Datasheet

The write-back memory section is the section where the DMAC stores the transfer descriptors for the
ongoing block transfers. WRBADDR points to the ongoing transfer descriptor of channel 0. All ongoing
transfer descriptors will be stored in a contiguous memory section where the transfer descriptors are
ordered according to their channel number. The figure below shows an example of linked descriptors on
DMA channel 0. For further details on linked descriptors, refer to 20.6.3.1 Linked Descriptors.
Figure 20-3. Memory Sections
Channel 0 – Descriptor n-1
Channel 0 – Last Descriptor
DESCADDR
DESCADDR
Device Memory Space
BASEADDR
Channel 0 – First Descriptor
Channel 1 – First Descriptor
Channel 2 – First Descriptor
Channel n – First Descriptor
Descriptor Section
WRBADDR Channel 0 Ongoing Descriptor
Channel 1 Ongoing Descriptor
Channel 2 Ongoing Descriptor
Channel n Ongoing Descriptor
Write-Back Section
Undefined
Undefined
Undefined
Undefined
Undefined
SRCADDR
DSTADDR
BTCTRL
DESCADDR
BTCNT
SRCADDR
DSTADDR
BTCTRL
DESCADDR
BTCNT
SRCADDR
DSTADDR
BTCTRL
0x00000000
BTCNT
The size of the descriptor and write-back memory sections is dependent on the number of the most
significant enabled DMA channel m, as shown below:
 = 128bits   + 1
For memory optimization, it is recommended to always use the less significant DMA channels if not all
channels are required.
The descriptor and write-back memory sections can either be two separate memory sections, or they can
share memory section (BASEADDR=WRBADDR). The benefit of having them in two separate sections, is
that the same transaction for a channel can be repeated without having to modify the first transfer
descriptor. The benefit of having descriptor memory and write-back memory in the same section is that it
requires less SRAM. In addition, the latency from fetching the first descriptor of a transaction to the first
burst transfer is executed, is reduced.
SAM D21 Family
DMAC – Direct Memory Access Controller
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 307