AM79C971VCW datasheet(64/265 Pages) AMD

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Am79C971

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 transmit 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 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/

NO_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 generation 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 Am79C971 controller to compute the ac-

tual num ber of pad byte s to be inser t ed . Th e

Am79C971 controller will append pad bytes dependent

on the actual number of bits transmitted onto the net-

work. Once the last data byte of the frame has com-

pleted, prior to appending the FCS, the Am79C971

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 Am79C971

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

Am79C971 controller regardless of the state of DXMT-

FCS or ADD_FCS/NO_FCS (TMD1, bit 29). Note that

the calculated FCS is transmitted most significant bit

Preamble

1010....1010

SFD

10101011

Destination

Address

Source

Address

Length

LLC

Data

Pad

FCS

Bytes

46 – 1500

Bytes

Bits

20550D-36

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