Digi NS9750 - Manual (Page 528)

D M A b u f f e r d e s c r i p t o r
5 0 4 N S 9 7 5 0 H a r d w a r e R e f e r e n c e
DMA buffer descriptor
All DMA channels operate using a buffer descriptor. Each DMA channel remains idle
until enabled through the DMA Channel Control register. When a DMA channel is
activated, it reads the DMA buffer descriptor pointed to by the Buffer Descriptor
Pointer register. When the current descriptor is retired, the next descriptor is
accessed from a circular buffer.
Each DMA buffer descriptor is four 32-bit words in length. Multiple buffer descriptors
are located in circular buffers of 1024 bytes, with a maximum of 64 buffer
descriptors. 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 encounters the W bit, the channel wraps around to the first descriptor.
Note:
Configuring a DMA channel for more than the maximum number of buffer
descriptors results in unpredictable behavior.
Figure 84 shows the DMA buffer descriptor. Table 303 explains each buffer descriptor
component.
Offset Description
0x00 Buffer descriptor pointer
0x01 Control register
0x02 Status register
0x03 Unused
0x04 Source Address register
0x05 Buffer Length register
0x06 Destination Address register
0x07 Control flags and transfer status
Table 302: DMA context memory entry