AM79C90PC datasheet(24/62 Pages) AMD

24 / 62 page

AMD

P R E L I M I N A R Y

Am79C90

Bit

Name

Description

PROM

PROMISCUOUS

mode.

When

PROM = 1, all incoming packets are

accepted.

14:08

RES

RESERVED. Read as zeroes. Write

as zeroes.

EMBA

Enable Modified Back-off Algorithm.

When set (EMBA=1), enables the

modified backoff algorithm. EMBA

is cleared by activation of the

RESET

pin or setting the STOP bit.

INTL

INTERNAL LOOPBACK is used with

the LOOP bit to determine where the

loopback is to be done. Internal loop-

back allows the chip to receive its

own transmitted packet. Since this

represents full duplex operation, the

packet size is limited to 8–32 bytes.

Internal loopback in the C-LANCE is

operational when the packets are

addressed to the node itself.

The C-LANCE will not receive any

packets externally when it is in inter-

nal loopback mode.

EXTERNAL LOOPBACK allows the

C-LANCE to transmit a packet

through the SIA transceiver cable

out to the Ethernet medium. It is

used to determine the operability of

all circuitry and connections be-

tween the C-LANCE and the physi-

cal medium. Multicast addressing in

external loopback is valid only when

DTCR = 1 (user needs to append the

4 bytes CRC).

In external loopback, the C-LANCE

also receives packets from other

nodes. The FIFOs READ/WRITE

pointers

may

misalign

the

C-LANCE under heavy traffic. The

packet could then be corrupted or

not received. Therefore, the external

loopback execution may need to be

repeated. See specific discussion

under “Loopback” in later section.

INTL is only valid if LOOP = 1; other-

wise, it is ignored.

LOOP

INTL

LOOPBACK

No loopback,

normal

External

Internal

DRTY

DISABLE RETRY. When DRTY = 1,

the C-LANCE will attempt only one

transmission of a packet. If there is a

collision on the first transmission at-

tempt, a Retry Error (RTRY) will be

reported in Transmit Message De-

scriptor 3 (TMD

3).

Bit

Name

Description

COLL

FORCE COLLISION. This bit allows

the collision logic to be tested. The

C-LANCE must be in internal loop-

back mode for COLL to be valid. If

COLL = 1, a collision will be forced

during the subsequent transmission

attempt. This will result in 16 total

transmission attempts with a retry er-

ror reported in TMD

DTCR

DISABLE TRANSMIT CRC. When

DTCR = 0, the transmitter will gener-

ate and append a CRC to the trans-

mitted packet. When DTCR = 1, the

CRC logic is allocated to the receiver

and no CRC is generated and sent

with the transmitted packet. The

ADD_FCS bit (bit 13, TMD

1) can be

used to override a DTCR=1 setting

on a per packet basis.

During loopback, DTCR = 0 will

cause a CRC to be generated on the

transmitted packet, but no CRC

check will be done by the receiver

since the CRC logic is shared and

cannot generate and check CRC at

the same time. The generated CRC

will be written into memory with the

data and can be checked by the host

software.

If DTCR = 1 during loopback, the

host software must append a CRC

value to the transmit data.

The receiver will check the CRC on

the received data and report any

errors.

LOOP

LOOPBACK allows the C-LANCE to

operate in full duplex mode for test

purposes. The packet size is limited

to 8–32 bytes.The received packet

can be up to 36 bytes (32 + 4 bytes

CRC) when DTCR = 0. During loop-

back, the runt packet filter is disabled

because the maximum packet is

forced to be smaller than the

minimum

size

Ethernet

packet

(64 bytes).

LOOP = 1 allows simultaneous

transmission and reception for a

message constrained to fit within the

Transmit FIFO. The C-LANCE waits

until the entire message is in the

Transmit FIFO before serial trans-

mission begins. The incoming data

stream fills the Receive FIFO. Mov-

ing the received message out of the

Receive FIFO to memory does not

begin until reception has ceased.

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