ATMEGA640 datasheet(30/449 Pages) ATMEL

30 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Figure 18. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1

(1)

Note:

1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper

sector) or SRW00 (lower sector).

The ALE pulse in period T7 is only present if the next instruction accesses the RAM

(internal or external).

Using all Locations of

External Memory Smaller than

64 KB

Since the external memory is mapped after the internal memory as shown in Figure 13,

the external memory is not addressed when addressing the first 8,704 bytes of data

space. It may appear that the first 8,704 bytes of the external memory are inaccessible

(external memory addresses 0x0000 to 0x21FF). 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 0xA1FF. Since the External Memory

Address bit A15 is not connected to the external memory, addresses 0x8000 to 0xA1FF

will appear as addresses 0x0000 to 0x21FF for the external memory. Addressing above

address 0xA1FF 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 0x2200

to 0xA1FF. This is illustrated in Figure 19.

Figure 19. Address Map with 32 KB External Memory

ALE

A15:8

Address

Prev. addr.

DA7:0

Address

Data

Prev. data

DA7:0 (XMBK = 0)

Data

Prev. data

Address

Data

Prev. data

Address

DA7:0 (XMBK = 1)

System Clock (CLKCPU)

0x0000

0x21FF

0x2200

0x7FFF

0x8000

0x90FF

0x9100

0x0000

0x7FFF

Internal Memory

AVR Memory Map

External 32K SRAM

External

Memory

ATMEGA640 Datasheet(PDF) 30 Page - ATMEL Corporation

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