LXT970ATC datasheet(38/74 Pages) INTEL

38 / 74 page

LXT970A — Dual-Speed Fast Ethernet Transceiver

Datasheet

2.8.1.5

SQE (10T Only)

When the SQE (heartbeat) function is enabled, the LXT970A asserts its COL output for 5-15 BT

after each packet. By default, the SQE function is disabled on the LXT970A. To enable SQE, set

bit 19.10 = 1. See Figure 30 on page 59 for SQE timing parameters.

2.8.1.6

Jabber (10T Only)

If the MAC transmission exceeds the jabber timer, the LXT970A disables the transmit and

loopback functions and asserts the COL pin. The LXT970A automatically exits jabber mode after

250-750 ms. This function can be disabled by setting bit 19.9 = 1. See Figure 31 on page 59 for

Jabber timing parameters.

2.8.2

PMA Layer

The Physical Medium Attachment (PMA) layer provides link and carrier status functions.

2.8.2.1

100TX Link Options

The LXT970A uses standard symbol error rate criteria to establish a link and has two options for

100TX and 100FX applications to break down a link:

• Standard (symbol error rate).

• Enhanced (scrambler lock).

Standard link criteria (17.2 = 0 default), takes down the link when the Bit Error Rate (BER)

exceeds the allowable standards. If the link goes down and auto-negotiation is enabled, the device

automatically restarts the auto-negotiation process. Otherwise the device continues to monitor the

line for a valid link. With this option both link up and link loss are based on symbol error rate.

Enhanced link-loss criteria (17.2 = 1) works independent of symbol error rate. The loss of

scrambler lock for more than 1 - 2 msec brings the link down. Enhanced link-loss criteria is not

available in 5B mode.

2.8.2.2

10T Link Test

For 10T operation, link is detected via Normal Link Pulses (NLPs). In 10T mode, the LXT970A

always transmits link pulses. If the Link Test Function is enabled, it monitors the connection for

link pulses. Once it detects 2 to 7 link pulses, data transmission will be enabled and remains

enabled as long as the link pulses or data transmission continues. Link failure occurs if Link Test is

enabled and link pulses are no longer received. When this condition occurs, the LXT970A returns

to the auto-negotiation phase if auto-negotiation is enabled. If the Link Test function is disabled,

the LXT970A transmits to the connection regardless of detected link pulses. The Link Test

function can be disabled by setting bit 19.8 = 1 or by setting MF0 to disable auto-negotiation and

setting CFG1 input High.

2.8.2.3

Carrier Sense (CRS)

For 100TX and 100FX links, a start-of-stream delimiter or /J/K symbol pair causes assertion of

carrier sense (CRS). An end-of-stream delimiter, or /T/R symbol pair causes deassertion of CRS.

The PMA layer also de-asserts CRS if IDLE symbols are received without /T/R, however, in this

case RX_ER asserts for one clock cycle when CRS is de-asserted.

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