Texas Instruments TMS320DM357 User Manual
3.3.2.2
CPPI Receive Buffer Descriptor
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USB Controller Host and Peripheral Modes Operation
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:
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.
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
through
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.
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.
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.
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.
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
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