5220-DS02-R datasheet(25/90 Pages) BOARDCOM

25 / 90 page

Preliminary Data Sheet

BCM5220

04/20/04

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

Document

5220-DS04-405-R

Reset

Page 13

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

RESET

There are two ways to reset the BCM5220. A hardware reset pin is provided that resets all internal nodes in the chip to a

known state. Reset# pin must be held low when powering the BCM5220 to get this chip into normal mode. If the BCM5220

is not provided a power-on reset, it is possible for the chip to power up in a low power mode. To exit this low power mode,

reset# pin must be held low for at least 20

µs. Hardware reset should always be applied to the BCM5220 after power-up.

The BCM5220 also has a software reset capability. To perform software reset, a 1 must be written to bit 15 of the MII Control

register. 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 the MII Control register reset bit

The BCM5220 clock input can be driven two ways: internal crystal oscillator or external oscillator. To take advantage of the

internal oscillator, attach a 25-MHz crystal between the XTALI and XTALO pins. Connect 27 pF capacitors from each pin to

ground. Alternatively, a 50% duty cycle 25-MHz clock can be directly applied to the XTALI pin. In this case, the XTALO output

must be left unconnected. No capacitors are used.

ISOLATE MODE

When the BCM5220 is put into isolate mode, all MII inputs (TXD[3:0], TXEN, and TXER) are ignored, and all MII outputs

(TXC, COL, CRS, RXC, RXDV, RXER, and RXD[3:0]) are set to high impedance. Only the MII management pins (MDC,

MDIO) operate normally. Upon resetting the chip, the isolate mode is off. Writing a 1 to bit 10 of the MII Control register puts

the transceiver into isolate mode. Writing a 0 to the same bit removes it from isolate mode.

LOOPBACK MODE

Loopback mode allows in-circuit testing of the BCM5220 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. The loopback mode is enabled by writing a 1 to bit 14 of the MII

Control register. To resume normal operation, bit 14 of the MII Control register must be 0.

Incoming packets on the cable are ignored in loopback mode. Because of this, the COL pin is normally not activated during

loopback mode. To test that the COL pin is actually working, the BCM5220 can be placed into collision test mode. This mode

is enabled by writing a 1 to bit 7 of the MII Control register. Asserting TXEN causes the COL output to go high, and

deasserting TXEN causes the COL output to go low.

While in loopback mode, several function bypass modes are also available that 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. All bypass modes can be accessed by writing bits of the Auxiliary Control register (10h).

5220-DS02-R 数据表(PDF) 25 Page - Broadcom Corporation.

5220-DS02-R 数据表(HTML) 25 Page - Broadcom Corporation.

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