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ATMEGA8515 Datasheet(PDF) 31 Page - ATMEL Corporation |
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ATMEGA8515 Datasheet(HTML) 31 Page - ATMEL Corporation |
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31 / 257 page  31 ATmega8515(L) 2512K–AVR–01/10 Special Function IO Register – SFIOR • Bit 6 – 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 5..3 – XMM2, XMM1, XMM0: External Memory High Mask When the External Memory is enabled, all Port C pins are used for the high address byte by default. If the full 64,928 bytes 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 4. As described in “Using all 64KB Locations of External Memory” on page 33, it is possible to use the XMMn bits to access all 64KB locations of the Exter- nal Memory. Using all Locations of External Memory Smaller than 64 KB Since the external memory is mapped after the internal memory as shown in Figure 11, the external memory is not addressed when addressing the first 608 bytes of data space. It may appear that the first 608 bytes of the external memory are inaccessible (external memory addresses 0x0000 to 0x025F). However, when connecting an exter- nal memory smaller than 64 KB, for example 32 KB, these locations are easily accessed simply by addressing from address 0x8000 to 0x825F. Since the External Memory Address bit A15 is not connected to the external memory, addresses 0x8000 to 0x825F will appear as addresses 0x0000 to 0x025F for the external memory. Addressing above address 0x825F is not recommended, since this will address an external memory loca- tion 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 0x0260 to 0x825F. This is illustrated in Figure 17. Bit 7 6 54321 0 – 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 Value 0 0 00000 0 Table 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 0 0 0 8 (Full 64,928 Bytes Space) None 001 7 PC7 010 6 PC7 - PC6 011 5 PC7 - PC5 100 4 PC7 - PC4 101 3 PC7 - PC3 110 2 PC7 - PC2 1 1 1 No Address High bits Full Port C |
Similar Part No. - ATMEGA8515_10 |
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Similar Description - ATMEGA8515_10 |
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