DP83821 datasheet(23/86 Pages) NSC

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3.0 Functional Description (Continued)

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3.13.2.1 Single Descriptor Packets

To represent a packet in a single descriptor, the MORE bit in the cmdsts field is set to 0.

Figure 3-10 Single Descriptor Packets

3.13.2.2 Multiple Descriptor Packets

A single packet may also cross descriptor boundaries. This

is indicated by setting the MORE bit in all descriptors

except the last one in the packet. Ethernet inter-networking

applications (bridges, switches, routers, etc.) can optimize

memory utilization by using a single small buffer per

receive descriptor, and allowing the DP83821 hardware to

use the minimum number of buffers necessary to store an

incoming packet.

Figure 3-11 Multiple Descriptor Packets

3.13.2.3 Descriptor Rings

The

simplest

and

recommended

organization

descriptors

fixed

ring

implementation.

initialization, the driver can set up a fixed list of descriptors

complete with links connecting the descriptors in a ring. All

descriptors will initially be owned by the producer of the

data (the driver for transmit, the DP83821 for receive). The

OWN bit is used by both driver and the DP83821 to

indicate data availability and to release descriptors back to

the producer. When using a descriptor ring, the driver

should never need to modify any fields of a descriptor it

does not own. For transmit, the driver should never assign

all descriptors to the device, reserving one descriptor to

terminate the list, preventing the device from wrapping

completely around the ring.

link

bufptr

MAC hdr

netwk hdr

data

single descriptor / single fragment

link

bufptr

MAC hdr

netwk hdr

data

multiple descriptor / single fragment

link

bufptr

link

bufptr

DP83821 Datasheet(PDF) 23 Page - National Semiconductor (TI)

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