AT90USB646 datasheet(35/434 Pages) ATMEL

35 / 434 page

7593A–AVR–02/06

AT90USB64/128

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 5-6. As described in

“Using all 64KB Locations of External Memory” on page 36, it is possible to use the XMMn bits to

access all 64KB locations of the External Memory.

5.5.8

Using all Locations of External Memory Smaller than 64 KB

Since the external memory is mapped after the internal memory as shown in Figure 5-4, the

external memory is not addressed when addressing the first 8,448/4,352 bytes (128/64Kbytes

version) of data space. It may appear that the first 8,448/4,352 bytes of the external memory are

inaccessible (external memory addresses 0x0000 to 0x10FF or 0x0000 to 0x20FF). However,

when connecting an external memory smaller than 64 KB, for example 32 KB, these locations

are easily accessed simply by addressing from address 0x8000 to 0xA1FF. Since the External

Memory Address bit A15 is not connected to the external memory, addresses 0x8000 to 0xA1FF

will appear as addresses 0x0000 to 0x21FF for the external memory. Addressing above address

0xA1FF is not recommended, since this will address an external memory location that is already

accessed by another (lower) address. To the Application software, the external 32 KB memory

will appear as one linear 32 KB address space from 0x2200 to 0xA1FF. This is illustrated in Fig-

ure 5-10.

Table 5-6.

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 56KB 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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