Texas Instruments TMS320DM357 Cppi Receive Buffer Descriptor, Receive Buffer Descriptor Word

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3.3.2.2CPPI Receive Buffer Descriptor

Rx buffer descriptors provide information about a single corresponding Rx data buffer. Every Rx buffer has a single Rx buffer descriptor that stores the following information:

∙Pointer to the data buffer

∙Pointer to the next buffer descriptor in the queue

∙Buffer length and offset to the first valid byte of buffer data

∙Start of DMA packet (SOP) indicator

∙End of DMA packet (EOP) indicator

∙Ownership (only valid with SOP)

∙End of queue (EOQ) (only valid on EOP)

∙Packet Length (only valid with SOP)

Receive buffer descriptors contain 16 bytes (4 words) and must begin on 16-byte aligned addresses. Receive buffer descriptors may be linked together to form packets. Buffer descriptor SOP and EOP bits are used to delimit packets. Packets in turn may be linked together to form receive queue. Each queue consists of a chain of buffer descriptors linked together by Next Descriptor Pointers. The last buffer descriptor in a queue has a zero Next Descriptor Pointer. Each descriptor points to a data buffer yielding a queue of buffers.

Four Words of Receive Buffer Descriptor are described in Table 10 through Table 13.

		Table 10. Receive Buffer Descriptor Word 0
Bits	Name	Description
31:0	Next Descriptor	The 32-bit word aligned memory address of the next buffer descriptor in the Rx queue. This is the
	Pointer	mechanism used to reference the next buffer descriptor from the current buffer descriptor. If the
		value of this pointer is zero then the current buffer is the last buffer in the queue. The software sets
		the Next Descriptor Pointer.

		Table 11. Receive Buffer Descriptor Word 1
Bits	Name	Description
31:0	Buffer Pointer	The Buffer Pointer is the byte aligned memory address of the buffer associated with the buffer
		descriptor. The software sets the Buffer Pointer.

		Table 12. Receive Buffer Descriptor Word 2
Bits	Name	Description
31:16	Buffer Offset	The Buffer Offset indicates how many unused bytes are at the start of the buffer (SOP buffers only).
		A value of zero indicates that there are no unused bytes at the start of the buffer and that valid data
		begins on the first byte of the buffer. A value of 000Fh (decimal 15) indicates that the first 15 bytes
		of the buffer are to be ignored by the DMA controller while transmitting and that valid buffer data
		starts on byte 16 of the buffer. The software sets the buffer offset to zero on buffer initialization and
		the DMA controller overwrites the zero value on SOP packets with the Rx DMA State buffer offset
		value.
15:0	Buffer Length	The Buffer Length field indicates how many valid data bytes are in the buffer. Unused or protocol
		specific bytes at the beginning of the buffer are not counted in the Buffer Length field. The software
		sets the buffer length on buffer initialization. The DMA controller will overwrite the software initialized
		value on an EOP buffer when the number of received data bytes is less than the host initiated value.
		The DMA controller will overwrite the host initialized value on SOP when the Buffer Offset is greater
		than zero, or the Packet Length is less than the buffer length.

SPRUGH3–November 2008	Universal Serial Bus (USB) Controller	63
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