
Functional Description
5-48
ADSP-BF59x Blackfin Processor Hardware Reference
ready to perform a transfer, then no transfer is performed, and the stream
selection unlocks and becomes free again on the next cycle.
If round-robin operation is used when only one MDMA stream is active,
one idle cycle will occur for each P MDMA data cycles, slightly lowering
the bandwidth by a factor of 1/(P+1). However if both MDMA streams
are used, memory DMA can operate continuously with zero additional
overhead for alternation of streams . (Other than overhead cycles normally
associated with reversal of read/write direction to memory). By selection
of various round-robin period values P, which limit how often the
MDMA streams alternate, maximal transfer efficiency can be maintained.
Traffic Control
In the Blackfin DMA architecture, there are two completely separate but
simultaneous prioritization processes—the DAB bus prioritization and the
memory bus (DCB and DEB) prioritization. Peripherals that are request-
ing DMA via the DAB bus, and whose data FIFOs are ready to handle the
transfer, compete with each other for DAB bus cycles. Similarly but sepa-
rately, channels whose FIFOs need memory service (prefetch or
post-write) compete together for access to the memory buses. MDMA
streams compete for memory access as a unit, and source and destination
may be granted together if their memory transfers do not conflict. In this
way, internal-to-external or external-to-internal memory transfers may
occur at the full system clock rate (
SCLK
). Examples of memory conflict
include simultaneous access to the same memory space and simultaneous
attempts to fetch descriptors. Special processing may occur if a peripheral
is requesting DMA but its FIFO is not ready (for example, an empty
transmit FIFO or full receive FIFO).
For more information, see “Tempo-
rary DMA Urgency” on page 5-44.
Traffic control is an important consideration in optimizing use of DMA
resources. Traffic control is a way to influence how often the transfer
direction on the data buses may change, by automatically grouping same
direction transfers together. The DMA block provides a traffic control
Содержание ADSP-BF59x Blackfin
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