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 – Res: Reserved Bits
These bits are reserved and will always read as zero. When writing to this address location,
write these bits to zero for compatibility with future devices.
• Bit 2:0 – XMM2, XMM1, XMM0: External Memory High Mask
When the External Memory is enabled, all Port C pins are default used for the high address byte.
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 9-3. 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 External Memory.
Table 9-3.
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
None
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
8 (Full 56KB space)
7
PC7
6
PC7 - PC6
PC7 - PC5
PC7 - PC4
PC7 - PC3
PC7 - PC2
Full Port C
5
4
3
2
No Address high bits
38
ATmega640/1280/1281/2560/2561
2549L–AVR–08/07
ATMEL [ ATMEL ]
