Datasheet

Figure 20-6. Dynamic (Round-Robin) Priority Scheduling
Channel N
Channel N
Channel 0
Channel x
Channel x+1
Channel x last acknowledge request
Channel (x+1) last acknowledge request
Channel 0
Channel x
Channel x+1
Channel x+2
Lowest Priority
Highest Priority
Highest Priority
Lowest Priority
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20.6.2.5 Data Transmission
Before the DMAC can perform a data transmission, a DMA channel has to be configured and enabled, its
corresponding transfer descriptor has to be initialized, and the arbiter has to grant the DMA channel
access as the active channel.
Once the arbiter has granted a DMA channel access as the active channel (refer to DMA Block Diagram
section) the transfer descriptor for the DMA channel will be fetched from SRAM using the fetch bus, and
stored in the internal memory for the active channel. For a new block transfer, the transfer descriptor will
be fetched from the descriptor memory section (BASEADDR); For an ongoing block transfer, the
descriptor will be fetched from the write-back memory section (WRBADDR). By using the data transfer
bus, the DMAC will read the data from the current source address and write it to the current destination
address. For further details on how the current source and destination addresses are calculated, refer to
the section on Addressing.
The arbitration procedure is performed after each burst transfer. If the current DMA channel is granted
access again, the block transfer counter (BTCNT) of the internal transfer descriptor will be decremented
by the number of beats in a burst transfer, the optional output event Beat will be generated if configured
and enabled, and the active channel will perform a new burst transfer. If a different DMA channel than the
current active channel is granted access, the block transfer counter value will be written to the write-back
section before the transfer descriptor of the newly granted DMA channel is fetched into the internal
memory of the active channel.
When a block transfer has come to its end (BTCNT is zero), the Valid bit in the Block Transfer Control
register will be cleared (BTCTRL.VALID=0) before the entire transfer descriptor is written to the write-
back memory. The optional interrupts, Channel Transfer Complete and Channel Suspend, and the
optional output event Block, will be generated if configured and enabled. After the last block transfer in a
transaction, the Next Descriptor Address register (DESCADDR) will hold the value 0x00000000, and the
DMA channel will either be suspended or disabled, depending on the configuration in the Block Action bit
group in the Block Transfer Control register (BTCTRL.BLOCKACT). If the transaction has further block
transfers pending, DESCADDR will hold the SRAM address to the next transfer descriptor to be fetched.
The DMAC will fetch the next descriptor into the internal memory of the active channel and write its
content to the write-back section for the channel, before the arbiter gets to choose the next active
channel.
SAM D21 Family
DMAC – Direct Memory Access Controller
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 310