19.4.3.1.2 Adding a Descriptor to a List

To add descriptors to a linked list, perform the following actions:

1. Enable the Suspend Interrupt for the DMA channel.
2. Enable the DMA channel.
3. Reserve memory space in SRAM to configure a new descriptor.
4. Configure the new descriptor:
   • Set the Next Descriptor Address (DESCADDR) register
   • Set the Block Transfer Destination Address (DSTADDR) register
   • Set the Block Transfer Source Address (SRCADDR) register
   • Configure the Block Transfer Control (BTCTRL) register and set the Descriptor Valid (VALID) bit
5. Clear the VALID bit for the existing list and for the descriptor that needs to be updated.
6. Read DESCADDR from the Write-Back memory.
   • If the DMA has not already fetched the descriptor that requires changes (i.e., DESCADDR is incorrect):
     • Update the DESCADDR location of the descriptor in the list
     • Clear the Channel Suspend flag (CHINTFLAG.SUSP) if the Suspend Block Action was enabled (BTCTRL.BLOCKACT = 0x2)
     • Set the Descriptor Valid bit
     • Issue a Resume software command (CHCTRLB.CMD = 0x02) if the channel was suspended
   • If the DMA is executing the same descriptor as the one which requires changes:
     • Issue the SUSPEND software command (CHCTRLB.CMD = 0x1) and wait for the Channel Suspend (CHINTFLAG.SUSP) interrupt flag to be set
     • Update the next descriptor address DESCRADDR in the Write-Back memory
     • Clear CHINTFLAG.SUSP and issue the Resume software command (CHCTRLB.CMD = 0x02)
     • Update the DESCADDR location of the descriptor in the list
     • Optionally clear the Suspend Block Action
     • Set the Descriptor Valid (VALID) bit
7. Repeat from step three if more descriptors are needed.