AM79C90PC datasheet(28/62 Pages) AMD

28 / 62 page

AMD

P R E L I M I N A R Y

Am79C90

Bit

Name

Description

OFLO

OVERFLOW error indicates that the

receiver has lost all or part of the in-

coming packet due to an inability to

store the packet in a memory buffer

before the internal Receive FIFO

overflowed. OFLO is valid only when

ENP is not set.

CRC

CRC indicates that the receiver has

detected a CRC error on the incom-

ing packet. CRC is valid only when

ENP is set and OFLO is not.

BUFF

BUFFER ERROR is set any time the

C-LANCE does not own the next

buffer while data chaining a received

packet. This can occur in either of

two ways: 1) the OWN bit of the next

buffer is zero, or 2) the Receive FIFO

overflow

occurred

before

the

C-LANCE

has

performed

lookahead poll of the next receive

descriptor.

If a Buffer Error occurs, an Overflow

Error may also occur internally in the

Receive FIFO, but will not be re-

ported in the descriptor status entry

unless both BUFF and OFLO errors

occur at the same time.

STP

START OF PACKET indicates that

this is the first buffer used by the

C-LANCE for this packet. It is used

for data chaining buffers.

ENP

END OF PACKET indicates that this

is the last buffer used by the

C-LANCE for this packet. It is used

for data chaining buffers. If both STP

and ENP are set, the packet fits into

one buffer and there is no data

chaining.

07:00

HADR

The HIGH ORDER 8 address bits of

the buffer pointed to by this descrip-

tor. This field is written by the host

and unchanged by the C-LANCE.

Receive Message Descriptor 2 (RMD2)

17881B-27

12 11

BCNT

Must be Ones

15:12

MUST BE ONES. This field is written

by the host and is not changed by the

C-LANCE.

11:00

BCNT

BUFFER BYTE COUNT is the length

of the buffer pointed to by this de-

scriptor, expressed as a two’s com-

plement number. This field is written

by the host and is not changed by the

C-LANCE. Minimum buffer size is 64

bytes for the first buffer of packet.

Receive Message Descriptor 3 (RMD3)

17881B-28

12 11

MCNT

RES

15:12

RES

RESERVED. Read as zeroes. Write

as zeroes.

11:00

MCNT

MESSAGE BYTE COUNT is the

length in bytes of the received mes-

sage. MCNT is valid only when ERR

is clear and ENP is set. MCNT is writ-

ten by the chip and cleared by the

host.

Transmit Message Descriptor Entry

Transmit Message Descriptor 0 (TMD0)

17881B-29

LADR

Bit

Name

Description

15:00

LADR

The LOW ORDER 16 address bits of

the buffer pointed to by this descrip-

tor. LADR is written by the host and is

not changed by the C-LANCE.

Transmit Message Descriptor 1 (TMD1)

ENP

STP

DEF

ONE

ADD_FCS

ERR

OWN

8 7

HADR

17881B-30

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