ADSP-BF537 Blackfin Processor Hardware Reference
5-47
Direct Memory Access
• Descriptor fetches consume one DMA memory cycle per 16-bit
word read from memory, but do not delay transfers on the DAB
bus.
• Initialization of a DMA channel stalls DMA activity for one cycle.
This occurs when
DMAEN
changes from 0 to 1 or when the
SYNC
bit
is set to 1 in the
DMAx_CONFIG
register.
Several of these factors may be minimized by proper design of the applica-
tion software. It is often possible to structure the software to avoid
internal and external memory conflicts by careful allocation of data buffers
within banks and pages, and by planning for low cache activity during
critical DMA operations. Furthermore, unnecessary MMR accesses can be
minimized, especially by using descriptors or autobuffering.
Efficiency loss caused by excessive direction changes (thrashing) can be
minimized by the processor’s traffic control features, described in the next
section.
The MDMA controllers are clocked by
SCLK
. If source and destination are
in different memory spaces (one internal and one external), the internal
and external memory transfers are typically simultaneous and continuous,
maintaining 100% bus utilization of the internal and external memory
interfaces. This performance is affected by core-to-system clock frequency
ratios. At ratios below about 2.5:1, synchronization and pipeline latencies
result in lower bus utilization in the system clock domain. At a clock ratio
of 2:1, for example, DMA typically runs at 2/3 of the system clock rate. At
higher clock ratios, full bandwidth is maintained.
If source and destination are in the same memory space (both internal or
both external), the MDMA stream typically prefetches a burst of source
data into the FIFO, and then automatically turns around and delivers all
available data from the FIFO to the destination buffer. The burst length is
dependent on traffic, and is equal to 3 plus the memory latency at the
DMA in
SCLK
s (which is typically 7 for internal transfers and 6 for exter-
nal transfers).
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 ...