A b u f f e r d e s c r i p t o r, Destination

E X T E R N A L D M A

DMA buffer descriptor

D M A b u f f e r d e s c r i p t o r

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All DMA channels use a buffer descriptor. When a DMA channel is activated, it reads the DMA buffer descriptor that the Buffer Descriptor Pointer register points to. A DMA buffer descriptor is always fetched using an AHB INCR4 transaction to maximize AHB bus bandwidth. When the current descriptor is retired, the next descriptor is accessed from a circular buffer.

Each DMA buffer requires four 32-bit words to describe a transfer. Multiple buffer descriptors are located in circular buffers of 4096 bytes. The DMA channel’s buffer descriptor pointer provides the first buffer descriptor address. Subsequent buffer descriptors are found adjacent to the first descriptor. The final buffer descriptor is defined with its W bit set. When the DMA channel finds the W bit, the channel wraps around to the first descriptor.

Each DMA channel can address a maximum of 256 buffer descriptors.

DMA buffer descriptor diagram

Important: A DMA channel configured for more than the maximum number of buffer descriptors operates in an unpredictable fashion.

		31	30	29	28		16	15		0
OFFSET	+ 0					Source address

OFFSET	+ 4					Reserved			Buffer length

OFFSET	+ 8					Destination address

OFFSET	+ C	W	I	L	F	Reserved			Status

Field descriptions follow.

Source address [pointer]

Buffer length

The source address pointer field identifies the starting location of the source data. The source address can be aligned to any byte boundary.

Note: Optimal performance is achieved when the source address is aligned on a word boundary.

Buffer length indicates the number of bytes to move between the source and the destination. After completing the transfer, the DMA controller updates this field with the actual number of bytes moved. This is useful for debugging error conditions or determining the number of bytes transferred before the DONE signal was asserted.

Destination	The description address pointer field identifies the starting location of the source
address [pointer]	data’s destination; that is, to where the source data needs to be moved. The
	destination address can be aligned to any byte boundary.

340Hardware Reference NS9215

NS9215 specifications

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Digi NS9215 manual A b u f f e r d e s c r i p t o r, DMA buffer descriptor diagram, Destination

Models: NS9215