ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 34

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Using all 64KB Locations of
External Memory
34
ATmega64(L)
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 controlled 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 examples
below.
Note:
Care must be exercised using this option as most of the memory is masked away.
Assembly Code Example
C Code Example
#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;
}
; 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
out
ldi
out
; release PC7:5
ldi
sts
; write 0xAA to address 0x0001 of external
; memory
ldi
sts
; re-enable PC7:5 for external memory
ldi
sts
; store 0x55 to address (OFFSET + 1) of
; external memory
ldi
sts
1. The example code assumes that the part specific header file is included.
r16, 0xFF
DDRC, r16
r16, 0x00
PORTC, r16
r16, (1<<XMM1)|(1<<XMM0)
XMCRB, r16
r16, 0xaa
0x0001+OFFSET, r16
r16, (0<<XMM1)|(0<<XMM0)
XMCRB, r16
r16, 0x55
0x0001+OFFSET, r16
(1)
(1)
2490G–AVR–03/04