LAN8742AI-CZ datasheet(34/124 Pages) MICROCHIP

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LAN8742A/LAN8742Ai

DS00001989A-page 34

 2013-2015 Microchip Technology Inc.

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, 25 MHz fun-

damental crystal. This type of crystal is inexpensive in comparison to 3rd overtone crystals that would normally be

required for 50 MHz. 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 50 MHz clock to the MAC.

Note:

The REF_CLK Out Mode is not part of the RMII Specification. To ensure proper system operation, a timing

analysis of the MAC and LAN8742A/LAN8742Ai must be performed.

Note:

In REF_CLK Out Mode, REFCLKO will not output when the device is in Energy Detect Power-Down Mode

or General Power-Down Mode.

FIGURE 3-8:

SOURCING REF_CLK FROM A 25 MHZ CRYSTAL

LAN8742A/LAN8742Ai

10/100 PHY

24-VQFN

RMII

TXP

TXN

Mag

RJ45

RXP

RXN

25 MHz

XTAL1/CLKIN

XTAL2

LED[2:1]

Interface

nRST

TXD[1:0]

RXD[1:0]

CRS_DV

RMII

MDIO

MDC

TXEN

REFCLKO

MAC

Capable of

accepting 50 MHz

clock

Note: nINT not available in

this configuration

RXER

REF_CLK

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