ATmega64A datasheet(27/392 Pages) ATMEL

27 / 392 page

8160C–AVR–07/09

ATmega64A

Figure 7-8.

External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 0

(1)

Note:

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

SRW00 (lower sector).

The ALE pulse in period T6 is only present if the next instruction accesses the RAM (internal

or external).

Figure 7-9.

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).

7.5.7

Using all Locations of External Memory Smaller than 64 KB

Since the external memory is mapped after the internal memory as shown in Figure 7-4 on page

23, 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 external 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 exter-

nal memory. Addressing above address 0x90FF is not recommended, since this will address an

external memory location that is already accessed by another (lower) address. To the Applica-

tion software, the external 32 KB memory will appear as one linear 32 KB address space from

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)

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)

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