Analog Devices ADSP-218x External Overlay Memory Technical Note

About this manual

This document provides technical guidance on expanding the memory capacity of the ADSP-2181 Digital Signal Processor (DSP) using external overlay memory. It addresses scenarios where the internal 16k x 24 program memory and 16k x 16 data memory are insufficient for specific applications.

The text details the organization of this external memory, which allows access to up to two additional pages of 8k each for both program and data storage. It explains the control mechanisms involving the MMAP pin and overlay bits (PMOVLAY and DMOVLAY) to switch between internal and external memory segments.

What's inside

• Memory Organization
• External Overlay Memory
• MMap Pin Control
• Overlay Bits
• BDMaA Transfer
• Linker Configuration
• Address Sharing
• Code Execution Safety

Frequently asked questions

What happens when 16k of program memory is not enough for an application?

The simple solution is to add more memory, specifically up to 2 pages of external overlay memory.

How much additional program and data memory can be accessed?

The user can access up to another 16k of program memory and another 16k of data memory.

Can external overlay memory be used to run code?

Yes, external program memory is 24 bits wide and capable of fitting code, similar to internal memory.

Is it safe to change the overlay bit while running code?

No, it is hazardous; only routines in internal memory should control the overlay bit to prevent execution errors.