LXT975 datasheet(22/74 Pages) INTEL

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LXT974/LXT975 — Fast Ethernet 10/100 Quad Transceivers

Datasheet

Transmit Clock

The LXT974/975 is the master clock source for data transmission. The LXT974/975 automatically

sets the speed of TX_CLK to match port conditions. If the port is operating at 100 Mbps, TX_CLK

is set to 25 MHz. If the port is operating at 10 Mbps, TX_CLK is set to 2.5 MHz. The transmit data

and control signals must always be synchronized to TX_CLK by the MAC. The LXT974/975

normally samples these signals on the rising edge of TX_CLK.

However, Advanced TX_CLK Mode is available by setting MII register bit 19.5=1. In this mode,

the LXT974/975 samples the transmit data and control signals on the falling edge of TX_CLK.

When operating under MDIO Control, the user can advance the transmit clock relative to

TXD<3:0> and TX_ER. When Advance TX_CLK Mode is selected, the LXT974/975 clocks TXD

data in on the falling edge of TX_CLK, instead of the rising edge. This mode provides an increase

in timing margins of TXD, relative to TX_CLK. Advance TX_CLK Mode is enabled when bit 19.5

= 1.

Transmit Enable

The MAC must assert TX_EN the same time as the first nibble of preamble, and de-assert TX_EN

after the last bit of the packet.

Receive Data Valid

The LXT974/975 asserts RX_DV when it receives a valid packet. Timing changes depend on line

operating speed:

• For 100TX and 100FX links, RX_DV is asserted from the first nibble of preamble to the last

nibble of the data packet.

• For 10BT links, the entire preamble is truncated. RX_DV is asserted with the first nibble of

the Start of Frame Delimiter (SFD) “5D” and remains asserted until the end of the packet.

Error Signals

Whenever the LXT974/975 receives an errored symbol from the network, it asserts RX_ER and

drives “1110” on the RXD pins.

When the MAC asserts TX_ER, the LXT974/975 drives “H” symbols out on the line.

Figure 6. MII Data Interface

LXT974/975

Media Access

Controller

MAC

RX_CLKn

RXD<3:0>n

RX_DVn

RX_ERn

TX_CLKn

TXD<3:0>n

TX_ENn

TX_ERn

CRSn

COLn

LXT975 Datasheet(PDF) 22 Page - Intel Corporation

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