ATmega128 datasheet(35/386 Pages) ATMEL

35 / 386 page

2467S–AVR–07/09

ATmega128

Using all 64KB

Locations of External

Memory

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

60KB of External Memory is available by default (address space 0x0000 to 0x10FF 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 releas-

ing the most significant bits for normal Port Pin operation, the Memory Interface will address

0x0000 - 0x1FFF. See the following code examples.

Note:

1. See “About Code Examples” on page 9.

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

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)

sts

XMCRB, 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)

sts

XMCRB, 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;

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

*p = 0xaa;

XMCRB = 0x00;

*p = 0x55;

}

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