ADM7008 datasheet(39/92 Pages) ETC1

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ADM7008

Function Description

3.1.9 Transmit Driver and Receiver

The ADM7008 integrates all the required signal conditioning functions in its 10Base-T

block such that external filters are not required. Only one isolation transformer and

impedance matching resistors are needed for the 10Base-T transmit and receive interface.

The internal transmit filtering ensures that all the harmonics in the transmission signal are

attenuated properly.

3.1.10 Smart Squelch

The smart squelch circuit is responsible for determining when valid data is present on the

differential receive. The ADM7008 implements an intelligent receive squelch on the

RXP/RXN 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 (as specified in the IEEE 802.3 10Base-T standard) to

determine the validity of data on the twisted-pair inputs.

The signal at the start of the packet is checked by the analog squelch circuit 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 150ns. Finally, the signal must

exceed the original squelch level within an additional 150ns to ensure that the input

waveform will not be rejected.

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.

Valid data is considered to be present until the squelch level has not been generated for a

time longer than 200 ns, indicating 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 detection. The receive squelch threshold level can be lowered for use in longer

cable applications. This is achieved by setting bit 7 of register address 10h.

3.1.11 Carrier Sense

Carrier Sense (CRS) is asserted due to receive activity once valid data is detected via the

smart squelch function. For 10 Mbps half duplex operation, CRS is asserted during either

packet transmission or reception. For 10 Mbps full duplex and repeater mode operations,

the CRS is asserted only due to receive activity.85

ADMtek Inc.

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