Texas Instruments TMS320TCI6486 manual Next Descriptor Pointer

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2.5.5.1Next Descriptor Pointer

The next descriptor pointer indicates the 32-bit word aligned memory address of the next buffer descriptor in the receive queue. The pointer creates a linked list of buffer descriptors. If the value of the pointer is zero, then the current buffer is the last buffer in the queue. The software application must set this value prior to adding the descriptor to the active receive list. This pointer is not altered by the EMAC.

The value of pNext should never be altered once the descriptor is in an active receive queue, unless its current value is NULL. If the pNext pointer is initially NULL, and more empty buffers can be added to the pool, the software application may alter this pointer to indicate 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. If the pNext value has already been read, the receiver will halt the receive channel in question, and the software application may restart it at that time. The software can detect this case by searching for an end-of-queue (EOQ) condition flag on the updated packet descriptor when it is returned by the EMAC.

2.5.5.2Buffer Pointer

The buffer pointer is the byte-aligned memory address of the memory buffer associated with the buffer descriptor. The software application must set this value prior to adding the descriptor to the active receive list. This pointer is not altered by the EMAC.

2.5.5.3Buffer Offset

This 16-bit field must be initialized to zero by the software application before adding the descriptor to a receive queue.

This field will be updated depending on the RXBUFFEROFFSET register setting. When the offset register is set to a non-zero value, the received packet is written to the packet buffer at an offset given by the value of the register, and this value is also written to the buffer offset field of the descriptor.

When a packet is fragmented over multiple buffers because it does not fit in the first buffer supplied, the buffer offset only applies to the first buffer in the list, which is where the start-of-packet (SOP) flag is set in the corresponding buffer descriptor. In other words, the buffer offset field is only updated by the EMAC on SOP descriptors.

The range of legal values for the BUFFEROFFSET register is 0 to (Buffer Length - 1) for the smallest value of buffer length for all descriptors in the list.

2.5.5.4Buffer Length

This 16-bit field has two functions:

•Before the descriptor is first placed on the receive queue by the application software, the software initializes the buffer length field with the physical size of the empty data buffer specified by the buffer pointer field.

•After the empty buffer has been processed by the EMAC and filled with received data bytes, the EMAC updates the buffer length field to reflect the actual number of valid data bytes written to the buffer.

2.5.5.5Packet Length

This 16-bit field specifies the number of data bytes in the entire packet. The software application initializes this value to zero for empty packet buffers. The EMAC fills in the value on the first buffer used for a given packet, as signified by the EMAC setting a start-of-packet (SOP) flag. The EMAC sets the packet length on all SOP buffer descriptors.

2.5.5.6Start-of-Packet (SOP) Flag

When set, this flag indicates that the descriptor points to the starting packet buffer of a new packet. For 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 has the EOP flag set. The software application initially clears this flag before adding the descriptor to the receive queue. The EMAC sets this bit on SOP descriptors.

38	C6472/TCI6486 EMAC/MDIO	SPRUEF8F –March 2006 –Revised November 2010
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