AM79C978AVCW datasheet(69/256 Pages) AMD

69 / 256 page

Am79C978A

Table 11.

Receive Address Match

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.

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 36 shows the byte/bit ordering of the received

length field for an IEEE 802.3-compatible frame format.

Since any valid Ethernet Type field value will always be

greater than a normal IEEE 802.3 Length field (

≥46),

the Am79C978A controller will not attempt to strip valid

Ethernet frames. Note that for some network protocols,

the value passed in the Ethernet Type and/or IEEE

802.3 Length field is not compliant with either standard

and may cause problems if pad stripping is enabled.

Receive FCS Checking

Reception and checking of the received FCS is per-

formed automatically by the Am79C978A controller.

Note that if the Automatic Pad Stripping feature is en-

abled, the FCS for padded frames will be verified

against the value computed for the incoming bit stream

including pad characters, but the FCS value for a pad-

ded frame will not be passed to the host. If an FCS

error is detected in any frame, the error will be reported

in the CRC bit in RMD1.

Receive Exception Conditions

Exception conditions for frame reception fall into

two distinct categories, i.e., those conditions which

are the result of normal network operation, and

those which occur due to abnormal network and/or

host related events.

Normal events which may occur and which are handled

autonomously by the Am79C978A controller are basi-

cally collisions within the slot time and automatic runt

packet rejection. The Am79C978A controller will

ensure that collisions that occur within 512 bit times

from the start of reception (excluding preamble) will be

automatically deleted from the receive FIFO with no

host intervention.

PAM

LAFM

BAM

DRCVBC

Comment

Frame accepted

due to PROM = 1

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

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