ADSP-BF537 Blackfin Processor Hardware Reference
5-53
Direct Memory Access
transfer corresponding to a count of 1, the MDMA stream selection is
passed automatically to the other stream with zero overhead, and the
MDMA_ROUND_ROBIN_COUNT
counter is reloaded with the period value
P
from
MDMA_ROUND_ROBIN_PERIOD
. In this cycle, if the other MDMA stream is
ready to perform a transfer, the stream selection is locked on the new
MDMA stream. If the other MDMA stream is not ready to perform a
transfer, then no transfer is performed, and on the next cycle the stream
selection unlocks and becomes free again.
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
bandwidth by a factor of
1/(P+1)
. If both MDMA streams are used, how-
ever, 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, for example).
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
Summary of Contents for Blackfin ADSP-BF537
Page 42: ...Contents xlii ADSP BF537 Blackfin Processor Hardware Reference ...
Page 90: ...Development Tools 1 32 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 138: ...Programming Examples 4 26 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 340: ...SDC Programming Examples 6 84 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 606: ...Programming Examples 9 94 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 660: ...Programming Examples 10 54 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 720: ...Electrical Specifications 11 60 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 840: ...Programming Examples 13 42 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 876: ...Programming Examples 14 36 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 938: ...Programming Examples 15 62 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 958: ...Programming Examples 17 12 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 986: ...Programming Examples 18 28 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 1162: ...G 26 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 1218: ...Index I 56 ADSP BF537 Blackfin Processor Hardware Reference ...