C2510Fx / CC2511Fx
controller sends a zero length data packet and
the USBCSIL.UNDERRUNbit will be asserted.
reduce the chance for this to happen by using
double buffering and use DMA to effectively
unload data packets.
Double buffering requires that a data packet is
loaded into the IN FIFO during the frame
preceding the frame where it should be sent. If
the first data packet is loaded before an IN
token is received, the data packet will be sent
during the same frame as it was loaded and
hence violate the double buffering strategy.
Thus, when double buffering is used, the
USBPOW.ISO_WAIT_SOF bit should be set to
1 to avoid this. Setting this bit will ensure that a
loaded data packet is not sent until the next
SOF token has been received.
An isochronous data packet in the OUT FIFO
may have bit errors. The hardware will detect
this condition and set USBCSOL.DATA_ERROR.
Firmware should therefore always check this
bit when unloading a data packet.
The AutoClear feature will typically not be used
for isochronous endpoints since the packet
size will increase or decrease from frame to
frame.
13.16.7
DMA
The AutoSet feature will typically not be used
for isochronous endpoints since the packet
size will increase or decrease from frame to
frame.
DMA should be used to fill the IN endpoint
FIFOs and empty the OUT endpoint FIFOs.
Using DMA will improve the read/write
performance significantly compared to using
the 8051 CPU. It is therefore highly
recommended to use DMA unless timing is not
critical or only a few bytes are to be
transferred.
13.16.6.7 Bulk/Interrupt OUT Endpoint
Interrupt OUT transfers occur at regular
intervals while bulk OUT transfers utilize
available
bandwidth
not
allocated
to
There are no DMA triggers for the USB
controller, meaning that DMA transfers must
be triggered by firmware.
isochronous, interrupt, or control transfers.
A Bulk/Interrupt OUT endpoint can be stalled
by setting the USBCSOL.SEND_STALL bit to
1. When the endpoint is stalled, the USB
The word size can be byte (8 bits) or word (16
bits). When word size transfer is used the
ENDIAN register must be set correctly (see
Section 11.2.3.6). The ENDIAN.USBRLE bit
selects whether a word is read as little or big
endian from the OUT FIFOs and the
ENDIAN.USBWLEbit selects whether a word is
written as little or big endian to the IN FIFOs.
Writing and reading words for the different
settings is shown in Figure 44 and Figure 45
respectively. Notice that the setting for these
bits will be used for all endpoints.
Consequently, it is not possible to have
multiple DMA channels active at once that use
different endianess. The ENDIANregister must
be configured to use big endian for both read
and write for a word size transfer to produce
the same result as a byte size transfer of an
even number of bytes. Word size transfers are
slightly more efficient than byte transfers.
controller will respond with
a
STALL
handshake when the host is done sending the
data packet. The data packet is discarded and
is not placed in the OUT FIFO. The USB
controller
will
assert
the
USBCSOL.SENT_STALL bit when the STALL
handshake is sent and generate an interrupt
request if the OUT endpoint interrupt is
enabled.
As the AutoSet feature is useful for bulk IN
endpoints, the AutoClear feature is useful for
OUT endpoints since many packets will be of
maximum size.
13.16.6.8 Isochronous OUT Endpoint
An Isochronous OUT endpoint is used to
transfer periodic data from the host to the USB
controller (one data packet every USB frame).
Refer to Section 13.5 for more details
regarding DMA.
If there is no buffer available when a data
packet
is
being
received,
the
USBCSOL.OVERRUN bit will be asserted and
the packet data will be lost. Firmware can
SWRS055D
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