ATMEGA640 datasheet(38/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

9.4.2

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 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

60Kbytes 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 9-4. As described in “Using all 64Kbytes Locations of External

Memory” on page 32, it is possible to use the XMMn bits to access all 64Kbytes locations of the External Memory.

Bit

210

(0x75)

XMBK

–

XMM2

XMM1

XMM0

XMCRB

Read/Write

R/W

Initial Value

Table 9-4.

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

8 (Full 56Kbytes 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

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