AM79C973VCW datasheet(74/304 Pages) AMD

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Am79C973/Am79C975

P R E L I M INARY

to be 64 bytes in the transmit FIFO to start a transmis-

sion.

Automatic Pad Generation

Transmit frames can be automatically padded to extend

them to 64 data bytes (excluding preamble). This al-

lows the minimum frame size of 64 bytes (512 bits) for

IEEE 802.3/Ethernet to be guaranteed with no software

intervention from the host/controlling process. Setting

the APAD_XMT bit in CSR4 enables the automatic

padding feature. The pad is placed between the LLC

data field and FCS field in the IEEE 802.3 frame. FCS

is always added if the frame is padded, regardless of

the state of DXMTFCS (CSR15, bit 3) or ADD_FCS

(TMD1, bit 29). The transmit frame will be padded by

bytes with the value of 00H. The default value of

APAD_XMT is 0, which will disable automatic pad gen-

eration after H_RESET.

It is the responsibility of upper layer software to cor-

rectly define the actual length field contained in the

message to correspond to the total number of LLC

Data bytes encapsulated in the frame (length field as

defined in the ISO 8802-3 (IEEE/ANSI 802.3) stan-

dard). The length value contained in the message is not

used by the Am79C973/Am79C975 controller to com-

pute the actual number of pad bytes to be inserted. The

Am79C973/Am79C975 controller will append pad

bytes dependent on the actual number of bits transmit-

ted onto the network. Once the last data byte of the

frame has completed, prior to appending the FCS, the

Am79C973/Am79C975 controller will check to ensure

that 544 bits have been transmitted. If not, pad bytes

are added to extend the frame size to this value, and

the FCS is then added. See Figure 33.

Figure 33. ISO 8802-3 (IEEE/ANSI 802.3) Data Frame

The 544 bit count is derived from the following:

Minimum frame size (excluding preamble/SFD,

including FCS)

64 bytes

512 bits

Preamble/SFD size 8 bytes

64 bits

FCS size

4 bytes

32 bits

At the point that FCS is to be appended, the transmitted

frame should contain:

Preamble/SFD + (Min Frame Size - FCS)

64 + (512-32) = 544 bits

A minimum length transmit frame from the Am79C973/

Am79C975 controller, therefore, will be 576 bits, after

the FCS is appended.

Transmit FCS Generation

Automatic generation and transmission of FCS for a

transmit frame depends on the value of DXMTFCS

(CSR15, bit 3). If DXMTFCS is cleared to 0, the trans-

mitter will generate and append the FCS to the trans-

mitted frame. If the automatic padding feature is

invoked (APAD_XMT is set in CSR4), the FCS will be

appended to frames shorter than 64 bytes by the

Am79C973/Am79C975 controller regardless of the

state of DXMTFCS or ADD_FCS (TMD1, bit 29). Note

that the calculated FCS is transmitted most significant

bit first. The default value of DXMTFCS is 0 after

H_RESET.

ADD_FCS (TMD1, bit 29) allows the automatic gener-

ation and transmission of FCS on a frame-by-frame

basis. DXMTFCS should be set to 1 in this mode. To

generate FCS for a frame, ADD_FCS must be set in all

descriptors of a frame (STP is set to 1). Note that bit 29

of TMD1 has the function of ADD_FCS if SWSTYLE

(BCR20, bits 7-0) is programmed to 0, 2, or 3.

Transmit Exception Conditions

Exception conditions for frame transmission fall into

two distinct categories: 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

include collisions within the slot time with automatic re-

try. The Am79C973/Am79C975 controller will ensure

that collisions which occur within 512 bit times from the

Preamble

1010....1010

SFD

10101011

Destination

Address

Source

Address

Length

LLC

Data

Pad

FCS

Bytes

46 – 1500

Bytes

Bits

21510D-38

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