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The previous example translates to the following multi-descriptor setup:
Transmit Case
Transmit setup is as follows:
•
One packet descriptor with packet length (this is NOT data buffer length, the term packet used here
is to mean a transfer length not USB packet) field of 608 bytes and a Data Buffer of size 256 Bytes
linked to the 1st host buffer descriptor.
•
Two buffer descriptors with first buffer descriptor (this is the one linked to the packet descriptor)
defining the second data buffer size of 256 Bytes which in turn is linked to the next (second) buffer
descriptor.
•
Second buffer fescriptor with a fata buffer size of 96 bytes (can be greater, the packet descriptor
contain the size of the packet) linked to no other descriptor (NULL).
Receive Case
For this example since each data buffer size is 256 bytes, we would require a minimum of three
descriptors that would define data buffer size of 608 bytes. The receive setup is as follows:
•
Two buffer descriptors with 256 bytes of data buffer size
•
One buffer descriptor with 96 bytes (can be greater) of data buffer size
Within the rest of this section, the following nomenclature is used:
BD — Buffer Descriptor or Host Buffer Descriptor
DB — Data Buffer Size of 256 Bytes
PBD — Pointer to Host Buffer Descriptor
PD — Host Packet Descriptor
PPD — Pointer to Host Packet Descriptor
TXSQ — Transmit Queue or Transmit Submit Queue (for USB0 EP1, use Queues 32 or 33, for USB1
EP1 use Queues 62 or 63)
TXCQ — Transmit Completion Queue or Transmit Return Queue (for USB0 Tx EP1, use Queue 93,
and for USB1 Tx EP1 use Queue 125)
RXCQ — Receive Completion Queue or Receive Return Queue (for USB0 Rx EP1, use Queue 109, for
USB1 Rx EP1 use Queue 141)
RXSQ — Receive Free/Buffer Descriptor Queue or Receive Submit Queue. (For USB0 Rx EP1 Queue
0 is used and for USB1 Rx EP1 Queue 16 should be used
20.4.9.1 Transmit USB Data Flow Using DMA
The transmit descriptors and queue status configuration prior to the transfer taking place is shown in
Figure
20-17.
SPRUGX9 – 15 April 2011
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Preliminary
Communications Port Programming Interface (CPPI) 4.1 DMA
© 2011, Texas Instruments Incorporated
1813
Universal Serial Bus (USB)
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