AX88796BLF datasheet(16/82 Pages) ASIX

16 / 82 page

ASIX ELECTRONICS CORPORATION

AX88796BLF / AX88796BLI

Initialization Of The Buffer Ring

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

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 - 5 Receive Buffer Ring At Initialization

Beginning Of Reception

When the first packet begins arriving the AX88796B 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 of comparison test 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 Register can be used to protect against overwriting any area in

the receive buffer that has not yet been 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 the next buffer is performed.

4000h

8000h

Page Start

Page Stop

…

n-2

n-1

Boundary Page

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