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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i 27 Revision 1.0 (05-28-09)
DATASHEET
4.7.1 REF_CLK In Mode
A 50MHz source of REF_CLK must be available external to the LAN8720 when using this mode. The
clock is driven to both the MAC and PHY as shown in Figure 4.4.
4.7.2 REF_CLK OUT Mode
To reduce BOM cost, the LAN8720 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 3
rd
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 4.5.
The LAN8720 is a small size, low pin count device. In order to optimize package size and cost, the
REFCLKO pin is multiplexed with the nINT pin. Therefore, in this specific mode of operation, the nINT
pin is disabled since REFCLKO is used to provide the 50MHz clock to the MAC.
Figure 4.4 External 50MHz clock sources the REF_CLK
LAN8720
10/100 PHY
24-QFN
RMII
TXP
TXN
Mag RJ45
RXP
RXN
XTAL1/CLKIN
XTAL2
TXD[1:0]
2
RXD[1:0]
CRS_DV
2
RMII
LED[2:1]
2
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