Architecture
1653
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Universal Serial Bus 2.0 (USB) Controller
Table 34-14. Host Packet Descriptor Word 6 (HPD 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 34-15. Host Packet Descriptor Word 7 (HPD Word 7)
Bits
Name
Description
31-22
Reserved
Reserved
21-0
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.
34.2.8.3 Host Buffer Descriptor (Non-SOP Descriptor)
The Host Buffer Descriptor is identical in size and organization to a Host Packet Descriptor but does not
include valid information in the packet level fields and does not include a populated region for protocol
specific information. The packet level fields is not needed since the SOP descriptor contain this
information and additional copy of this data is not needed/necessary.
Host Buffer Descriptors are designed to be linked onto a Host Packet Descriptor or another Host Buffer
Descriptor to provide support for unlimited scatter / gather type operations. Host Buffer Descriptors provide
information about a single corresponding data buffer. Every Host buffer descriptor stores the following
information:
•
Pointer to the first valid byte in the data buffer
•
Length of the data buffer
•
Pointer to the next buffer descriptor in the packet
Host Buffer Descriptors always contain 32 bytes of required information. Since it is a requirement that it is
possible to convert a Host descriptor between a Buffer Descriptor and a Packet Descriptor (by filling in the
appropriate fields) in practice, Host Buffer Descriptors will be allocated using the same sizes as Host
Packet Descriptors. In addition, 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 always 32 bytes.
(For more information on Descriptor Size, see
).
The Host Buffer Descriptor layout is shown in
and described in
to
.