ATMEGA8515-16PI(2010) View Datasheet(PDF) - Atmel Corporation

Part Name

Description

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ATMEGA8515-16PI
(Rev.:2010)

8-bit Microcontroller with 8K Bytes In-System Programmable Flash

Atmel Corporation 

ATmega8515(L)

Using all 64KB Locations of

External Memory

Since the External Memory is mapped after the Internal Memory as shown in Figure 11,

only 64,928 bytes of External Memory is available by default (address space 0x0000 to

0x025F is reserved for Internal Memory). However, it is possible to take advantage of

the entire External Memory by masking the higher address bits to zero. This can be

done by using the XMMn bits and control by software the most significant bits of the

address. By setting Port C to output 0x00, and releasing the most significant bits for nor-

mal Port Pin operation, the Memory Interface will address 0x0000 - 0x1FFF. See code

example below.

Assembly Code Example(1)

; OFFSET is defined to 0x2000 to ensure

; external memory access

; Configure Port C (address high byte) to

; output 0x00 when the pins are released

; for normal Port Pin operation

ldi r16, 0xFF

out DDRC, r16

ldi r16, 0x00

out PORTC, r16

; release PC7:5

ldi r16, (1<<XMM1)|(1<<XMM0)

out SFIOR, r16

; write 0xAA to address 0x0001 of external

; memory

ldi r16, 0xaa

sts 0x0001+OFFSET, r16

; re-enable PC7:5 for external memory

ldi r16, (0<<XMM1)|(0<<XMM0)

out SFIOR, r16

; store 0x55 to address (OFFSET + 1) of

; external memory

ldi r16, 0x55

sts 0x0001+OFFSET, r16

C Code Example(1)

#define OFFSET 0x2000

void XRAM_example(void)

{

unsigned char *p = (unsigned char *) (OFFSET + 1);

DDRC = 0xFF;

PORTC = 0x00;

SFIOR = (1<<XMM1) | (1<<XMM0);

*p = 0xaa;

SFIOR = 0x00;

*p = 0x55;

}

Note: 1. See “About Code Examples” on page 7.

Care must be exercised using this option as most of the memory is masked away.

33

2512K–AVR–01/10

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