Rainbow Electronics ATmega8515L - Manual (Page 29)

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ATmega8515(L)
2512A–AVR–04/02
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 6..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 60KB 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 30, it is possible to use the XMMn bits to access all 64KB locations of the External
Memory.
Bit 7 6 5 4 3 2 1 0
– 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 Value 0 0 0 0 0 0 0 0
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 60 KB space) None
0 0 1 7 PC7
0 1 0 6 PC7 - PC6
0 1 1 5 PC7 - PC5
1 0 0 4 PC7 - PC4
1 0 1 3 PC7 - PC3
1 1 0 2 PC7 - PC2
1 1 1 No Address High bits Full Port C