Motorola DSP56305 User Manual download pdf (Page 53)

4-4 Optimizing DSP56300/DSP56600 Applications MOTOROLA

Using the DMA

Using Slow, Low-Cost Memories

Another possible application of this kind is in a multi-tasking

operating system: the DMA can be periodically activated by the

timer, and load the program of the next process, while the core

executes another code segment.

4.3 USING SLOW, LOW-COST MEMORIES

In many systems, data that is stored in external memory is not

frequently used, and can be loaded at a relatively slow rate. In

principle, this permit the use of slow, low-cost memories.

Interfacing such memories, however, can sometime require glue

hardware and management software, thus cutting possible savings

and performance gain. The DSP56300 was designed specifically

with features to support the use of low-cost memories, thus

reducing the overall system cost and software development time.

The External Memory Interface (DSP56300 only) supports glueless

connection to various types of external memory devices (DRAMs,

SRAMs and SSRAMs), and has the following supporting features:

• Programmable number of wait states

• Specialized address attributes pins, which can be used as

programmable chip-selects, masking address ranges and

memory spaces (x, y or p); each may support a different

memory type

• On-chip DRAM controller with programmable in-page and

out-of-page wait states and refresh control

More detailed information on these and other features could be

found in Section 2 of the

DSP56300 Family Manual

The parallel operation of the DMA, as in the example above, is

especially suited to load data slowly from an external memory

device. The resulting wait states do not effect program execution at

the core, as long as the core does not also attempt to use the external

port. For applications in which such contentions may occur, the user

can assign priorities between the core and each DMA channel, and

change them dynamically. For details, see Section 8 of the

DSP56300 Family Manual

The DMA and BIU have a specialized Packing mode to support

external 8-bit memory devices. In this mode, each external DMA

access is translated to three hardware accesses to consecutive

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Motorola DSP56305 User Manual Page 53