Datasheet

ATmega8515(L)

2512G–AVR–03/05

Special Function IO Register –

SFIOR

• Bit 6 – XMBK: External Memory Bus Keeper Enable

Writing XMBK to one enables the Bus Keeper on the AD7:0 lines. When the Bus Keeper

is enabled, AD7:0 will keep the last driven value on the lines even if the XMEM interface

has tri-stated the lines. Writing XMBK to zero disables the Bus Keeper. XMBK is not

qualified with SRE, so even if the XMEM interface is disabled, the Bus Keepers are still

activated as long as XMBK is one.

• Bit 5..3 – XMM2, XMM1, XMM0: External Memory High Mask

When the External Memory is enabled, all Port C pins are used for the high address

byte by default. If the full 64,928 bytes address space is not required to access the

External Memory, some, or all, Port C pins can be released for normal Port Pin function

as described in Table 4. As described in “Using all 64KB Locations of External Memory”

on page 32, it is possible to use the XMMn bits to access all 64KB locations of the Exter-

nal Memory.

Using all Locations of

External Memory Smaller than

64 KB

Since the external memory is mapped after the internal memory as shown in Figure 11,

the external memory is not addressed when addressing the first 608 bytes of data

space. It may appear that the first 608 bytes of the external memory are inaccessible

(external memory addresses 0x0000 to 0x025F). 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 0x825F. Since the External Memory

Address bit A15 is not connected to the external memory, addresses 0x8000 to 0x825F

will appear as addresses 0x0000 to 0x025F for the external memory. Addressing above

address 0x825F 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 0x0260

to 0x825F. This is illustrated in Figure 17.

Bit 76543210

– XMBK XMM2 XMM1 XMM0 PUD – PSR10 SFIOR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Table 4. 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

0 0 0 8 (Full 64,928 Bytes Space) None

0017 PC7

0106 PC7 - PC6

0115 PC7 - PC5

1004 PC7 - PC4

1013 PC7 - PC3

1102 PC7 - PC2

1 1 1 No Address High bits Full Port C