ATMEGA640 datasheet(31/449 Pages) ATMEL

31 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Using all 64KB Locations of

External Memory

Since the External Memory is mapped after the Internal Memory as shown in Figure 13,

only 56KB of External Memory is available by default (address space 0x0000 to 0x21FF

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 set-

ting Port C to output 0x00, and releasing the most significant bits for normal Port Pin

operation, the Memory Interface will address 0x0000 - 0x2FFF. See the following code

examples.

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

Note:

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

Assembly Code Example

(1)

; OFFSET is defined to 0x4000 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 PC7:6

ldi

r16, (1<<XMM1)

sts

XMCRB, r16

; write 0xAA to address 0x0001 of external

; memory

ldi

r16, 0xaa

sts

0x0001+OFFSET, r16

; re-enable PC7:6 for external memory

ldi

r16, (0<<XMM1)

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 0x4000

void

XRAM_example(void)

{

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

DDRC = 0xFF;

PORTC = 0x00;

XMCRB = (1<<XMM1);

*p = 0xaa;

XMCRB = 0x00;

*p = 0x55;

}

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