ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 30

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Using all Locations of
External Memory Smaller than
64 KB
30
ATmega640/1280/1281/2560/2561
Figure 18. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1
Note:
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
System Clock (CLK
DA7:0 (XMBK = 0)
DA7:0 (XMBK = 1)
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper
DA7:0
A15:8
CPU
ALE
WR
RD
)
sector) or SRW00 (lower sector).
The ALE pulse in period T7 is only present if the next instruction accesses the RAM
(internal or external).
Prev. addr.
Prev. data
Prev. data
Prev. data
0x0000
0x21FF
0x2200
0x7FFF
0x8000
0x90FF
0x9100
T1
Address
Address
AVR Memory Map
Internal Memory
Address
T2
External
Memory
XX
Address
T3
Data
Data
Data
T4
External 32K SRAM
T5
T6
0x0000
0x7FFF
2549K–AVR–01/07
(1)
T7