DP83936AVF datasheet(13/104 Pages) NSC

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30 Functional Description (Continued)

The signal at the start of the 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 the next 150 ns time period to ensure that the

input waveform will not be rejected The checking proce-

dure typically results in the loss of three bits at the begin-

ning of each packet

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

In the reduced squelch mode the operation is identical ex-

cept that the lower squelch levels shown in

Figure 3-2 are

used

Valid data is considered to be present until either squelch

level has not been generated for a time period of more than

150 ns indicating the End of Packet Once good data has

been detected the squelch levels are reduced to minimize

the effect of noise causing premature End of Packet detec-

tion

Collision

A collision is detected by the TPI module when

the receive and transmit channels are simultaneously ac-

tive 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 in-

correctly 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) is generated

which typically consists of 10 cycles of a 10 MHz signal

This 10 MHz signal also called the Heartbeat ensures the

continued functioning of the collision circuitry

Link DetectorGenerator

The link generator is a timer cir-

cuit that generates a link pulse produced by the transmitter

section as defined by the 10BASE-T specification The

100 ns wide pulse is transmitted on the TXOa output every

16 ms in the absence of transmit data

This link pulse is used to check the integrity of the connec-

tion to the remote MAU The link detection circuit checks for

valid pulses that are received from the remote unit If valid

link pulses are not received the link detector will disable the

transmit receive and collision detection functions

The LINKLED 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 by

asserting the LNKDIS input pin

Jabber

The jabber timer monitors the transmitter and dis-

ables 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

asserted This signal has to be deasserted for approximate-

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

transmit outputs

Transmitter

The transmitter consists of four signals the

true and complement Manchester encoded data (TXOg)

and these signals delayed by 50 ns (TXOdg)

These four signals are resistively combined (see Section

82) TXOa with TXOdb and TXOb with TXOda in a con-

figuration referred to as pre-emphasis This digital pre-em-

phasis is required to compensate for the low-pass filter ef-

fects of the twisted pair cable which causes greater attenua-

tion to the 10 MHz (50 ns) pulses of the Manchester encod-

ed waveform than the 5 MHz (100 ns) pulses

TLF12597 – 6

FIGURE 3-3 Typical Summed Transmit Waveform

The signal with pre-emphasis is generated by resistively

combining TXOa and TXOdb

(Figure 3-3) This signal

along with its complement is passed to the transmit filter

Status Information

Status information is provided by the

SONIC-T Controller on the RXLED

TXLED

COLED

LINKLED and POLED outputs as described in the pin de-

scription table These outputs

(Figure 3-4) are suitable for

driving status LEDs

TLF12597 – 7

FIGURE 3-4 Typical SONIC-T LED Connection

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