AM79C971VCW datasheet(67/265 Pages) AMD

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Am79C971

If DRCVBC (CSR15, bit 14) is cleared to 0, only BAM,

but not LAFM will be set when a Broadcast frame is re-

ceived, even if the Logical Address Filter is pro-

grammed in such a way that a Broadcast frame would

pass the hash filter. If DRCVBC is set to 1 and the Log-

ical Address Filter is programmed in such a way that a

Broadcast frame would pass the hash filter, LAFM will

be set on the reception of a Broadcast frame.

When the Am79C971 controller operates in promiscu-

ous mode and none of the three match bits is set, it is

an indication that the Am79C971 controller only ac-

cepted the frame because it was in promiscuous mode.

When the Am79C971 controller is not programmed to

be in promiscuous mode, but the EADI interface is en-

abled, then when none of the three match bits is set, it

is an indication that the Am79C971 controller only ac-

cepted the frame because it was not rejected by driving

the EAR pin LOW within 64 bytes after SFD.

See Table 6 for receive address matches.

Automatic Pad Stripping

During reception of an IEEE 802.3 frame, the pad field

can be stripped automatically. Setting ASTRP_RCV

(CSR4, bit 0) to 1 enables the automatic pad stripping

feature. The pad field will be stripped before the frame

is passed to the FIFO, thus preserving FIFO space for

additional frames. The FCS field will also be stripped,

since it is computed at the transmitting station based on

the data and pad field characters, and will be invalid for

a receive frame that has had the pad characters

stripped.

Table 6.

Receive Address Match

The number of bytes to be stripped is calculated from

the embedded length field (as defined in the ISO 8802-

3 (IEEE/ANSI 802.3) definition) contained in the frame.

The length indicates the actual number of LLC data

bytes contained in the message. Any received frame

which contains a length field less than 46 bytes will have

the pad field stripped (if ASTRP_RCV is set). Receive

frames which have a length field of 46 bytes or greater

will be passed to the host unmodified.

Figure 34 shows the byte/bit ordering of the received

length field for an IEEE 802.3-compatible frame format.

Figure 34.

IEEE 802.3 Frame And Length Field Transmission Order

PAM

LAF

BAM

DRC

VBC

Comment

Frame accepted due

to PROM = 1 or no

EADI reject

Physical address

match

Logical address filter

match;

frame is not of type

broadcast

Logical address filter

match;

frame can be of type

broadcast

Broadcast frame

Preamble

1010....1010

SFD

10101011

Destination

Address

Source

Address

Length

LLC

Data

Pad

FCS

Bytes

46 – 1500

Bytes

Bits

Start of Frame

at Time = 0

Increasing Time

Bit

Most

Significant

Byte

Least

Significant

Byte

1 – 1500

Bytes

45 – 0

Bytes

20550D-37

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