Datasheet

30
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
T1 T2 T3
Write
Read
WR
T7
A15:8
AddressPrev. addr.
DA7:0
Address DataPrev. data XX
RD
DA7:0 (XMBK = 0)
DataPrev. data Address
DataPrev. data Address
DA7:0 (XMBK = 1)
System Clock (CLK
CPU
)
T4 T5 T6
0x0000
0x21FF
0x2200
0x7FFF
0x8000
0x90FF
0x9100
0x0000
0x7FFF
Internal Memory
AVR Memory Map
External 32K SRAM
External
Memory