DP83907 datasheet(42/70 Pages) NSC

42 / 70 page

60 Operation of DP83907 (Continued)

63 PACKET RECEPTION

The Local DMA receive channel uses a Buffer Ring Struc-

ture comprised of a series of contiguous fixed length 256

byte (128 word) buffers for storage of received packets The

location of the Receive Buffer Ring is programmed in two

registers a Page Start and a Page Stop Register Ethernet

packets consist of a distribution of shorter link control pack-

ets and longer data packets the 256 byte buffer length pro-

vides a good compromise between short packets and long-

er packets to most efficiently use memory In addition these

buffers provide memory resources for storage of back-to-

back packets in loaded networks The assignment of buffers

for storing packets is controlled by Buffer Management Log-

ic in the DP83907 The Buffer Management Logic provides

three basic functions linking receive buffers for long pack-

ets recovery of buffers when a packet is rejected and recir-

culation of buffer pages that have been read by the host

At initialization a portion of the 64k byte (or 32k word) ad-

dress space is reserved for the receive buffer ring Two

eight bit registers

the Page Start Address Register

(PSTART) and the Page Stop Address Register (PSTOP)

define the physical boundaries of where the buffers reside

The DP83907 treats the list of buffers as a logical ring

whenever the DMA address reaches the Page Stop Ad-

dress the DMA is reset to the Page Start Address

TLF12082 – 24

FIGURE 29 Buffer Ring at Initialization

Initialization of the Buffer Ring

Two static registers and two working registers control the

operation of the Buffer Ring These are the Page Start Reg-

ister Page Stop Register (both described previously) the

Current Page Register and the Boundary Pointer Register

The Current Page Register points to the first buffer used to

store a packet and is used to restore the DMA for writing

status to the Buffer Ring or for restoring the DMA address in

the event of a Runt packet a CRC or Frame Alignment

error The Boundary Register points to the first packet in the

Ring not yet read by the host If the local DMA address ever

reaches the Boundary reception is aborted The Boundary

Pointer is also used to initialize the Remote DMA for remov-

ing a packet and is advanced when a packet is removed A

simple analogy to remember the function of these registers

is that the Current Page Register acts as a Write Pointer and

the Boundary Pointer acts as a Read Pointer

Note 1

At initialization the Page Start Register value should be loaded into

both the Current Page Register and the Boundary Pointer Register if using

the Send Packet Command If using manual Remote Reads the Current

Page Register must always remain 1 a Boundary Register

Note 2

The Page Start Register must not be initialized to 00H

Beginning of Reception

When the first packet begins arriving the DP83907 begins

storing the packet at the location pointed to by the Current

Page Register An offset of 4 bytes is saved in this first

buffer to allow room for storing receive status correspond-

ing to this packet

TLF12082-25

FIGURE 30 Received Packet Enters the Buffer Pages

DP83907 Datasheet(PDF) 42 Page - National Semiconductor (TI)

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