Datasheet

36
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
XMCRB – External Memory
Control Register B
• Bit 7– 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 – Res: Reserved Bits
These bits are reserved and will always read as zero. When writing to this address loca-
tion, write these bits to zero for compatibility with future devices.
• Bit 2:0 – XMM2, XMM1, XMM0: External Memory High Mask
W hen 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 Mem-
ory, some, or all, Port C pins can be released for normal Port Pin function as described
in Table 9. As described in “Using all 64KB Locations of External Memory” on page 31,
it is possible to use the XMMn bits to access all 64KB locations of the External Memory.
Bit 7654 3 210
(0x75) XMBK – – – – XMM2 XMM1 XMM0 XMCRB
Read/Write R/W RRR RR/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 9. Port C Pins Released as N ormal Port Pins when the External Memory is
Enabled
XMM2 XMM1 XMM0 # Bits for External Memory Address Released Port Pins
0 0 0 8 (Full 56KB space) None
0017 PC7
0106 PC7 - PC6
0115 PC7 - PC5
1004 PC7 - PC4
1013 PC7 - PC3
1102 PC7 - PC2
111No Address high bits Full Port C