E t h e r n e t f r o n t - e n d ( E F E )

The MAC forwards good Ethernet packets through address filtering. Packets can be filtered based on station address, broadcast, and select multicast packets. Those packets that pass through address filtering are moved into system memory.

Receive buffer descriptor selection

When an Ethernet frame is received, the Ethernet receiver DMA channel searches the four buffer descriptors — starting with A and continuing with B, C, and D, as necessary — for an appropriately sized buffer. The search stops when the DMA channel finds an available buffer that is large enough to hold the entire Ethernet receive frame. The search also stops when the DMA channel finds a Control register whose Channel Enable (CE) bit is zero.

Figure 22 shows receiver buffer size selection.

2048-byte

receive

FIFO

RX DMA channel

DMA buffer

descriptor

list "A" smallest

DMA

buffer

descriptor

list

"B"

DMA buffer descriptor list

"C"

DMA buffer

descriptor

list

"D"

largest

Figure 22: Receive buffer size selection

1 5 2

N S 7 5 2 0 H a r d w a r e R e f e r e n c e , R e v . D 0 3 / 2 0 0 6

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Digi NS7520 manual Receive buffer descriptor selection, Dma