CS89712 datasheet(53/170 Pages) CIRRUS

53 / 170 page

CS89712

DS502PP2

inating

the

need

for

external

filters

filter/transformer hybrid. On-chip filters are gm/c

implementations of fifth-order Butterworth low-

pass filters. Internal tuning circuits keep the gm/c

ratio tightly controlled, even when large tempera-

ture, supply, and IC process variations occur. The

nominal 3 dB cutoff frequency of the filters is

16 MHz, and the nominal attenuation at 30 MHz

(3rd harmonic) is -27 dB.

2.28.2

Transmitter

When

configured

for

10BASE-T

operation,

Manchester encoded data from the ENDEC is fed

into the transmitter’s predistortion circuit where

initial wave shaping and preequalization is per-

formed. The output of the predistortion circuit is

fed into the transmit filter where final wave shaping

occurs and unwanted noise is removed. The signal

then passes to the differential driver where it is am-

plified and driven out of the TXD+/TXD- pins.

In the absence of transmit packets, the transmitter

generates link pulses in accordance with section

14.2.1.1. of the Ethernet standard. Transmitted link

pulses are positive pulses, one bit time wide, typi-

cally generated at a rate of one every 16 ms. The

16 ms timer starts whenever the transmitter com-

pletes an End-of-Frame (EOF) sequence. Thus,

there is a link pulse 16 ms after an EOF unless there

is another transmitted packet. Figure 18 diagrams

the operation of the Link Pulse Generator.

RXSQL

Link Pulse

Detector

TX Pre-

Distortion

RX Squelch

Comparator

TX Filters

Filter Tuning

RX Filters

TX Drivers

RXD-

RXD+

TXD-

TXD+

ENDEC

LinkOK

(to MAC)

10BASE-T Transceiver

Figure 17. 10BASE-T Transceiver

Time

Link

Pulse

Link

Pulse

16ms

Less Than

16ms

Packet

Figure 18. Link Pulse Transmission

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