AM79C976 datasheet(87/309 Pages) AMD

87 / 309 page

8/01/00

Am79C976

P R E L I M INARY

1. FORCE_FD (CMD3, bit 12),

2. FORCE_SPEED (CTRL2, bits 18-16),

3. FORCE_LINK_STAT (CMD3, bit 11), and

4. Force Pause Ability (FPA, FLOW_CONTROL, bit

20).

These bits set up the duplex mode, speed, and flow

control ability in the MAC and put the MAC into the Link

Pass state.

Regulating Network Traffic

The Am79C976 device provides two hardware mecha-

nisms for regulating network traffic: 802.3x Flow Con-

trol and collision-based back pressure. 802.3x Flow

Control applies to full-duplex operation only, while back

pressure applies to half-duplex operation only. 802.3x

Flow Control works by sending and receiving MAC

Control PAUSE frames, which cause the receiving sta-

tion to postpone transmissions for a time determined by

the contents of the PAUSE frame. Back pressure forces

collisions to occur when other nodes attempt to trans-

mit, thereby preventing other nodes from transmitting

for periods of times determined by the back-off algo-

rithm.

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The format of a MAC Control Pause frame is shown in

Table 12.

When a network station that supports IEEE 802.3x

Flow Control receives a pause frame, it must suspend

transmissions after the end of any frame that was being

transmitted when the pause frame arrived. The length

of time for which the station must suspend transmis-

sions is given in the request_operand field of the pause

frame. This pause time is given in units of slot times.

For 10-Mbps and 100-Mbps 802.3 networks, one slot

time is 512 bit times. The request_operand field is in-

terpreted as Big-Endian data--octet 17 is the most sig-

nificant byte and octet 18 is the least significant byte.

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The Am79C976 device supports collision-based back

pressure for congestion control when the device is op-

erating in half-duplex mode. Back pressure is enabled

when the device is operating in half duplex mode and

either the Flow Control Command bit (FCCMD,

FLOW_CONTROL, bit 16) is set or the FC Pin Enable

bit (FCPEN, FLOW_CONTROL, bit 17) is set and the

FC pin is asserted.

When the MAC begins receiving a frame that passes

the address matching criteria and if back pressure is

enabled, the MAC will intentionally cause a collision by

transmitting a “phantom” frame consisting of a continu-

ous stream of alternating 1s and 0s. The length of the

phantom frame is 568 bits so that it will be interpreted

as a runt frame.

Back pressure does not affect the transmission of a

frame. The MAC will only force a collision when it be-

gins to receive a new frame.

The generation of a Back-Pressure collision causes the

XmtBackPressure MIB Counter to increment.

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Traffic regulation can be controlled either by external

hardware or by CPU commands. Traffic regulation is af-

fected by the following:

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)ORZ &RQWURO )&SLQ

)&3LQ (QDEOH)&3(1ELW

)ORZ &RQWURO &RPPDQG )&&0'ELW

)L[HG /HQJWK 3DXVH ),;3 ELW

ELW3$86( /HQJWK 5HJLVWHU

1HJRWLDWH 3DXVH $ELOLW\ 13$ELW

)RUFH 3DXVH $ELOLW\ )3$ ELW

The duplex mode affects the type of traffic regulation

that is used. In full-duplex mode the FC pin and the

FCPEN, FCCMD, and FIXP bits control the transmis-

sion of pause frames. In half-duplex mode the same pin

and bits control the assertion of back pressure. Also, in

half-duplex mode the Am79C976 device does not

respond to received pause frames.

The Am79C976 device includes support for two styles

of full-duplex flow control. In one style, which is similar

to an XON-XOFF protocol, a pause frame whose

request_operand field (bytes 17 and 18) contains

0FFFFh is sent to prevent the link partner from transmit-

ting. Later, a pause frame whose request_operand field

contains 0 is sent to allow the link partner to resume

transmissions. This style of flow control is selected by

clearing the Fixed Length Pause bit (FIXP) to 0.

Table 12. MAC Control Pause Frame Format

Octet

Numbers

Field Name

Value

1-6

Destination

Address

01-80-C2-00-00-01

7-12

Source Address

Sender’s physical

address

13-14

Length/Type

88-08

15-16

MAC Control

Opcode

00-01

17-18

Request_operand

Pause time measured in

slot times

19-60

Pad

Zeros

61-64

FCS

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