Rainbow-electronics ATmega8515L Uživatelský manuál stáhnout pdf (Strana 29)

ATmega8515(L)

2512A–AVR–04/02

Special Function IO Register –

SFIOR

•Bit6–XMBK: External Memory Bus Keeper Enable

Writing XMBK to one enables the BusKeeper on theAD7:0 lines. When the BusKeeper

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 BusKeeper. XMBK is not

qualifiedwithSRE, soevenif the XMEM interfaceisdisabled, the BusKeepers are still

activated aslong asXMBKis one.

• Bit 6..3 – XMM2, XMM1, XMM0: External Memory High Mask

When the ExternalMemory is enabled, all Port Cpins areusedfor the high address

byte by default. If the full 60KB address spaceis not required to access the External

Memory, some, or all, Port Cpinscan be releasedfor normal PortPinfunction as

described in Table4.Asdescribed in “Using all 64KB Locations ofExternalMemory”on

page 30, it ispossibletousethe XMMn bits to access all 64KB locations of the External

Memory.

Bit 76543 210

– 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 Value00000000

Table 4. Port C Pins Released asNormal PortPinswhen the ExternalMemory is

Enabled

XMM2 XMM1 XMM0 # Bits for External Memory Address Released Port Pins

0008(Full 60 KB space)None

0017 PC7

0106 PC7 - PC6

0115 PC7 - PC5

1004 PC7 - PC4

1013 PC7 - PC3

1102 PC7 - PC2

111NoAddress HighbitsFull Port C

1 2 ... 24 25 26 27 28 29 30 31 32 33 34 ... 222 223

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Rainbow-electronics ATmega8515L Uživatelský manuál Strana 29