5228-DS06-R datasheet(37/100 Pages) BOARDCOM

37 / 100 page

Data Sheet

BCM5228

7/12/04

Broa d c om Co rp o r a t i o n

Document

5228-DS09-405-R

Operational Description

Page 25

Se cti on 4: Op er a t ion a l D e s c r i p ti on

RESETTING THE BCM5228

There are two ways to reset each transceiver in the BCM5228. A hardware reset pin has been provided which resets all

internal nodes inside the chip to a known state. The reset pulse must be asserted for at least 2 µs. Hardware reset should

always be applied to a BCM5228 after power-up and after all clocks to the chip are running.

Each transceiver in the BCM5228 also has an individual software reset capability. To perform software reset, a 1 must be

written to bit 15 of the MII Control register 0 (see “MII Register Map Summary” on page 37) of the transceiver. This bit is self-

clearing, meaning that a second write operation is not necessary to end the reset. There is no effect if a 0 is written to this bit.

LOOPBACK MODE

The loopback mode allows in-circuit testing of the BCM5228 chip. All packets sent in through the TXD pins are looped-back

internally to the RXD pins, and are not sent out to the cable. Incoming packets on the cable are ignored.

The loopback mode can be entered by writing a 1 to bit 14 of the MII Control register or by writing a 1 to bit 8 and bit 7 of

shadow register 1Dh. In order to resume normal operation the bits must be 0.

Several function bypass modes are also supported which can provide a number of different combinations of feedback paths

during loopback testing. These bypass modes include: bypass scrambler, bypass MLT3 encoder and bypass 4B5B encoder.

FULL-DUPLEX MODE

The BCM5228 supports full-duplex operation. While in full-duplex mode, a transceiver can simultaneously transmit and

receive packets on the cable. By default, each transceiver in the BCM5228 powers up in half-duplex mode.

When auto-negotiation is disabled, full-duplex operation can be enabled either by a pin (FDXEN) or by an MII register bit

(register 0 bit 8).

When auto-negotiation is enabled in DTE mode, full-duplex capability is advertised by default but can be overridden by a

write to the auto-negotiation Advertisement register (04h).

100BASE-FX MODE

Any of the BCM5228F transceivers can interface with an external 100BASE-FX fiber-optic driver and receiver instead of the

magnetics module used with twisted-pair cable. The differential transmit and receive data pairs operate at PECL voltage

levels instead of those required for twisted-pair transmission, if the termination scheme recommended in the application note

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