DP83905AVQB datasheet(20/80 Pages) NSC

20 / 80 page

40 Functional Description (Continued)

TLF11498 – 11

FIGURE 14 Twisted Pair Squelch Waveform

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 later Finally the signal must exceed the original

squelch level within a further 150 ns to ensure that the input

waveform will not be rejected The checking procedure re-

sults in the loss of typically three bits at the beginning 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 14 are

used

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

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 10 Base-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 by

setting the GDLNK bit of Configuration Register B

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

Transmitter

The transmitter consists of four signals the true and compli-

ment Manchester encoded data (TXOg) and these signals

delayed by 50 ns (TXOdg)

These four signals are resistively combined TXOa with

TXOdb and TXOb with TXOda This is known as digital

pre-emphasis and is required to compensate for the twisted

pair cable which acts like a low pass filter causing greater

attenuation to the 10 MHz (50 ns) pulses of the Manchester

encoded waveform than the 5 MHz (100 ns) pulses

An example of how these siqnals are combined is shown in

the following diagram

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