
Functional Description
5-20
ADSP-BF59x Blackfin Processor Hardware Reference
When
DMAx_CONFIG
is written directly by software, the DMA controller
recognizes this as the special startup condition that occurs when starting
DMA for the first time on this channel or after the engine has been
stopped (
FLOW
= 0).
When the descriptor fetch is complete and
DMAEN
= 1, the
DMACFG
descrip-
tor element that was read into
DMAx_CONFIG
assumes control. Before this
point, the direct write to
DMAx_CONFIG
had control. In other words, the
WDSIZE
,
DI_EN
,
DI_SEL
,
SYNC
, and
DMA2D
fields will be taken from the
DMACFG
value in the descriptor read from memory, while these field values
initially written to the
DMAx_CONFIG
register are ignored.
As
Figure 5-1 on page 5-18
and
Figure 5-2 on page 5-19
show, at startup
the
FLOW
and
NDSIZE
bits in
DMAx_CONFIG
determine the course of the
DMA setup process. The
FLOW
value determines whether to load more cur-
rent registers from descriptor elements in memory, while the
NDSIZE
bits
detail how many descriptor elements to fetch before starting DMA. DMA
registers not included in the descriptor are not modified from their prior
values.
If the
FLOW
value specifies small or large descriptor list modes, the
DMAx_NEXT_DESC_PTR
is copied into
DMAx_CURR_DESC_PTR
. Then, fetches of
new descriptor elements from memory are performed, indexed by
DMAx_CURR_DESC_PTR
, which is incremented after each fetch. If
NDPL
and/or
NDPH
is part of the descriptor, then these values are loaded into
DMAx_NEXT_DESC_PTR
, but the fetch of the current descriptor continues
using
DMAx_CURR_DESC_PTR
. After completion of the descriptor fetch,
DMAx_CURR_DESC_PTR
points to the next 16-bit word in memory past the
end of the descriptor.
If neither
NDPH
nor
NDPL
are part of the descriptor (that is, in descriptor
array mode,
FLOW
= 4), then the transfer from
NDPH
/
NDPL
into
DMAx_CURR_DESC_PTR
does not occur. Instead, descriptor fetch indexing
begins with the value in
DMAx_CURR_DESC_PTR
.
Содержание ADSP-BF59x Blackfin
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