CS8900A datasheet(36/128 Pages) CIRRUS

36 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

3.11.2 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 14 diagrams

the operation of the Link Pulse Generator.

If no link pulses are being received on the receiver,

the 10BASE-T transmitter is internally forced to an

inactive state unless bit DisableLT in register 19

(Test Control register) is set to one.

3.11.3 Receiver

The 10BASE-T receive section consists of the re-

ceive filter, squelch circuit, polarity detection and

correction circuit, and link pulse detector.

3.11.3.1 Squelch Circuit

The 10BASE-T squelch circuit determines when

valid data is present on the RXD+/RXD- pair. In-

coming signals passing through the receive filter

are tested by the squelch circuit. Any signal with

amplitude less than the squelch threshold (either

positive or negative, depending on polarity) is re-

jected.

3.11.3.2 Extended Range

The CS8900A supports an Extended Range feature

that reduces the 10BASE-T receive squelch thresh-

old by approximately 6 dB. This allows the

CS8900A to operate with 10BASE-T cables that

are longer than 100 meters (100 meters is the max-

imum length specified by the Ethernet standard).

The exact additional distance depends on the qual-

ity of the cable and the amount of electromagnetic

noise in the surrounding environment. To activate

this feature, the host must set the LoRxSquelch bit

(Register 13, LineCTL, Bit E).

3.11.4 Link Pulse Detection

To prevent disruption of network operation due to

a faulty link segment, the CS8900A continually

monitors the 10BASE-T receive pair (RXD+/

RXD-) for packets and link pulses. After each

packet or link pulse is received, an internal Link-

Loss timer is started. As long as a packet or link

pulse is received before the Link-Loss timer finish-

es (between 25 and 150 ms), the CS8900A main-

tains normal operation. If no receive activity is

detected, the CS8900A disables packet transmis-

sion to prevent "blind" transmissions onto the net-

work (link pulses are still sent while packet

transmission is disabled). To reactivate transmis-

sion, the receiver must detect a single packet (the

packet itself is ignored), or two link pulses separat-

Time

Link

Pulse

Link

Pulse

16ms

Less Than

16ms

Packet

Figure 14. Link Pulse Transmission

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