ATMEGA640 datasheet(31/436 Pages) MICROCHIP

31 / 436 page

ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Figure 9-6.

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 sec-

tor).

The ALE pulse in period T7 is only present if the next instruction accesses the RAM (internal or external).

9.1.5

Using all Locations of External Memory Smaller than 64Kbytes

Since the external memory is mapped after the internal memory as shown in Figure 9-1 on page 27, 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 external memory smaller than 64Kbytes, for example 32Kbytes, these locations are easily accessed

simply by addressing from address 0x8000 to 0xA1FF. Since the External Memory Address bit A15 is not con-

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

32Kbytes memory will appear as one linear 32Kbytes address space from 0x2200 to 0xA1FF. This is illustrated in

Figure 9-7.

Address Map with 32Kbytes 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

0xFFFF

0x2200

0x7FFF

0x8000

0x90FF

0x9100

0x0000

0x7FFF

Internal Memory

(Do Not Use)

AVR Memory Map

External 32K SRAM

External

Memory

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