DP83902A datasheet(9/70 Pages) NSC

9 / 70 page

40 Functional Description (Refer toFigure 1)

TWISTED PAIR INTERFACE (TPI) MODULE

The TPI consists of five main logical functions

a) The Smart Squelch responsible for determining when

valid data is present on the differential receive inputs

(RXIg)

b) The Collision function checks for simultaneous transmis-

sion and reception of data on the TXOg and RXIg pins

c) The Link DetectorGenerator checks the integrity of the

cable connecting the two twisted pair MAUs

d) The Jabber disables the transmitter if it attempts to trans-

mit a longer than legal packet

e) The Tx Driver

Pre-emphasis transmits Manchester en-

coded data to the twisted pair network via the summing

resistors and transformerfilter

SMART SQUELCH

The ST-NIC implements an intelligent receive squelch on

the RXIg differential inputs to ensure that impulse noise on

the receive inputs will not be mistaken for a valid signal

The squelch circuitry employs a combination of amplitude

and timing measurements to determine the validity of data

on the twisted pair inputs There are two squelch levels

which are selectable via the SQSEL pin One mode is

10BASE-T compatible and the second is reduced squelch

mode

The diagram shows the 10BASE-T mode operation of the

smart squelch

The signal at the start of packet is checked by the smart

squelch and any pulses not exceeding the squelch level

(either positive or negative depending upon polarity) will be

rejected Once this first squelch level is overcome correctly

the opposite squelch level must then be exceeded within

150 ns Finally the signal must exceed the original squelch

level within a further 150 ns to ensure that the input wave-

form will not be rejected The checking procedure results in

the loss of typically three bits at the beginning of each pack-

Only after all these conditions have been satisfied will a

control signal be generated to indicate to the remainder of

the circuitry that valid data is present At this time the smart

squelch circuitry is reset

Valid data is considered to be present until either squelch

level has not been generated for a time longer than 150 ns

indicating End of Packet Once good data has been detect-

ed the squelch levels are reduced to minimize the effect of

noise causing premature End of Packet detection

The reduced squelch mode functions the same as the

10BASE-T mode except that only the lower level is used for

both turn-on and turn-off

COLLISION

A collision is detected by the TPI module when the receive

and transmit channels are active simultaneously If the TPI

is receiving when a collision is detected it is reported to the

controller immediately If however the TPI is transmitting

when a collision is detected the collision is not reported until

seven bits have been received while in the collision state

This prevents a collision being reported incorrectly due to

noise on the network The signal to the controller remains

for the duration of the collision

Approximately 1 ms after the transmission of each packet a

signal called the Signal Quality Error (SQE) consisting of

typically 10 cycles of 10 MHz is generated This 10 MHz

signal also called the Heartbeat ensures the continued

functioning of the collision circuitry

LINK DETECTORGENERATOR

The link generator is a timer circuit that generates a link

pulse as defined by the 10BASE-T specification that will be

generated by the transmitter section The pulse which is

100 ns wide is transmitted on the TXOa output every

16 ms in the absence of transmit data

The pulse is used to check the integrity of the connection to

the remote MAU The link detection circuit checks for valid

pulses from the remote MAU and if valid link pulses are not

received the link detector will disable the transmit receive

and collision detection functions

The GDLNK output can directly drive a LED to show that

there is a good twisted pair link For normal conditions the

LED will be on The link integrity function can be disabled as

described in the Pin Description Section

JABBER

The jabber timer monitors the transmitter and disables the

transmission if the transmitter is active for greater than

26 ms The transmitter is then disabled for the whole time

that the ENDEC module’s internal transmit enable is assert-

ed This signal has to be deasserted for approximately

750 ms (the unjab time) before the Jabber re-enables the

transmit outputs

TRANSMIT DRIVER

The transmitter consists of four signals the true and com-

plement Manchester encoded data (TXOg) and these sig-

nals delayed by 50 ns (TXOdg)

TLF11157 – 5

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