DP83907 datasheet(17/70 Pages) NSC

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40 Functional Description (Continued)

Receiver and Smart Squelch

The DP83907 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 voltage level op-

tions for the smart squelch One mode 10BASE-T mode

uses levels that meet the 10BASE-T specification The sec-

ond mode reduced squelch mode uses a lower squelch

threshold level and can be used in longer cable applica-

tions where due to attenuation smaller signal levels may be

present The squelch level mode can be selected in the

DP83907 configuration registers

Figure 14 shows the operation of the smart squelch in

10BASE-T mode

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 the figure 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 msec 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 asserted

This signal has to be deasserted for approximately 750 ms

(the unjab time) before the Jabber re-enables the transmit

outputs

TLF12082 – 9

FIGURE 14 Twisted Pair Squelch Waveform

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