AM79C975 datasheet(77/304 Pages) AMD

77 / 304 page

Am79C973/Am79C975

P R E L I M INARY

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

Since any valid Ethernet Type field value will always be

greater than a normal IEEE 802.3 Length field (Š46),

the Am79C973/Am79C975 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 Am79C973/Am79C975

controller. Note that if the Automatic Pad Stripping fea-

ture is enabled, the FCS for padded frames will be ver-

ified against the value computed for the incoming bit

stream including pad characters, but the FCS value for

a padded frame will not be passed to the host. If an

FCS error is detected in any frame, the error will be re-

ported 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 Am79C973/Am79C975 controller

are basically collisions within the slot time and auto-

ma tic r u n t packe t r e jection . T h e Am 79C9 73/

Am79C975 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. The receive

FIFO will delete any frame that is composed of fewer

than 64 bytes provided that the Runt Packet Accept

(RPA bit in CSR124) feature has not been enabled and

the network interface is operating in half-duplex mode,

or the full-duplex Runt Packet Accept Disable bit (FDR-

PAD, BCR9, bit 2) is set. This criterion will be met re-

gardless of whether the receive frame was the first (or

only) frame in the FIFO or if the receive frame was

queued behind a previously received message.

Abnormal network conditions include:

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

21510D-39

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