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Peripheral Architecture
2.5.4.1Next Descriptor Pointer
The next descriptor pointer points to the
The value of pNext should never be altered once the descriptor is in an active transmit queue, unless its current value is NULL. If the pNext pointer is initially NULL, and more packets need to be queued for transmit, the software application may alter this pointer to point to a newly appended descriptor. The EMAC will use the new pointer value and proceed to the next descriptor unless the pNext value has already been read. In this latter case, the transmitter will halt on the transmit channel in question, and the software application may restart it at that time. The software can detect this case by checking for an end of queue (EOQ) condition flag on the updated packet descriptor when it is returned by the EMAC.
2.5.4.2Buffer Pointer
The buffer pointer is the
2.5.4.3Buffer Offset
This
Note that this value is only checked on the first descriptor of a given packet (where the start of packet (SOP) flag is set). It can not be used to specify the offset of subsequent packet fragments. Also, since the buffer pointer may point to any
The range of legal values for this field is 0 to (Buffer Length – 1).
2.5.4.4Buffer Length
This
2.5.4.5Packet Length
This
2.5.4.6Start of Packet (SOP) Flag
When set, this flag indicates that the descriptor points to a packet buffer that is the start of a new packet. In the case of a single fragment packet, both the SOP and end of packet (EOP) flags are set. Otherwise, the descriptor pointing to the last packet buffer for the packet sets the EOP flag. This bit is set by the software application and is not altered by the EMAC.
SPRU941A
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