Texas Instruments TMS320DM646x manual Example 2. Receive Buffer Descriptor in C Structure Format

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Architecture

Example 2. Receive Buffer Descriptor in C Structure Format

/*

//EMAC Descriptor

//The following is the format of a single buffer descriptor

//on the EMAC.

*/
typedef struct _EMAC_Desc {
struct _EMAC_Desc *pNext;		/* Pointer to next descriptor in chain	*/
Uint8	*pBuffer;	/* Pointer to data buffer	*/
Uint32	BufOffLen;	/* Buffer Offset(MSW) and Length(LSW)
*/
Uint32	PktFlgLen;	/* Packet Flags(MSW) and Length(LSW)	*/
} EMAC_Desc;
/* Packet Flags			*/
#define EMAC_DSC_FLAG_SOP		0x80000000u
#define EMAC_DSC_FLAG_EOP		0x40000000u
#define EMAC_DSC_FLAG_OWNER		0x20000000u
#define EMAC_DSC_FLAG_EOQ		0x10000000u
#define EMAC_DSC_FLAG_TDOWNCMPLT		0x08000000u
#define EMAC_DSC_FLAG_PASSCRC		0x04000000u
#define EMAC_DSC_FLAG_JABBER		0x02000000u
#define EMAC_DSC_FLAG_OVERSIZE		0x01000000u
#define EMAC_DSC_FLAG_FRAGMENT		0x00800000u
#define EMAC_DSC_FLAG_UNDERSIZED		0x00400000u
#define EMAC_DSC_FLAG_CONTROL		0x00200000u
#define EMAC_DSC_FLAG_OVERRUN		0x00100000u
#define EMAC_DSC_FLAG_CODEERROR		0x00080000u
#define EMAC_DSC_FLAG_ALIGNERROR		0x00040000u
#define EMAC_DSC_FLAG_CRCERROR		0x00020000u
#define EMAC_DSC_FLAG_NOMATCH		0x00010000u

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.

Whether or not this field is updated depends on the setting of the RXBUFFEROFFSET register. 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.

28 Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) SPRUEQ6–December 2007

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