AM79C972BVCW datasheet(63/130 Pages) AMD

63 / 130 page

Am79C972

byte ordering is such that the first byte received from

the network (after the SFD) must match the least signif-

icant byte of CSR12 (PADR[7:0]), and the sixth byte re-

ceived must match the most significant byte of CSR14

(PADR[47:40]).

When DRCVPA (CSR15, bit 13) is set to 1, the

Am79C972 controller will not accept unicast frames.

If the incoming frame is multicast, the Am79C972 con-

troller performs a calculation on the contents of the

destination address field to determine whether or not to

accept the frame. This calculation is explained in the

section that describes the Logical Address Filter

(LADRF).

When all bits of the LADRF registers are 0, no multicast

frames are accepted, except for broadcast frames.

Although broadcast frames are classified as special

multicast frames, they are treated differently by the

Am79C972 controller hardware. Broadcast frames are

always accepted, except when DRCVBC (CSR15, bit

14) is set and there is no Logical Address match.

None of the address filtering described above applies

when the Am79C972 controller is operating in the pro-

miscuous mode. In the promiscuous mode, all properly

formed packets are received, regardless of the con-

tents of their destination address fields. The promiscu-

ous mode overrides the Disable Receive Broadcast bit

(DRCVBC bit l4 in the MODE register) and the Disable

Receive Physical Address bit (DRCVPA, CSR15, bit

13).

The Am79C972 controller operates in promiscuous

mode when PROM (CSR15, bit 15) is set.

In addition, the Am79C972 controller provides the Ex-

ternal Address Detection Interface (EADI) to allow ex-

ternal address filtering. See the section External

Address Detection Interface for further detail.

The receive descriptor entry RMD1 contains three bits

that indicate which method of address matching

caused the Am79C972 controller to accept the frame.

Note that these indicator bits are only available when

the Am79C972 controller is programmed to use 32-bit

structures for the descriptor entries (BCR20, bit 7-0,

SWSTYLE is set to 2 or 3).

PAM (RMD1, bit 22) is set by the Am79C972 controller

when it accepted the received frame due to a match of

the frame’s destination address with the content of the

physical address register.

LAFM (RMD1, bit 21) is set by the Am79C972 control-

ler when it accepted the received frame based on the

value in the logical address filter register.

BAM (RMD1, bit 20) is set by the Am79C972 controller

when it accepted the received frame because the

frame’s destination address is of the type ’Broadcast’.

If DRCVBC (CSR15, bit 14) is cleared to 0, only BAM,

but not LAFM will be set when a Broadcast frame is re-

ceived, even if the Logical Address Filter is pro-

grammed in such a way that a Broadcast frame would

pass the hash filter. If DRCVBC is set to 1 and the Log-

ical Address Filter is programmed in such a way that a

Broadcast frame would pass the hash filter, LAFM will

be set on the reception of a Broadcast frame.

When the Am79C972 controller operates in promiscu-

ous mode and none of the three match bits is set, it is

an indication that the Am79C972 controller only ac-

cepted the frame because it was in promiscuous mode.

When the Am79C972 controller is not programmed to

be in promiscuous mode, but the EADI interface is en-

abled, then when none of the three match bits is set, it

is an indication that the Am79C972 controller only ac-

cepted the frame because it was not rejected by driving

the EAR pin LOW within 64 bytes after SFD.

See Table 6 for receive address matches.

Table 6.

Receive Address Match

Automatic Pad Stripping

During reception of an IEEE 802.3 frame, the pad field

can be stripped automatically. Setting ASTRP_RCV

(CSR4, bit 0) to 1 enables the automatic pad stripping

feature. The pad field will be stripped before the frame

is passed to the FIFO, thus preserving FIFO space for

additional frames. The FCS field will also be stripped,

since it is computed at the transmitting station based on

the data and pad field characters, and will be invalid for

a receive frame that has had the pad characters

stripped.

The number of bytes to be stripped is calculated from

the embedded length field (as defined in the ISO 8802-

3 (IEEE/ANSI 802.3) definition) contained in the frame.

The length indicates the actual number of LLC data

bytes contained in the message. Any received frame

which contains a length field less than 46 bytes will have

the pad field stripped (if ASTRP_RCV is set). Receive

PAM

LAF

MBAM

DRC

VBC

Comment

Frame accepted due to

PROM = 1 or no EADI

reject

Physical address match

Logical address filter

match;

frame is not of type

broadcast

Logical address filter

match;

frame can be of type

broadcast

Broadcast frame

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