
ADSP-BF59x Blackfin Processor Hardware Reference
5-45
Direct Memory Access
is not too large a fraction of the total, then all peripherals’ requests should
be granted as required.
Occasionally, instantaneous DMA traffic might exceed the available band-
width, causing congestion. This may occur if L1 or external memory is
temporarily stalled, perhaps for an SDRAM page swap or a cache line fill.
Congestion might also occur if one or more DMA channels initiates a
flurry of requests, perhaps for descriptor fetches or to fill a FIFO in the
DMA or in the peripheral.
If congestion persists, lower priority DMA peripherals may become
starved for data. Even though the peripheral’s priority is low, if the neces-
sary data transfer does not take place before the end of the peripheral’s
regular interval, system failure may result. To minimize this possibility,
the DMA unit detects peripherals whose need for data has become urgent,
and preferentially grants them service at the highest priority.
A DMA channel’s request for memory service is defined as urgent if both:
• The channel’s FIFO is not ready for a DAB bus transfer (that is, a
transmit FIFO is empty or a receive FIFO is full), and
• The peripheral is asserting its DMA request line.
Descriptor fetches may be urgent if they are necessary to initiate or con-
tinue a DMA work unit chain for a starving peripheral.
DMA requests from an MDMA channel become urgent when handshaked
operation is enabled and the
DMARx
edge count exceeds the value stored in
the
HMDMAx_ECURGENT
register. If handshaked operation is disabled, soft-
ware can control urgency of requests directly by altering the
DRQ
bit field
in the
HMDMAx_CONTROL
register.
Содержание ADSP-BF59x Blackfin
Страница 64: ...Development Tools 1 22 ADSP BF59x Blackfin Processor Hardware Reference...
Страница 74: ...Processor Specific MMRs 2 10 ADSP BF59x Blackfin Processor Hardware Reference...
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Страница 244: ...Programming Examples 6 40 ADSP BF59x Blackfin Processor Hardware Reference...
Страница 345: ...ADSP BF59x Blackfin Processor Hardware Reference 9 9 Core Timer Unique Information for the ADSP BF59x Processor None...
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Страница 398: ...Unique Information for the ADSP BF59x Processor 11 42 ADSP BF59x Blackfin Processor Hardware Reference...
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Страница 700: ...Programming Examples 16 78 ADSP BF59x Blackfin Processor Hardware Reference...
Страница 738: ...Boundary Scan Architecture B 8 ADSP BF59x Blackfin Processor Hardware Reference...