AM79C976 datasheet(85/309 Pages) AMD

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8/01/00

Am79C976

P R E L I M INARY

the external PHY status register), and the register is al-

ways enabled. This means that if the Auto-Poll State

Machine is enabled, it will always poll register 1 of the

default PHY and will interrupt the host CPU when it de-

tects a change in that register.

In addition to the PHY address, register number, and

enable bit, the Auto-Poll Registers contain two other

control bits for each of the 5 user-selected registers.

T hese bit s ar e t he Pr ea mb le Su ppr ession

(AP_PRE_SUP) and Default PHY (AP_DFLT_PHY)

bits.

If the Preamble Suppression bit is set, the Auto-Poll

sends management frames to the corresponding regis-

ter with no preamble field. The host CPU should only

set the Preamble Suppression bit for registers in PHY

devices that are known to be able to accept manage-

ment frames without preambles. For PHY devices that

comply with Clause 22 of IEEE Std 802.3, bit 6 of PHY

ment frames with the preamble suppressed.

If the Default PHY bit (AP_DFLT_PHY) is set, the cor-

responding Preamble Suppression bit and PHY ad-

dress field are ignored. In this case the Auto-Poll State

Machine uses the default PHY address from the

AP_PHY0_ADDR field, and suppresses the preamble

if the Network Port Manager logic has determined that

the default PHY device accepts management frames

with no preamble. If the Network Port Manager logic

has not determined that the default PHY device ac-

cepts management frames with no preamble, the Auto-

Poll State Machine does not suppress the preamble

when accessing the selected register.

The Auto-Poll State Machine is enabled when the Auto-

Poll External PHY (APEP) bit (CMD3, bit 24) is set to 1.

If APEP is cleared to 0, the Auto-Poll machine does not

poll any PHY registers regardless of the state of the en-

able bits in the Auto-Poll registers. The APEP bit has no

effect on the Network Port Manager, which may poll the

default PHY even when the state of the APEP bit is 0.

The Auto-Poll’s frequency of generating MII manage-

ment frames can be adjusted by setting of the APDW

bits (BCR32, bits 10-8). The delay can be adjusted

from 0 MDC periods to 2048 MDC periods.

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The Am79C976 controller is unique in that it does not

require software intervention to control and configure

an external PHY attached to the MII. This feature was

included to ensure backwards compatibility with exist-

ing software drivers. The Am79C976 controller will op-

erate with existing PCnet drivers from revision 2.5

upward (although older drivers will report incorrect sta-

tistics for the Am79C976 device). The heart of this au-

tomatic configuration system is the Network Port

Manager.

The Network Port Manager initiates auto-negotiation in

the external PHY when necessary and monitors the re-

sults. When auto-negotiation is complete, the Network

Port Manager sets up the MAC to be consistent with

the negotiated configuration. The Network Port Man-

ager auto-negotiation sequence requires that the exter-

nal PHY respond to the auto-negotiation request within

4 seconds. Otherwise, system software will be required

to properly control and configure the external PHY at-

tached to the MII. After auto negotiation is complete,

the Network Port Manager generates MII management

frames about once every 900 ms to monitor the status

of the external PHY.

The Network Port Manager is enabled when the Dis-

able Port Manager (DISPM) bit (CMD3, bit 14) is

cleared to 0.

Auto-Negotiation

The external PHY and its link partner may have one or

more of the following capabilities: 100BASE-T4,

100BASE-TX Full-/Half-Duplex, 10BASE-T Full-/Half-

Duplex, and MAC Control PAUSE frame processing.

During the auto-negotiation process the two PHY de-

vices exchange information about their capabilities and

then select the best mode of operation that is common

to both devices. The modes of operation are prioritized

according to the order shown in Table 11 (with the high-

est priority shown at the top of the table).

Table 11. Auto-Negotiation Capabilities

Auto-Negotiation goes further by providing a message-

based communication scheme called, Next Pages, be-

fore connecting to the Link Partner. The Network Port

Manager does not support this feature. However, the

host CPU can disable the Network Port Manager and

manage Next Pages by accessing the PHY device

through the PHY Access Register. The host CPU can

disable the Network Port Manager by setting the Dis-

able Port Manager (DISPM) bit (CMD3, bit 14) to 1.

(The DISPM bit corresponds to the Disable Auto-Nego-

tiation Auto Setup (DANAS) bit in BCR32 of older

PCnet family devices.)

Network Speed

Physical Network Type

200 Mbps

100BASE-X, Full Duplex

100 Mbps

100BASE-T4, Half Duplex

100 Mbps

100BASE-X, Half Duplex

20 Mbps

10BASE-T, Full Duplex

10 Mbps

10BASE-T, Half Duplex

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