Datasheet

ManualsBrandsAtmel ManualsAVR MicrocontrollersAVR RISC Microcontroller Flash 128KB TQFP 8MHz

2549O–AVR–05/12

ATmega640/1280/1281/2560/2561

Figure 9-5. 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 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 sector).

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

28, 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 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 location that is already accessed by another (lower) address. To

ALE

T1 T2 T3

Write

Read

A15:8

AddressPrev. addr.

DA7:0

Address DataPrev. data XX

DA7:0 (XMBK = 0)

DataPrev. data Address

DA7:0 (XMBK = 1)

System Clock (CLK

CPU

)

T4 T5

ALE