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SOP | EOP
60
0 60
pBuffer
pNext
Packet A
60 bytes
0
SOP
Packet B
Fragment 1
512 bytes
512
1514
pBuffer
pNext
EOP
0
0
−−−
Packet B
Fragment 3
500 bytes
502
pBuffer
−−−
500
pNext
−−−
pBuffer
pNext
Packet B
Fragment 2
502 bytes
SOP | EOP
0
Packet C
1514 bytes
1514
pBuffer
pNext (NULL)
1514
EMAC Functional Architecture
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For example, consider three packets to be transmitted, Packet A is a single fragment (60 bytes), Packet B
is fragmented over three buffers (1514 bytes total), and Packet C is a single fragment (1514 bytes).
Figure 11 shows the linked list of descriptors to describe these three packets.
Figure 11. Typical Descriptor Linked List
2.5.2 Transmit and Receive Descriptor Queues
The EMAC module processes descriptors in linked list chains (Section 2.5.1). The lists controlled by the
EMAC are maintained by the application software through the head descriptor pointer (HDP) registers.
Since the EMAC supports eight channels for both transmit and receive, there are eight head descriptor
pointer registers for both.
They are designated as follows:
• TXnHDP: Transmit Channel n DMA Head Descriptor Pointer Register
• RXnHDP: Receive Channel n DMA Head Descriptor Pointer Register
After an EMAC reset, and before enabling the EMAC for send or receive, you must initialize all 16 head
descriptor pointer registers to zero.
The EMAC uses a simple system to determine ownership of a descriptor (either the EMAC or the
application software). There is a flag in the descriptor flags field called OWNER. When this flag is set, the
EMAC owns the referenced packet.
NOTE: Ownership is assigned on a packet-based granularity, not on descriptor granularity. Thus,
only SOP descriptors use the OWNER flag. The EMAC patches the SOP descriptor of the
corresponding packet and clears the OWNER flag as packets are processed. This means
that the EMAC is finished processing all descriptors up to and including the first with the
EOP flag set. This indicates that you have reached the end of the packet. This may only be
one descriptor with both the SOP and EOP flags set.
32
C6472/TCI6486 EMAC/MDIO SPRUEF8F–March 2006–Revised November 2010
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