DP83902A datasheet(15/70 Pages) NSC

15 / 70 page

70 Packet Reception (Continued)

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

At initialization the Page Start Register value should be loaded into

both Current Page Register and the Boundary Pointer Register

Note

The Page Start Register must not be initialized to 00H

BEGINNING OF RECEPTION

When the first packet begins arriving the ST-NIC 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

Buffer Ring at Initialization

TLF11157 – 10

Received Packet Enters the Buffer Pages

TLF11157 – 11

DP83902A датащи(PDF) 15 Page - National Semiconductor (TI)