34
2513L–AVR–03/2013
ATmega162/V
Using all 64KB
Locations of External
Memory
Since the external memory is mapped after the internal memory as shown in
Figure 11, only
64,256 Bytes of external memory are available by default (address space 0x0000 to 0x04FF 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 normal Port Pin operation, the Memory Interface will
address 0x0000 - 0x1FFF. See code example below.
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.
; 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
#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;
}