AM79C940KCSLASHW datasheet(61/144 Pages) AMD

61 / 144 page

Am79C940

to the inability of the host/con-

troller to read data fast enough

to keep pace with the receive se-

rial bit stream and the latency

provided by the Receive FIFO it-

self. OFLO is indicated on the re-

ceive frame that caused the

overflow

condition;

complete

frames in the Receive FIFO are

not affected. While the Receive

FIFO is in the overflow condition,

it ignores additional receive data

on the network. The internal ad-

dress detect logic will continue

to operate and the Missed Pack-

et Count (MPC in register 24) will

be incremented for each packet

which passes the address match

criteria, and complete without

collision.

Bit 6

CLSN

Collision Flag. Indicates that the

receive operation suffered a col-

lision during reception of the

frame. If CLSN is set, it indicates

that the receive frame suffered a

late collision, since a frame ex-

periencing collision within the

slot time will be automatically

deleted from the RCVFIFO (pro-

viding LLRCV in the Receive

Frame

Control

cleared). Note that if the LLRCV

bit is enabled, the late collision

threshold is effectively moved

from the normal 64–byte (512–

bit) level to the 12-byte (96–bit)

level. Runt packets suffering a

collision will be flushed from the

RCVFIFO

regardless

the

state of the RPA bit (User Test

OFLO is set.

Bit 5

FRAM

Framing Error flag. Indicates

that

the

received

frame

contained a non-integer multiple

of bytes and an FCS error. If

there was no FCS error then

FRAM will not be set. FRAM is

not

valid

during

internal

loopback. FRAM will not be set if

OFLO is set.

Bit 4

FCS

FCS Error flag. Indicates that

there is an FCS error in the

frame.

The

receive

FCS

computed and checked normally

when ASTRP RCV = 1, but is not

passed to the host. FCS will not

be set if OFLO is set.

Bit 3-0

RCVCNT

The Receive Message Byte

[11:8]

Count indicates the number of

whole bytes in the received

message

from

the

network.

RCVCNT is 12 bits long, and

valid (accurate) only when there

are no errors reported in the

Receive Status (RCVSTS). If a

late collision is detected (CLSN

set in RCVSTS), the count is an

indication of the length (in byte

times) of the duration of the

receive activity including the

collision. RCVCNT [7:0] corre-

spond to bits 7-0 in RFS0 of the

Receive

Frame

Status.

RCVCNT [11–0} will be invalid

when OFLO is set.

RFS2—Runt Packet Count (RNTPC)

Bit

Name

Description

Bit 7-0

RNTPC

The

Runt

Packet

Count

indicates

[7–0]

the number of runt packets

received,

addressed

this

node, since the last successfully

received packet. The value does

not roll over after 255 runt

packets have been detected,

and will remain frozen at the

maximum count.

RFS3—Receive Collision Count (RCVCC)

Bit

Name

Description

Bit 7–0 RCVCC

The Receive Collision Count in-

[7–0]

dicates the number of collisions

detected on the network since

the last successfully received

packet. The value does not roll

over after 255 collisions have

been detected, and will remain

frozen at the maximum count.

FIFO Frame Count (FIFOFC)

(REG ADDR 7)

Bit

Name

Description

Bit 7–4

RCVFC

Receive

Frame

Count.

The

(read

[3–0]

only) count of the frames in the

Receive FIFO. A frame is count-

ed when the last byte is put in

the FIFO. The counter is decre-

mented when the last byte of the

frame is read. If the RCVFC

reaches its maximum value of

RNTPC [7–0]

RCVCC [7–0]

RCVFC[3–0]

XMTFC[3–0]

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