Rainbow-electronics ATmega8515L Manuel d'utilisateur télécharger pdf (Page 30)

ATmega8515(L)

2512A–AVR–04/02

Using all 64KB Locations of

External Memory

Sincethe ExternalMemory is mapped after the InternalMemory asshowninFigure11,

only 60KB ofExternalMemory is available by default (address space0x0000 to 0x025F

isreservedforInternalMemory). However, it ispossibletotakeadvantage of the entire

ExternalMemory by masking the higher address bits to zero. Thiscan be done by using

the XMMn bits andcontrolbysoftwarethemost significant bits of theaddress. By set-

ting Port C to output 0x00, andreleasing themost significant bitsfor normal PortPin

operation, the Memory Interface will address 0x0000 - 0x1FFF. See codeexample

below.

Note: 1. Theexample codeassumes that the part specificheaderfileis included.

Caremust beexercised using this option as mostof thememory 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)

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

CCode 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;

}

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Rainbow-electronics ATmega8515L Manuel d'utilisateur Page 30

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