E.5.3.2 isochronous transfers, E.5.3.2, Isochronous transfers – Motorola USB08 User Manual
Page 184
Designer Reference Manual
USB08 Evaluation Board
184
Universal USB Device Driver (USBIO)
MOTOROLA
Universal USB Device Driver (USBIO)
A read operation will be completed if the whole buffer is filled or a short
packet is transmitted. A short packet is a packet that is shorter than the
FIFO size of the endpoint. For more information on receiving short
packets see below. To read a data packet with a length of zero, the
buffer size has to be at least one byte. A read operation with a NULL
buffer will be completed with success by the system without performing
a read operation of the USB.
The behavior of the read operation depends on the state of the flag
USBIO_SHORT_TRANSFER_OK
of the related pipe. This setting may be
changed by using the IOCTL_USBIO_SET_PIPE_PARAMETERS
(
) operation. The default state is defined by the registry
parameter
ShortTransferOk
. If the flag
USBIO_SHORT_TRANSFER_OK
is set a read operation that returns a
data packet that is shorter than the FIFO size of the endpoint is
completed with success. Otherwise, every data packet from the endpoint
that is smaller than the FIFO size causes an error.
E.5.3.2 Isochronous Transfers
For isochronous transfers the data buffer that is passed to the
ReadFile
or
WriteFile
function has to contain a header that
describes the location and the size of the data packets to be transferred.
Therefore, the buffer that follows the header is divided into packets.
Each packet is transmitted within an USB frame (normally 1 ms). The
size of the packet can be different in each frame. This allows to support
any data rate of the isochronous data stream.
The structure of the data buffer is shown in
. The buffer
contains an USBIO_ISO_TRANSFER_HEADER (
) structure
of variable size at offset zero and the data packets. The header contains
an USBIO_ISO_TRANSFER (
) structure that provides general
information about the transfer buffer. An important member of this
structure is the NumberOfPackets parameter. This parameter specifies
the number of data packets contained in the transfer buffer. The
maximum number of packets that can be used in a single transfer is
limited by the registry parameter
MaxIsoPackets
. Each data packet
has to be described by an USBIO_ISO_PACKET (
) structure