AM79C976 datasheet(70/309 Pages) AMD

70 / 309 page

Am79C976

8/01/00

P R E L I M INARY

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 after H_RESET, which will disable au-

tomatic pad generation.

If automatic pad generation is disabled, the software is

responsible for insuring that the minimum frame size

requirement is met. The hardware can reliably transmit

frames ranging in size from 16 to 65536 octets.

It is the responsibility of upper layer software to cor-

rectly define the actual length/type field contained in

the message to correspond to the total number of LLC

Data bytes encapsulated in the frame (length/type field

as defined in the IEEE 802.3 standard). The length

value contained in the message is not used by the

Am79C976 controller to compute the actual number of

pad bytes to be inserted. The Am79C976 controller 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 appending the

FCS, the Am79C976 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 3030.

)LJXUH ,62 ,((($16, 'DWD )UDPH

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 Am79C976

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

appended.

7UDQVPLW)&6 *HQHUDWLRQ

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 transmitter modifies the frame data

because of automatic padding or VLAN tag manipula-

tion, the FCS will be appended by the Am79C976 con-

troller 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.

When DXMTFCS is set to 1, the ADD_FCS (TMD1, bit

29) allows the automatic generation and transmission

of FCS on a frame-by-frame basis. When DXMTFCS is

set to 1, a valid FCS field is appended only to those

frames whose TX descriptors have their ADD_FCS bits

set to 1. If a frame is split into more than one buffer, the

ADD_FCS bit is ignored in all descriptors except for the

first.

7UDQVPLW(UURU &RQGLWLRQV

The Am79C976 transmitter detects the following error

conditions and increments the appropriate error

counters when they occur:

/RVVRI FDUULHU

/DWH FROOLVLRQ

7UDQVPLW),)28QGHUIORZ

Late collision errors can only occur when the device is

operating in half-duplex mode. Loss of carrier and

transmit FIFO underflow errors are possible when the

device is operating in half- or full-duplex mode.

When an error occurs in the middle of a multi-buffer

frame transmission, the appropriate error counter will

be incremented, and the transmission will be aborted

with an inverted FCS field appended to the frame. The

OWN bit(s) in the current and subsequent descriptor(s)

will be cleared until the STP (the next frame) is found.

Preamble

1010....1010

SFD

10101011

Destination

Address

Source

Address

Length/

Type

LLC

Data

Pad

FCS

Bytes

46 – 1500

Bytes

Bits

AM79C976 Datenblatt(PDF) 70 Page - Advanced Micro Devices

AM79C976 Datenblatt(HTML) 70 Page - Advanced Micro Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datenblatt Download

Link URL