DP83821 datasheet(20/86 Pages) NSC

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3.0 Functional Description (Continued)

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Table 3-2 DP83821 Descriptor Format

In 32-bit addressing mode, the link and bufptr fields are 32-

bit fields. The DP83821 supports an optional extended

status field which supports VLAN and IP functions. To

enable

the

extsts

field,

software

should

set

the

EXTSTS_EN bit in the CFG register.

Some of the bit definitions in the cmdsts field are common

to both receive and transmit descriptors:

Table 3-3

cmdsts Common Bit Definitions

Table 3-4 Transmit

cmdsts Bit Definitions

offset

tag

description

0000h

link

32- or 64-bit "link" field to the next descriptor in the linked list. Bits 2-0 must be 0, as

descriptors must be aligned on 64-bit boundaries.

0004h or

0008h

bufptr

32- or 64-bit pointer to the first fragment or buffer. In transmit descriptors, the buffer can

begin on any byte boundary. In receive descriptors, the buffer must be aligned on a 64-bit

boundary.

0008h or

0010h

cmdsts

32-bit Command/Status Field (bit-encoded)

000ch or

0014h

extsts

OPTIONAL 32-bit Extended Status Field. Contains VLAN and IP information.

bit

tag

description

usage

OWN

Descriptor Ownership

Set to 1 by the data producer of the descriptor to transfer

ownership to the data consumer of the descriptor. Set to 0 by

the data consumer of the descriptor to return ownership to

the data producer of the descriptor. For transmit descriptors,

the driver is the data producer, and the DP83821 is the data

consumer. For receive descriptors, the DP83821 is the data

producer, and the driver is the data consumer.

More descriptors

Set to 1 to indicate that this is NOT the last descriptor in a

packet (there are MORE to follow). When 0, this descriptor is

the last descriptor in a packet. Completion status bits are

only valid when this bit is zero.

INTR

Interrupt

Set to 1 by software to request a "descriptor interrupt" when

DP83821 transfers the ownership of this descriptor back to

software.

SUPCRC

INCCRC

Suppress CRC / Include

CRC

In transmit descriptors, this indicates that CRC should not be

appended by the MAC. On receives, this bit will be set based

on the RXCFG:INCCRC bit.

Packet OK

In the last descriptor in a packet, this bit indicates that the

packet was either sent or received successfully.

26-16

---

The usage of these bits differ in receive and transmit

descriptors. See below for details.

15-0

SIZE

Descriptor Byte Count

Set to the size in bytes of the data.

bit

tag

description

usage

TXA

Transmit Abort

Transmission of this packet was aborted.

TFU

Transmit FIFO Underrun

The transmit FIFO was exhausted during the transmission of

this packet.

CRS

Carrier Sense Lost

Carrier was lost during the transmission of this packet. This

condition is not reported if TXCFG:CSI is set.

Transmit Deferred

Transmission of this packet was deferred.

Excessive Deferral

The length of deferral during the transmission of this packet

was excessive.

OWC

Out of Window Collision

The MAC encountered an "out of window" collision during

the transmission of this packet.

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