ATMEGA64-16AI datasheet(35/393 Pages) ATMEL

35 / 393 page

ATmega64(L)

2490L–AVR–10/06

Using all Locations of

External Memory Smaller than

64 KB

Since the external memory is mapped after the internal memory as shown in Figure 11,

the external memory is not addressed when addressing the first 4,352 bytes of data

space. It may appear that the first 4,352 bytes of the external memory are inaccessible

(external memory addresses 0x0000 to 0x10FF). However, when connecting an exter-

nal memory smaller than 64 KB, for example 32 KB, these locations are easily accessed

simply by addressing from address 0x8000 to 0x90FF. Since the External Memory

Address bit A15 is not connected to the external memory, addresses 0x8000 to 0x90FF

will appear as addresses 0x0000 to 0x10FF for the external memory. Addressing above

address 0x90FF is not recommended, since this will address an external memory loca-

tion that is already accessed by another (lower) address. To the Application software,

the external 32 KB memory will appear as one linear 32 KB address space from 0x1100

to 0x90FF. This is illustrated in Figure 17. Memory configuration B refers to the

ATmega103 compatibility mode, configuration A to the non-compatible mode.

When the device is set in ATmega103 compatibility mode, the internal address space is

4,096 bytes. This implies that the first 4,096 bytes of the external memory can be

accessed at addresses 0x8000 to 0x8FFF. To the Application software, the external 32

KB memory will appear as one linear 32 KB address space from 0x1000 to 0x8FFF.

Figure 17. Address Map with 32 KB External Memory

0x0000

0x10FF

0xFFFF

0x1100

0x7FFF

0x8000

0x90FF

0x9100

0x0000

0x10FF

0x1100

0x7FFF

Memory Configuration A

Memory Configuration B

Internal Memory

(Unused)

AVR Memory Map

External 32K SRAM

External

Memory

0x0000

0x0FFF

0xFFFF

0x1000

0x7FFF

0x8000

0x8FFF

0x9000

0x0000

0x0FFF

0x1000

0x7FFF

Internal Memory

(Unused)

AVR Memory Map

External 32K SRAM

External

Memory

ATMEGA64-16AI Datasheet(PDF) 35 Page - ATMEL Corporation

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