Functional Description
5-54
ADSP-BF537 Blackfin Processor Hardware Reference
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 5-50.
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
mechanism controlled by the
DMA_TC_PER
and
DMA_TC_CNT
registers. This
mechanism performs the optimization without real-time processor inter-
vention, and without the need to program transfer bursts into the DMA
work unit streams. Traffic can be independently controlled for each of the
three buses (DAB, DCB, and DEB) with simple counters. In addition,
alternation of transfers among MDMA streams can be controlled with the
MDMA_ROUND_ROBIN_COUNT
field of the
DMA_TC_CNT
register. See
DMA Priority and Scheduling” on page 5-51
.
Using the traffic control features, the DMA system preferentially grants
data transfers on the DAB or memory buses which are going in the same
read/write direction as the previous transfer, until either the traffic control
counter times out, or until traffic stops or changes direction on its own.
When the traffic counter reaches zero, the preference is changed to the
opposite flow direction. These directional preferences work as if the prior-
ity of the opposite direction channels were decreased by 16.
For example, if channels 3 and 5 were requesting DAB access, but lower
priority channel 5 is going “with traffic” and higher priority channel 3 is
going “against traffic,” then channel 3’s effective priority becomes 19, and
channel 5 would be granted instead. If, on the next cycle, only channels 3
and 6 were requesting DAB transfers, and these transfer requests were
both “against traffic,” then their effective priorities would become 19 and
22, respectively. One of the channels (channel 3) is granted, even though
its direction is opposite to the current flow. No bus cycles are wasted,
other than any necessary delay required by the bus turnaround.
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 ...