DP83901AV datasheet(12/62 Pages) NSC

12 / 62 page

70 Packet Reception (Continued)

LINKING RECEIVE BUFFER PAGES

If the length of the packet exhausts the first 256-byte 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 com-

parisons 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 sec-

ond comparison tests for equality between the DMA ad-

dress 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 ring 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

Linking Buffers

Before the DMA can enter the next contiguous 256-byte

buffer the address is checked for equality to PSTOP and to

the Boundary Pointer If neither are reached the DMA is

allowed to use the next buffer

Linking Receive Buffer Pages

TLF10469 – 10

1) Check for e to PSTOP

2) Check for e to Boundary

DP83901AV датащи(PDF) 12 Page - National Semiconductor (TI)