ENC28J60-CSLASHSP datasheet(55/96 Pages) MICROCHIP

55 / 96 page

Preliminary

DS39662B-page 53

ENC28J60

9.0

DUPLEX MODE

CONFIGURATION AND

NEGOTIATION

The ENC28J60 does not support automatic duplex

negotiation. If it is connected to an automatic duplex

negotiation enabled network switch or Ethernet control-

ler, the ENC28J60 will be detected as a half-duplex

device. To communicate in Full-Duplex mode, the

ENC28J60 and the remote node (switch, router or

Ethernet controller) must be manually configured for

full-duplex operation.

9.1

Half-Duplex Operation

The ENC28J60 operates in Half-Duplex mode when

MACON3.FULDPX = 0 and PHCON1.PDPXMD = 0. If

only one of these two bits is set, the ENC28J60 will be

in an indeterminate state and not function correctly.

Since switching between Full and Half-Duplex modes

may result in this indeterminate state, the host control-

ler

should

not

transmit

any

packets

(maintain

ECON1.TXRTS clear) and packet reception should be

disabled (ECON1.RXEN and ESTAT.RXBUSY should

be clear) during this period.

In Half-Duplex mode, only one Ethernet controller may

be transmitting on the physical medium at any time. If

the host controller sets the ECON1.TXRTS bit,

requesting that a packet be transmitted while another

Ethernet

controller

already

transmitting,

the

ENC28J60 will delay, waiting for the remote transmitter

to stop. After the transmission stops, the ENC28J60

will attempt to transmit its packet. If another Ethernet

controller starts transmitting at approximately the same

time that the ENC28J60 starts transmitting, the data on

the wire will become corrupt and a collision will occur.

The hardware will handle this condition in one of two

ways:

If the collision occurs before the number of bytes

specified

the

“Collision

Window”

MACLCON2

were

transmitted,

the

ECON1.TXRTS bit will remain set, a random

exponential back off delay will elapse as defined

by the IEEE 802.3 specification and then a new

attempt to transmit the packet from the begin-

ning will occur. The host controller will not need

to intervene. If the number of retransmission

attempts already matches the “Retransmission

Maximum” (RETMAX) defined in MACLCON1,

the packet will be aborted and ECON1.TXRTS

will be cleared. The host controller will then be

responsible for taking appropriate action. The

host controller will be able to determine that the

packet was aborted instead of being success-

fully transmitted by reading the ESTAT.TXABRT

flag. For more information, see Section 7.1

“Transmitting Packets”. A transmit abort will

cause the transmit error interrupt.

If the collision occurs after the number of bytes

specified

the

“Collision

Window”

MACLCON2 were transmitted, the packet will be

immediately aborted without any retransmission

attempts.

Ordinarily,

802.3

compliant

networks which are properly configured, this late

collision will not occur. User intervention may be

required to correct the issue. This problem may

occur as a result of a full-duplex node attempting

to transmit on the half-duplex medium. Alter-

nately, the ENC28J60 may be attempting to

operate in Half-Duplex mode while it may be

connected to a full-duplex network. Excessively

long cabling and network size may also be a

possible cause of late collisions.

When set in Half-Duplex mode, the Reset default

configuration will loop transmitted packets back to

itself. Unless the receive filter configuration filters these

packets out, they will be written into the circular receive

buffer, just as any other network traffic. To stop this

behavior,

the

host

controller

should

set

the

PHCON2.HDLDIS bit.

9.2

Full-Duplex Operation

The ENC28J60 operates in Full-Duplex mode when

MACON3.FULDPX = 1 and PHCON1.PDPXMD = 1. If

only one of these two bits is clear, the ENC28J60 will

be in an indeterminate state and not function correctly.

Since switching between Full and Half-Duplex modes

may result in this indeterminate state, the host control-

ler

should

not

transmit

any

packets

(maintain

ECON1.TXRTS clear) and packet reception should be

disabled (ECON1.RXEN and ESTAT.RXBUSY should

be clear) during this period.

In Full-Duplex mode, packets will be transmitted

simultaneously while packets may be received. Given

this, it is impossible to cause any collisions when trans-

mitting packets. Several configuration fields, such as

“Retransmission Maximum” (RETMAX) in MACLCON1

and “Collision Window” (COLWIN) in MACLCON2, will

not be used.

When set in Full-Duplex mode, the Reset default

configuration will not loop transmitted packets back to

itself. If loopback is desired for diagnostic purposes, the

PHCON1.PLOOPBK bit should be set by the host

controller. Enabling loopback in Full-Duplex mode will

disable the twisted pair output driver and ignore all

incoming data, thus dropping any link (if established).

All packets received as a result of the loopback config-

uration will be subject to all enabled receive filters, just

as ordinary network traffic would be.

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