AT90CAN64 datasheet(34/428 Pages) ATMEL

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7679H–CAN–08/08

AT90CAN32/64/128

4.5.8

Using all Locations of External Memory Smaller than 64 KB

Since the external memory is mapped after the internal memory as shown in Figure 4-4, the

external memory is not addressed when addressing the first “ISRAM size” bytes of data space. It

may appear that the first “ISRAM size” bytes of the external memory are inaccessible (external

memory addresses 0x0000 to “ISRAM end”). However, when connecting an external memory

smaller than 64 KB, for example 32 KB, these locations are easily accessed simply by address-

ing from address 0x8000 to “ISRAM end + 0x8000”. Since the External Memory Address bit A15

is not connected to the external memory, addresses 0x8000 to “ISRAM end + 0x8000” will

appear as addresses 0x0000 to “ISRAM end” for the external memory. Addressing above

address “ISRAM end + 0x8000” is not recommended, since this will address an external mem-

ory location 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 “XMem start” to

“XMem start + 0x8000”. This is illustrated in Figure 4-10.

Figure 4-10. Address Map with 32 KB External Memory

Table 4-5.

Port C Pins Released as Normal Port Pins when the External Memory is Enabled

XMM2

XMM1

XMM0

# Bits for External Memory Address

Released Port Pins

8 (Full External Memory Space)

None

001

PC7

010

PC7 .. PC6

011

PC7 .. PC5

100

PC7 .. PC4

101

PC7 .. PC3

110

PC7 .. PC2

No Address high bits

Full Port C

(Unused)

Internal Memory

0x0000

XMem start

ISRAM end

0xFFFF

AVR Memory Map

External Memory

0x8000

0x7FFF

XMem start + 0x8000

ISRAM end + 0x8000

External 32K SRAM (Size=0x8000)

0x7FFF

0x0000

XMem start

ISRAM end

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