RM0390 Rev 4 217/1328
RM0390 Direct memory access controller (DMA)
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In all cases, the burst size multiplied by the data size must not exceed the FIFO size (data
size can be: 1 (byte), 2 (half-word) or 4 (word)).
Incomplete burst transfer at the end of a DMA transfer may happen if one of the following
conditions occurs:
• For the AHB peripheral port configuration: the total number of data items (set in the
DMA_SxNDTR register) is not a multiple of the burst size multiplied by the data size.
• For the AHB memory port configuration: the number of remaining data items in the
FIFO to be transferred to the memory is not a multiple of the burst size multiplied by the
data size.
In such cases, the remaining data to be transferred is managed in single mode by the DMA,
even if a burst transaction is requested during the DMA stream configuration.
Note: When burst transfers are requested on the peripheral AHB port and the FIFO is used
(DMDIS = 1 in the DMA_SxCR register), it is mandatory to respect the following rule to
avoid permanent underrun or overrun conditions, depending on the DMA stream direction:
If (PBURST × PSIZE) = FIFO_SIZE (4 words), FIFO_Threshold = 3/4 is forbidden with
PSIZE = 1, 2 or 4 and PBURST = 4, 8 or 16.
This rule ensures that enough FIFO space at a time is free to serve the request from the
peripheral.
FIFO flush
The FIFO can be flushed when the stream is disabled by resetting the EN bit in the
DMA_SxCR register and when the stream is configured to manage peripheral-to-memory or
memory-to-memory transfers. If some data are still present in the FIFO when the stream is
disabled, the DMA controller continues transferring the remaining data to the destination
(even though stream is effectively disabled). When this flush is completed, the transfer
complete status bit (TCIFx) in the DMA_LISR or DMA_HISR register is set.
The remaining data counter DMA_SxNDTR keeps the value in this case to indicate how
many data items are currently available in the destination memory.
Note that during the FIFO flush operation, if the number of remaining data items in the FIFO
to be transferred to memory (in bytes) is less than the memory data width (for example 2
bytes in FIFO while MSIZE is configured to word), data is sent with the data width set in the
MSIZE bit in the DMA_SxCR register. This means that memory is written with an undesired
Half-word
1/4
Forbidden
Forbidden
Forbidden
1/2 1 burst of 4 beats
3/4
Forbidden
Full 2 bursts of 4 beats 1 burst of 8 beats
Word
1/4
Forbidden
Forbidden
1/2
3/4
Full 1 burst of 4 beats
Table 34. FIFO threshold configurations (continued)
MSIZE FIFO level MBURST = INCR4 MBURST = INCR8 MBURST = INCR16
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