Preliminary
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Communications Port Programming Interface (CPPI) 4.1 DMA
Table 20-20. Buffer Descriptor Word 5 (BD5) Bit Field Descriptions
Bits
Field Name
Description
The 32-bit word aligned memory address of the next buffer descriptor in
the packet. If the value of this pointer is zero, then the current descriptor
31-0
Next description pointer
is the last descriptor in the packet. The CPU initializes this field for
transmitted packets. The DMA overwrites this field upon packet reception.
Table 20-21. Buffer Descriptor Word 6 (BD6) Bit Field Descriptions
Bits
Field Name
Description
31-22
Reserved
Reserved
The buffer length field indicates the original size of the buffer in bytes.
This value is not overwritten during reception. This value is read by the Rx
DMA to determine the actual buffer size as allocated by the CPU at
21-0
Original buffer 0 length
initialization. Since the buffer length in Word 3 is overwritten by the Rx
port during reception, this field is necessary to permanently store the
buffer size information.
Table 20-22. Buffer Descriptor Word 7 (BD7) Bit Field Descriptions
Bits
Field Name
Description
The buffer pointer is the byte-aligned memory address of the buffer
associated with the buffer descriptor. This value is not overwritten during
reception. This value is read by the Rx DMA to determine the actual
31-0
Original buffer 0 pointer
buffer location as allocated by the CPU at initialization. Since the buffer
pointer in Word 4 is overwritten by the Rx port during reception, this field
is necessary to permanently store the buffer pointer information.
20.4.2.3 Teardown Descriptor
The Teardown Descriptor is not like the packet or buffer descriptors since it is not used to describe
either a packet or a buffer. The teardown descriptor is always 32 bytes long and is comprised of 4 bytes
of actual teardown information and 28 bytes of pad. The teardown descriptor layout and associated bit
field descriptions are shown in the Figure below and Tables that follows. Since the 5 LSBs of the
descriptor pointers are used in CPPI 4.1 for the purpose of indicating the length of the descriptor, the
minimum size of a descriptor is 32 bytes.
The teardown descriptor is used to describe a channel halt and teardown event. Channel teardown
ensures that when a connection is no longer needed that the hardware can be reliably halted and any
remaining packets which had not yet been transmitted can be reclaimed by the host without the
possibility of losing buffer or descriptor references (which results in a memory leak).
Theteardown descriptor contains the following information:
•
Indicator which identifies the descriptor as a teardown packet descriptor
•
DMA controller number where teardown
•
Channel number within DMA where teardown occurred
•
Indicator of whether this teardown was for the Tx or Rx channel
1801
SPRUGX9 – 15 April 2011
Universal Serial Bus (USB)
© 2011, Texas Instruments Incorporated
Содержание TMS320C6A816 Series
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