ML4658CP datasheet(13/17 Pages) MICRO-LINEAR

13 / 17 page

ML4658

When data is received by the chip while transmitting, a

collision condition exits. This will cause the collision

oscillator to turn on within 9 bit times. The data on the DI

AUI pair (Rx+, Rx–) changes from Tx+, Tx– to RxTP+,

RxTP–, when entering the collision state. During a

collision, if the receive data (RxTP+, RxTP–) drops out

before the transmit data (Tx+, Tx–), Rx+, Rx– will switch

back to Tx+, Tx–.

SQE TEST FUNCTION (SIGNAL QUALITY ERROR)

The SQE test function allows the DTE to determine

whether the collision detect circuitry is functional. After

each transmission, during the inter-packet gap time, the

collision oscillator will be activated for typically 1µs. The

SQE test will not be activated if the chip is in the link fail

state, or the Jabber state.

For SQE to operate, the SQEN pin must be tied to VCC or

BIAS. The SQE test can be disabled by tying the SQEN

pin to 1.2V or ground. This allows the chip to be

interfaced to a repeater.

JABBER FUNCTION

The Jabber function prevents a babbling transmitter from

bringing down the network. Within the transceiver is a

Jabber timer that starts at the beginning of each

transmission and resets at the end of each transmission. If

the transmission lasts longer than 20ms the jabber logic

disables the transmitter, and turns on the collision

oscillator COL+, COL–. When Tx+ and Tx– finally go

idle, a second timer measures 0.5 seconds of idle on Tx+

and Tx– before re-enabling the transmitter and turning off

the collision oscillator. If transmission starts up again

before 0.5 seconds has expired, the timer is reset and

measures another 0.5 seconds of idle time.

Even though the transmitter is disabled during jabber, Link

Pulses are still transmitted if the, Link Test is enabled.

Jabber can be disabled by placing 1.2V on the SQEN/

LTD/JABD pin. This is useful for measuring jitter

performance on the transmitter.

LINK TEST FUNCTION

Transmission — Whenever data is not being delivered to

the twisted pair link, the idle signal is applied. The idle

signal is a sequence of Link Pulses separated by a 16ms

period of silence. The idle signal starts with a period of

silence after a packet transmission ends. The link test

pulse is a single high pulse with the same amplitude

requirements as the data signal.

Reception — The transceiver monitors the receive twisted

pair input for packet and link pulse activity. If neither a

packet nor a link test pulse is received for 50 to 150ms,

the transceiver enters the Link Test Fail state and inhibits

transmission and reception. Link pulses received with the

wrong polarity will be ignored and cause the chip to go

into link test fail.

A DTE can determine that the transceiver is in Link Test

Fail one of two ways: it can monitor the LTF pin if the

transceiver is internal, or it can monitor loopback. If the

MAU is on-board the LTF pin can be sampled to

determine that the transceiver is in the link fail state. If

the MAU is external the DTE can monitor carrier sense

during transmission. A loss of carrier sense is an indication

of Link Test Fail State, since in Link Test Fail, loopback is

disabled. Note that jabber also disables loopback but with

Jabber the collision signal will be on.

When a packet, or two consecutive link test pulses is

received from the twisted pair input, the transceiver will

exit the Link Test Fail state upon transmit and receive

data being idle, and re-enable transmission and reception.

Link test pulses that do not occur within at most 25 to

150ms of each other are not considered consecutive.

In addition, detected pulses that occur within a time

between 2 to 7ms of a previous pulse will be considered

as noise by the link test circuitry.

POLARLITY CIRCUITRY

The ML4658 offers automatic polarity correction. The

POLRD pin is used to report the status of the receive pair

polarity. This pin reflects the true status of the receive

polarity regardless of whether the part has autopolarity

correction or not.

Automatic Polarity Correction — ML4658 — In the link

OK state, receive polarity is updated when two

consecutive frames are received with the same Start of

Idle polarity. In the Link Test Fail state the part will use

either the Start of Idle signal or link pulses to correct the

receive polarity.

In the case where the part is powered up with the receive

polarity reversed and no frames are received, the part will

go into Link Test Fail. After two link pulses are received

with the same polarity, the part will exit Link Test Fail and

correct the receive polarity. The POLRD pin will continue

to reflect the true polarity of the receive pair.

LED DRIVERS

The ML4658 has six LED drivers for transmit, receive,

collision, Link Test Fail, reverse polarity, and jabber. The

LEDs are normally off except for LTF which is normally on

and active high. The LEDs are tied to their respective pins

through a 510

Ω resistor to 5 Volts.

The XMT, RCV and CLSN pins have pulse stretchers on

them which enables the LEDs to be visible. When

transmission or reception occurs, the LED XMT, RCV or

CLSN status pins will activate low for 100ms. If another

transmit, receive or collision condition occurs during the

first 100ms, the LED timer will reset and begin timing

again for 100ms. The LEDs will remain on for consecutive

frames. The JAB, POLRD, and LTF LEDs do not have pulse

stretchers on them since their conditions occur long

enough for the eye to see.

SYSTEM DESCRIPTION (Continued)

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