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Architecture
1655
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Universal Serial Bus 2.0 (USB) Controller
Table 34-20. Host Buffer Descriptor Word 4 (HBD Word 4)
Bits
Name
Description
31-0
Buffer 0 Pointer
The Buffer Pointer is the byte aligned memory address of the buffer associated with
the buffer descriptor. The CPU initializes this field for transmitted packets; the DMA
overwrites this field on packet reception.
Table 34-21. Host Buffer Descriptor Word 5 (HBD Word 5)
Bits
Name
Description
31-0
Next Descriptor Pointer
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 is the last descriptor in the
packet. The CPU initializes this field for transmitted packets; the DMA overwrites this
field on packet reception.
Table 34-22. Host Buffer Descriptor Word 6 (HBD Word 6)
Bits
Name
Description
31-22
Reserved
Reserved
21-0
Original Buffer 0 Length
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 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 34-23. Host Buffer Descriptor Word 7 (HBD Word 7)
Bits
Name
Description
31-0
Original Buffer 0 Pointer
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 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.
34.2.8.4 Teardown Descriptor
The Teardown Descriptor is not like the Host 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 is shown in
and described in
and
. 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).
The Teardown Descriptor contains the following information:
•
Indicator which identifies the descriptor as a Teardown Packet Descriptor
•
DMA Controller Number where teardown occurred
•
Channel number within DMA where teardown occurred
•
Indicator of whether this teardown was for the Tx or Rx channel