DMA controller (DMA)
RM0090
318/1731
DocID018909 Rev 11
10.3.11
Single and burst transfers
The DMA controller can generate single transfers or incremental burst transfers of 4, 8 or 16
beats.
The size of the burst is configured by software independently for the two AHB ports by using
the MBURST[1:0] and PBURST[1:0] bits in the DMA_SxCR register.
The burst size indicates the number of beats in the burst, not the number of bytes
transferred.
To ensure data coherence, each group of transfers that form a burst are indivisible: AHB
transfers are locked and the arbiter of the AHB bus matrix does not degrant the DMA master
during the sequence of the burst transfer.
Depending on the single or burst configuration, each DMA request initiates a different
number of transfers on the AHB peripheral port:
•
When the AHB peripheral port is configured for single transfers, each DMA request
generates a data transfer of a byte, half-word or word depending on the PSIZE[1:0] bits
in the DMA_SxCR register
•
When the AHB peripheral port is configured for burst transfers, each DMA request
generates 4,8 or 16 beats of byte, half word or word transfers depending on the
PBURST[1:0] and PSIZE[1:0] bits in the DMA_SxCR register.
The same as above has to be considered for the AHB memory port considering the
MBURST and MSIZE bits.
In direct mode, the stream can only generate single transfers and the MBURST[1:0] and
PBURST[1:0] bits are forced by hardware.
The address pointers (DMA_SxPAR or DMA_SxM0AR registers) must be chosen so as to
ensure that all transfers within a burst block are aligned on the address boundary equal to
the size of the transfer.
The burst configuration has to be selected in order to respect the AHB protocol, where
bursts must
not
cross the 1 KB address boundary because the minimum address space that
can be allocated to a single slave is 1 KB. This means that the 1 KB address boundary
should not be crossed by a burst block transfer, otherwise an AHB error would be
generated, that is not reported by the DMA registers.
Table 47. Restriction on NDT versus PSIZE and MSIZE
PSIZE[1:0] of DMA_SxCR
MSIZE[1:0] of DMA_SxCR
NDT[15:0] of DMA_SxNDTR
00 (8-bit)
01 (16-bit)
must be a multiple of 2
00 (8-bit)
10 (32-bit)
must be a multiple of 4
01 (16-bit)
10 (32-bit)
must be a multiple of 2