Analog Devices ADSP-BF59x Blackfin, Hardware Reference Manual, Page: 123 / 772

ADSP-BF59x Blackfin Processor Hardware Reference 5-23
Direct Memory Access
registers using the new contents of
DMAx_CURR_DESC_PTR
, and incre-
ments
DMAx_CURR_DESC_PTR
. The first descriptor element that is
loaded is a new 16-bit value for the lower 16 bits of
DMAx_NEXT_DESC_PTR
, followed by the rest of the descriptor ele-
ments. The high 16 bits of
DMAx_NEXT_DESC_PTR
will retain their
former value. This supports a shorter, more efficient descriptor
than the large descriptor list model, which is suitable whenever the
application can place the channel’s descriptors in the same 64K
byte range of memory.
If
FLOW
= 7 (large descriptor list) the DMA controller copies the
32-bit
DMAx_NEXT_DESC_PTR
into
DMAx_CURR_DESC_PTR
. Next, the
DMA controller fetches a descriptor from memory into the DMA
registers using the new contents of
DMAx_CURR_DESC_PTR
, and incre-
ments
DMAx_CURR_DESC_PTR
. The first descriptor element that is
loaded is a new 32-bit value for the full
DMAx_NEXT_DESC_PTR
, fol-
lowed by the rest of the descriptor elements. The high 16 bits of
DMAx_NEXT_DESC_PTR
may differ from their former value. This sup-
ports a fully flexible descriptor list which can be located anywhere
in internal memory or external memory.
• If it is necessary to link from a descriptor chain whose descriptors
are in one 64K byte area to another chain whose descriptors are
outside that area, only the descriptor containing the link to the new
64K byte range needs to use
FLOW
= 7. All descriptors that reference
the same 64K byte area may use
FLOW
= 6.
• If
FLOW
= 4, 6, or 7 (descriptor array, small descriptor list, or large
descriptor list, respectively), the DMA controller clears the
DFETCH
bit in the
DMAx_IRQ_STATUS
register.
• If
FLOW
= any value but 0 (Stop), the DMA controller begins the
next work unit for that channel, which must contend with other
channels for priority on the memory buses. On the first memory
transfer of the new work unit, the DMA controller updates the