Rainbow Electronics ATmega128L - Manual (Page 32)

32
ATmega128(L)
2467B–09/01
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 set-
ting Port C to output 0x00, and releasing the most significant bits for normal Port Pin
operation, the Memory Interface will address 0x0000-0x1FFF. See the following code
examples.
Note: 1. The example code assumes that the part specific header file is included.
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<
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<
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<
*p = 0xaa;
XMCRB = 0x00;
*p = 0x55;
}