock

All RMII signals are

synchronous to the supplied

clock

REF_CLK

RXER

LAN8720AI Datasheet(PDF) 34 Page - SMSC Corporation

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