19

ML2652/ML2653

2–7ms apart in the link fail state, the device ignores the link

pulses and resets the number of consecutive link pulses to

zero. After the link pulse fail state is exited, transmission and

reception can be resumed.

Link pulse status is indicated by the LTP pin. LTP is a dual

function input/output pin that acts both as an active low link

test pass output and a link test disable input. The pin consists

of an open drain output with an internal pull-up resistor. If

the pin is tied to GND, the pin acts as an input and the link

test function is disabled. If the pin is not tied to GND, the pin

acts as an active low link test pass output and can drive an

LED from VCC or another digital output. Thus, the LED is lit

when the link test is passing.

JABBER

The transmit section contains a jabber detect circuit.

Jabber is a fault condition characterized by a babbling

transmitter. The ML2652 and ML2653 detect jabber when

a transmission packet exceeds 20–150ms in length. If

jabber detect occurs, the transmit output is disabled, the

collision signal COL is sent over the controller interface,

and the JAB pin is pulled low. The device remains in the

jabber detect state until there is at least 250–750ms of

continuous non-transmission. Note that link pulses

continue to be transmitted even when the device is in the

jabber condition.

The jabber detection circuitry can be disabled (only on the

ML2652) with the JABDIS pin for testing and diagnostic

purposes. Disabling jabber means that a jabber condition

is never recognized, even when it occurs. JABDIS is an

active high jabber disable input and has an internal pull-

down resistor to GND.

COLLISION

Collision occurs whenever the DTE card is transmitting

and receiving data simultaneously. However, the collision

circuit on the ML2652 operates differently depending on

whether twisted pair interface or AUI is being used.

When the twisted pair interface is used, collision occurs

whenever the device is transmitting and receiving data

simultaneously, that is when both RxE and TxE are active.

The collision state is indicated by COL and CLS pins. COL

is used to signal collision to the controller. CLS is an active

low open drain output. CLS is activated during Jabber, but

not during SQE test while COL is activated during both.

When the AUI is used (ML2652 only), collision is no

longer detected from simultaneous transmission and

reception, but the collision state is determined when a

collision signal is present on the AUI collision inputs, CI+

and CI–. A 10 MHz square wave has to be applied to this

input in order for the device to signal the collision state on

COL and CLS.

The CLS pin is an output that indicates collision activity.

The pin consists of an open drain output with an internal

pull-up resistor and can drive an LED from VCC or another

digital input. In order to make an LED visible, CLS has an

internal blinker circuit that generates a 100ms blink (50ms

high, 50ms low) that is triggered when a collision starts. At

the completion of the 100ms blink period, if collision is in

progress, another 100ms blink is generated.

SQE TEST

When the twisted pair interface is used, the device tests

the collision circuitry at the end of each transmission by

sending a 1µs collision pulse over the COL pin. This is

known as SQE (signal quality error) test and is shown in

the transmit timing diagram in Figure 1. The SQE test is

disabled if the device is in jabber detect state or link pulse

fail condition.

When AUI is used (ML2652), the SQE test pulse is

generated by an external MAU and the external MAU

sends the SQE test pulse to the ML2652 via the collision

inputs , CI+ and CI–. The ML2652 then relays the collision

signal to the controller via the COL and CLS output pins.

RECEIVE POLARITY DETECT AND AUTO CORRECTION

The ML2652 and ML2653 contain an auto-polarity circuit

that detects the polarity of the receive twisted pair leads,

Rx+ and RX-and internally reverses the leads if their

polarity is incorrect.

When the device is powered up, it is assumed that the

polarity is correct and no polarity correction occurs. Then

receive polarity is continuously monitored by checking the

polarity of the SOI and link pulses since they are always

positive pulses. If either 2 consecutive SOI or 4

consecutive link pulses have incorrect RX± polarity, then

the auto-polarity circuit internally reverses the Rx+ and

Rx– connections.

AUI (APPLIES ONLY TO ML2652)

The ML2652 can be used with an external MAU via the

Attachment Unit Interface (AUI). When the AUI is used,

the internal MAU functions and twisted pair interface are

disabled, and the device only uses the manchester

encoder and decoder functions, as shown in the block

diagram. The AUI consists of three differential signal pairs:

DI, DO, and CI. The function of each pair is described

below.

The DO+ and DO– are differential outputs to the external

MAU which contain the transmit data output from the

Manchester encoder. The DO+ and DO– output drivers

are capable of driving 50 meters of 78 ohm cable with less

than 5ns rise and fall time and less than ±0.5ns of jitter. In

addition, at the end of transmission, the AUI output driver

inserts a 200ns minimum pulse and meets the turnoff and

idle characteristics specified in IEEE 802.3–1988. An

external 78 ohm resistor across DO+ and DO– is required

as shown in Figure 12 to develop the proper output levels

from the internal current sources. The DO+ and DO–