80221 datasheet(51/91 Pages) LSI

51 / 91 page

80220/80221

4-51

MD400159/E

mission is inhibited. The default value of this bit when the

device powers up or is reset is dependent on the physical

device address. If the device address latched into MDA[4:0]

at reset is 11111, it is assumed that the device is being

used in applications where there maybe more than one

device sharing the MII bus, like external PHY's or adapter

cards, so the device powers up with the MII interface

disabled. If the device address latched into MDA[4:0] at

reset is not 11111, it is assumed that the device is being

used in application where it is the only device on the MII

bus, like hubs, so the device powers up with the MII

interface enabled.

5.7.5 Receive Output Enable

The receive output enable pin, RX_EN, forces the receive

and collision MII/FBI outputs into the high impedance

state.

More specifically, when RX_EN is deasserted,

RX_CLK, RXD[3:0], RX_DV, RX_ER, and COL are placed

in high impedance.

RX_EN can be used to "wire OR" the outputs of many

80220/80221 devices in multiport applications where only

one device may be receiving at a time, like a repeater. By

monitoring CRS from each individual port, the repeater

can assert only the one RX_EN to that 80220/80221

device which is receiving data. The method will reduce, by

8 per device, the number of pins and PCB traces required

by a repeater core IC.

The RX_EN function can be enabled by appropriately

setting the R/J Configuration select bit in the MI serial port

Configuration 2 register. When this bit is set, the RX_EN/

JAM pin becomes RX_EN.

5.8 FBI CONTROLLER INTERFACE

The FBI (Five Bit Interface) controller interface has the

same characteristics of the MII except that the data path is

five bits wide, instead of 4 bits wide per the MII. The five

bit wide data path is automatically enabled when the 4B5B

encoder is bypassed. Because of this encoder/decoder

bypass, the FBI is used primarily for repeaters or other

applications where the full PHY is not needed. For more

details about the FBI, see the Repeater Applications

section.

5.9 REPEATER APPLICATIONS

5.9.1 MII Based Repeaters

The 80220/80221 can be used as the physical interface for

MII based repeaters by using the standard MII as the

interface to the repeater core.

For most repeaters, it is necessary to disable the internal

CRS loopback. This can be done be setting the TX_EN to

CRS loopback disable bit in the MI serial port Configura-

tion 1 register.

For some particular types of repeaters, it may be desirable

to either enable or disable AutoNegotiation, force Half

Duplex operation, and enable either 100 Mbps or 10 Mbps

operation. All of these modes can be configured by setting

the appropriate bits in the MI serial port Control register.

The 80221 has a RPTR pin which will automatically

configure the device for one common type of repeater

application. When the RPTR pin is asserted, (1) TX_EN to

CRS loopback is disabled, (2) AutoNegotiation is disabled,

(3) Half Duplex operation is selected, and (4) 100 Mbps

operation is selected.

The MII requires 16 signals between the 80220/80221 and

a repeater core. The MII signal count to a repeater core will

be 16 multiplied by the number of ports, which can be quite

large. The signal count between the 80220/80221 and

repeater core can be reduced by 8 per device by sharing

the receive output pins and using RX_EN to enable only

that port where CRS is asserted. Refer to the Controller

Interface section within the Applications section for more

details about RX_EN.

5.9.2 Non-MII Based Repeaters

The FBI interface available on the 80220/80221 can be

used to connect to non-MII based repeaters that employ

the industry popular five bit wide interface.

Since the FBI is a 5 bit wide interface, it requires that the

4B5B encoder/decoder be bypassed. The FBI is automati-

cally selected on the 80220/80221 when the 4B5B en-

coder/decoder is bypassed. The 4B5B encoder/decoder

can be bypassed by setting the bypass encoder/decoder

select bit in the MI serial port Configuration 1 register.

Some applications may also require the scrambler/

descrambler to be bypassed. This can be done by setting

the bypass scrambler/descrambler select bit in the MI

serial port Configuration 1 register.

For most repeaters, it is necessary to disable the internal

CRS loopback. This can be done be setting the TX_EN to

CRS loopback disable bit in the MI serial port Configura-

tion 1 register.

For some particular types of repeaters, it may be desirable

to either enable or disable AutoNegotiation, force Half

Duplex operation, and enable either 100 Mbps or 10 Mbps

operation. All of these modes can be configured by setting

the appropriate bits in the MI serial port Control register.

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