AM79C976 datasheet(80/309 Pages) AMD

80 / 309 page

Am79C976

8/01/00

P R E L I M INARY

Table 9. VLAN Tag Control Command

Table 10. VLAN Tag Type

Loopback Operation

Loopback is a mode of operation intended for system

diagnostics. In this mode, the transmitter and receiver

are both operating at the same time so that the control-

ler receives its own transmissions. The controller pro-

vides two basic types of loopback. In internal loopback

mode, the transmitted data is looped back to the re-

ceiver inside the controller without actually transmitting

any data to the external network. The receiver will

move the received data to the next receive buffer,

where it can be examined by software. Alternatively, in

external loopback mode, data can be transmitted to

and received from the external network.

The external loopback through the MII requires a two-

step operation. The external PHY must be placed into

a loop-back mode by writing to the PHY Access Regis-

ter. Then the Am79C976 controller must be placed into

an external loopback mode by setting EXLOOP

(CMD2, bit 3).

The internal loopback through the MII is controlled by

INLOOP (CMD2, bit 4). When set to 1, this bit will

cause the internal portion of the MII data port to loop-

back on itself. The MII management port (MDC, MDIO)

is unaffected by the INLOOP bit.

The internal MII interface is mapped in the following

way:

7KH 7;'>@ QLEEOH GDWD SDWK LV ORRSHG EDFN RQWR

WKH 5;'>@ QLEEOHGDWD SDWK

7;B&/. LVORRSHGEDFN DV 5;B&/.

7;B(1LV ORRSHG EDFN DV5;B'9

7;B(1 LV 25¶G ZLWK WKH &56 SLQ DQG LV ORRSHG EDFN

DV&56

7KH &2/ LQSXW LVGULYHQ ORZ

7;B(5 LV QRW GULYHQ E\ WKH $P& DQG WKHUH

IRUH QRWORRSHG EDFN

During the internal loopback, the TX_EN and TXD pins

will be active. Internal loopback should not be used on

a live network because collisions will not be handled

correctly. The wire should be disconnected or the PHY

isolated before using internal loopback.

0LVFHOODQHRXV/RRSEDFN )HDWXUHV

All transmit and receive function programming, such as

automatic transmit padding and receive pad stripping,

operates identically in loopback as in normal operation.

Runt Packet Accept is internally enabled regardless of

the state of the RPA bit in CSR124 when any loopback

mode is invoked. This is for backwards compatibility

with the C-LANCE (Am79C90) software.

The C-LANCE controller and the half-duplex members

of the PCnet family of devices place certain restrictions

on FCS generation and checking, and on testing multi-

cast address detection. Since the Am79C976 controller

has two FCS generators, these restrictions do not

apply to the Am79C976 controller. On receive, the

Am79C976 controller provides true FCS status. The

descriptor for a frame with an FCS error will have the

FCS bit (RMD1, bit 27) set to 1. The FCS generator on

the transmit side can still be disabled by setting DXMT-

FCS (CSR15, bit 3) to 1.

In internal loopback operation, the Am79C976 control-

ler provides a special mode to test the collision logic.

When FCOLL (CSR15, bit 4) is set to 1, a collision is

forced during every transmission attempt. This will re-

sult in a Retry error.

Full-Duplex Operation

The Am79C976 controller supports full-duplex opera-

tion on both network interfaces. Full-duplex operation

allows simultaneous transmit and receive activity on the

TXD[3:0] and RXD[3:0] pins of the MII port. Full-duplex

operation is enabled by the FDEN bit located in BCR9

for all ports. Full-duplex operation is also enabled

through Auto-Negotiation when DANAS (BCR 32, bit 7)

is not enabled on the MII port and the ASEL bit is set,

and both the external PHY and its link partner are ca-

pable of Auto-Negotiation and full-duplex operation.

When operating in full-duplex mode, the following

changes to the device operation are made:

The MAC engine changes for full-duplex operation are

as follows:

&KDQJHV WR WKH 7UDQVPLW'HIHUUDOPHFKDQLVP

TCC

(TMD2[17:16])

Action

Transmit data in buffer unaltered

Delete Tag Header

Insert Tag Header containing TCI

field from descriptor.

Replace TCI field from buffer with TCI

data from descriptor.

(RMD1[19:18])

Description

Reserved

Frame is untagged

Frame is priority-tagged

Frame is VLAN-tagged

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