AX88796ALF datasheet(25/68 Pages) ASIX

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AX88796ALF

3-in-1 Local Bus Fast Ethernet Controller

INITIALIZATION OF THE BUFFER RING

Two static registers and two working registers control the operation of the Buffer Ring. These are the Page

Start Register, 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 removing 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.

Buffer #1

Buffer #2

Buffer #3

…

Buffer #n

Physical Memory Map

Logic Receive Buffer Ring

Fig - 9

Receive Buffer Ring At Initialization

BEGINNING OF RECEPTION

When the first packet begins arriving the AX88796A and begins storing the packet at the location pointed to

by the Current Page Register. An offset of 4 bytes is reserved in this first buffer to allow room for storing

receives status corresponding to this packet.

LINKING RECEIVE BUFFER PAGES

If the length of the packet exhausts the first 256 bytes buffer, the DMA performs a forward link to the next

buffer to store the remainder of the packet. For a maximal length packet the buffer logic will link six buffers to

store the entire packet. Buffers cannot be skipped when linking; a packet will always be stored in contiguous

buffers. Before the next buffer can be linked, the Buffer Management Logic performs two comparisons. The

first comparison tests for equality between the DMA address of the next buffer and the contents of the Page

Stop Register. If the buffer address equals the Page Stop Register, the buffer management logic will restore the

DMA to the first buffer in the Receive Buffer Ring value programmed in the Page Start Address Register. The

second comparison test for equality between the DMA address of the next buffer address and the contents of

the Boundary Pointer Register. If the two values are equal the reception is aborted. The Boundary Pointer

read. When linking buffers, buffer management will never cross this pointer, effectively avoiding any

overwrites. If the buffer address does not match either the Boundary Pointer or Page Stop Address, the link to

4000h

8000h

Page Start

Page Stop

…

n-2

n-1

Boundary Page

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