AM79C978VCSLASHW datasheet(69/261 Pages) AMD

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Am79C978

Automatic pad field insertion is controlled by the

APAD_XMT bit in CSR4.

The disable FCS generation/transmission feature can

be programmed as a static feature or dynamically on a

frame-by-frame basis.

Transmit FIFO Watermark (XMTFW) in CSR80 sets

the point at which the BMU requests more data from

the transmit buffers for the FIFO. A minimum of

XMTFW empty spaces must be available in the trans-

mit FIFO before the BMU will request the system bus in

order to transfer transmit frame data into the transmit

FIFO.

Transmit Start Point (XMTSP) in CSR80 sets the point

when the transmitter actually attempts to transmit a

frame onto the media. A minimum of XMTSP bytes

must be written to the transmit FIFO for the current

frame before transmission of the current frame will be-

gin. (When automatically padded packets are being

sent, it is conceivable that the XMTSP is not reached

when all of the data has been transferred to the FIFO.

In this case, the transmission will begin when all of the

frame data has been placed into the transmit FIFO.)

The default value of XMTSP is 01b, meaning there has

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

sion.

Automatic Pad Generation

Transmit frames can be automatically padded to ex-

tend them to 64 data bytes (excluding preamble). This

allows the minimum frame size of 64 bytes (512 bits)

for IEEE 802.3/Ethernet to be guaranteed with no soft-

ware intervention from the host/controlling process.

Setting the APAD_XMT bit in CSR4 enables the auto-

matic 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) standard). The

length value contained in the message is not used by

the Am79C978 controller to compute the actual num-

ber of pad bytes to be inserted. TheAm79C978 control-

ler will append pad bytes dependent on the actual

number of bits transmitted onto the network. Once the

last data byte of the frame has completed, prior to ap-

pending the FCS, the Am79C978 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 35.

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

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 theAm79C978

controller, therefore, will be 576 bits after the FCS is ap-

pended.

Figure 35.

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

Preamble

1010....1010

SFD

10101011

Destination

Address

Source

Address

Length

LLC

Data

Pad

FCS

Bytes

46 – 1500

Bytes

Bits

22206B-38

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