Dma Functional Description - ST STM32L4x6 Reference Manual

RM0351
10.4

DMA functional description

The DMA controller performs direct memory transfer by sharing the system bus with the
®
Cortex -M4 core. The DMA request may stop the CPU access to the system bus for some
bus cycles, when the CPU and DMA are targeting the same destination (memory or
peripheral). The bus matrix implements round-robin scheduling, thus ensuring at least half
of the system bus bandwidth (both to memory and peripheral) for the CPU.
10.4.1 DMA transactions
After an event, the peripheral sends a request signal to the DMA Controller. The DMA
controller serves the request depending on the channel priorities. As soon as the DMA
Controller accesses the peripheral, an Acknowledge is sent to the peripheral by the DMA
Controller. The peripheral releases its request as soon as it gets the Acknowledge from the
DMA Controller. Once the request is de-asserted by the peripheral, the DMA Controller
release the Acknowledge. If there are more requests, the peripheral can initiate the next
transaction.
In summary, each DMA transfer consists of three operations:
•
The loading of data from the peripheral data register or a location in memory addressed
through an internal current peripheral/memory address register. The start address used
for the first transfer is the base peripheral/memory address programmed in the
DMA_CPARx or DMA_CMARx register
•
The storage of the data loaded to the peripheral data register or a location in memory
addressed through an internal current peripheral/memory address register. The start
address used for the first transfer is the base peripheral/memory address programmed
in the DMA_CPARx or DMA_CMARx register
•
The post-decrementing of the DMA_CNDTRx register, which contains the number of
transactions that have still to be performed.
10.4.2 Arbiter
The arbiter manages the channel requests based on their priority and launches the
peripheral/memory access sequences.
The priorities are managed in two stages:
•
Software: each channel priority can be configured in the DMA_CCRx register. There
are four levels:
–
–
–
–
•
Hardware: if 2 requests have the same software priority level, the channel with the
lowest number will get priority versus the channel with the highest number. For
example, channel 2 gets priority over channel 4.
Very high priority
High priority
Medium priority
Low priority
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