5 non-isochronous endpoint operation – NXP Semiconductors LPC24XX UM10237 User Manual
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NXP Semiconductors
UM10237
Chapter 13: LPC24XX USB device controller
14.4.11 LS_byte_extracted
Used in ATLE mode. When set, this bit indicates that the Least Significant Byte (LSB) of
the transfer length has been extracted. The extracted size is reflected in the
DMA_buffer_length field, bits 23:16.
14.4.12 MS_byte_extracted
Used in ATLE mode. When set, this bit indicates that the Most Significant Byte (MSB) of
the transfer size has been extracted. The size extracted is reflected in the
DMA_buffer_length field, bits 31:24. Extraction stops when LS_Byte_extracted and
MS_byte_extracted bits are set.
14.4.13 Present_DMA_count
The number of bytes transferred by the DMA engine. The DMA engine updates this field
after completing each packet transfer.
For isochronous endpoints, Present_DMA_count is the number of packets transferred; for
non-isochronous endpoints, Present_DMA_count is the number of bytes.
14.4.14 Message_length_position
Used in ATLE mode. This field gives the offset of the message length position embedded
in the incoming data packets. This is applicable only for OUT endpoints. Offset 0 indicates
that the message length starts from the first byte of the first packet.
14.4.15 Isochronous_packetsize_memory_address
The memory buffer address where the packet size information along with the frame
number has to be transferred or fetched. See
. This is applicable to
isochronous endpoints only.
14.5 Non-isochronous endpoint operation
14.5.1 Setting up DMA transfers
Software prepares the DMA Descriptors (DDs) for those physical endpoints to be enabled
for DMA transfer. These DDs are present in the USB RAM. The start address of the first
DD is programmed into the DMA Description pointer (DDP) location for the corresponding
endpoint in the UDCA. Software then sets the EPxx_DMA_ENABLE bit for this endpoint in
the USBEpDMAEn register (
).The DMA_mode bit field in the descriptor
is set to ‘00’ for normal mode operation. All other DD fields are initialized as specified in
DMA operation is not supported for physical endpoints 0 and 1 (default control endpoints).
14.5.2 Finding DMA Descriptor
When there is a trigger for a DMA transfer for an endpoint, the DMA engine will first
determine whether a new descriptor has to the fetched or not. A new descriptor does not
have to be fetched if the last packet transferred was for the same endpoint and the DD is
not yet in the retired state. An internal flag called DMA_PROCEED is used to identify this
condition (see