Cypress EX-USB FX3, Руководство программиста

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FX3 Programmers Manual, Doc. # 001-64707 Rev. *C 41
FX3 Overview
Figure 3-14. .DMA Configurations
The buffers do not reside in the peripherals; they are created (memory allocated) in the Sysmem
area and a pointer is made available to the associated socket. Therefore any DMA access to the
Sysmem needs to go through the DMA and the System AHB, the widths of which directly determine
the DMA bandwidth.
Certain peripherals may contain tens of sockets, each associated with different buffer chains. To
achieve reasonably high bandwidth on DMA transfers over sockets while maintaining reasonable
AHB bus width, Socket requests for buffer reads/ writes are not directly time multiplexed; rather the
sockets are grouped into threads, the requests of which are time multiplexed. Only one socket in a
group can actively execute at any instant of time. The thread handling the socket group needs to be
reconfigured every time a different socket in the group needs to execute. The thread-socket relations
are handled by the thread controller. The DMA adapter block converts read/write queries on the
threads to AHB requests and launches them on to the AHB. A single thread controller and DMA
adapter block may be shared by more than one peripheral.
A socket of a peripheral can be configured to either write to or read from buffers, not both. As a
convention, sockets used to write into system buffers are called 'Producers' and the direction of data
flow in this case is referred to as 'Ingress'. Conversely, sockets used to read out system buffers are
called 'Consumers' and the direction of data flow in this case is referred to as 'Egress'. Every socket
has a set of registers, which indicate the status of the socket such as number of bytes transferred
over the socket, current sub buffer being filled or emptied, location of the current buffer in memory,
and no buffer available.
A DMA transfer in FX3 can be envisioned as a flow of data from a producer socket on one peripheral
to a consumer socket on the other through buffers in the Sysmem. Specific DMA instructions called
'Descriptors' enable synchronizing between the sockets. Every buffer created in the Sysmem has a
descriptor associated with it that contains essential buffer information such as its address, empty/full
status and the next buffer/descriptor in chain. Descriptors are placed at specific locations in the
Sysmem which are regularly monitored and updated by the peripherals' sockets.
In a typical DMA transaction, the data source-peripheral sends data over the producing socket,
which fills the current buffer of the latter. Upon the last byte of the producer's current buffer being
written, the producer socket updates the descriptor of its current buffer and loads the next buffer in
chain. This process goes on until either the buffer chain ends or the next buffer in chain is not free
(empty to produce into). The corresponding consumer socket waits until the buffer it points to
becomes available (gets filled to consume from). When available, the data destination-peripheral is
notified to start reading the buffer over the consumer socket. When the last byte of the current buffer
is read, the consumer socket updates the descriptor of its current buffer and loads the next buffer in
(C) multiple buffers of
different sizes
chained to create
a large buffer
(B) multiple buffers of
the same size
chained to create
a large buffer
(A) a single large
buffer of fixed size
(D) a single large
circular buffer
(E)
circular buffer
using multiple
sub buffers