Rainbow-electronics ATmega128L Bedienungsanleitung PDF herunterladen (Seite 31)

ATmega128(L)

2467B–09/01

• Bit 0 - Res: Reserved Bit

This is a reserved bit and will always read as zero. When writing to this address location,

write this bit to zero for compatibility with future devices.

External Memory Control

• 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..4 - Res: Reserved Bits

These are reserved bits 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

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 mem-

ory, some, or all, Port C pins can be released for normal Port Pin function as described

Table 5. 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.

Bit 76543210

XMBK ––––XMM2 XMM1 XMM0 XMCRB

Read/WriteR/WRRRRR/WR/WR/W

Initial value00000000

Table 5. 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

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

1 2 ... 26 27 28 29 30 31 32 33 34 35 36 ... 330 331

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Rainbow-electronics ATmega128L Bedienungsanleitung Seite 31