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Rabbit 6000 User’s Manual
digi.com
260
24.3.5 Buffer Descriptor Modes
Flags in the control byte of a buffer descriptor (which gets loaded into DyCR) describe whether to halt on
completion of the transfer (or load another descriptor) and whether the next descriptor is adjacent in mem-
ory (which implies that the current descriptor is only 12 bytes long) or located at the link address. Each
descriptor can also be set to generate an interrupt on completion of the transfer. By using these options in
various ways, the Rabbit 6000 DMA can be operated in a number of conventional DMA modes.
The most common options are described here; others are certainly possible by different use of the available
linking methods.
24.3.5.1 Single Buffer
In the simplest mode, a single descriptor is set to halt and interrupt on completion.
24.3.5.2 Buffer Array
In this mode, an array of 12-byte descriptors is set up adjacent in memory; only the last buffer is set to halt
on completion. The last buffer is also typically set to interrupt on completion, but other buffer descriptors
in the array can also generate interrupts.
The advantage of the buffer array is that its descriptors require less memory than a full 16-byte descriptor.
The simplest version of the buffer array is a double buffer, which is frequently used to provide a reserve
buffer in case the application is slow in handling the first buffer once received (in this case, both buffers
are enabled to interrupt on completion).
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