Datasheet
Figure 12-8 External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 0
(1)
ALE
T1 T2 T3
Write
Read
WR
T6
A15:8
Addre ss
Prev. addr.
DA7:0
Addre ss DataPrev. data
XX
RD
DA7:0 (XMBK = 0)
Data
Prev. data Addre ss
DataPrev. data Addre s s
DA7:0 (XMBK = 1)
System Clock (CLK
CPU
)
T4 T5
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 12-9 External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1
(1)
ALE
T1 T2 T3
Write
Read
WR
T7
A15:8
Address
Prev. a ddr.
DA7:0
Address DataPrev. da ta
XX
RD
DA7:0 (XMBK = 0)
Data
Prev. da ta Address
DataPrev. da ta Addre ss
DA7:0 (XMBK = 1)
Syste m Clock (CLK
CPU
)
T4 T5 T6
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).
12.6.8. Using all Locations of External Memory Smaller than 64 Kbytes
Since the external memory is mapped after the internal memory as shown in Figure 12-4 External
Memory with Sector Select on page 35, 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 64K bytes, for example 32K bytes, 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
external 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 Application
software, the external 32K bytes memory will appear as one linear 32K bytes address space from 0x1100
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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