ATmega640/1280/1281/2560/2561
file, the next 64 location the standard I/O Memory, then 416 locations of Extended I/O memory
and the next 8,192 locations address the internal data SRAM.
An optional external data SRAM can be used with the ATmega640/1280/1281/2560/2561. This
SRAM will occupy an area in the remaining address locations in the 64K address space. This
area starts at the address following the internal SRAM. The Register file, I/O, Extended I/O and
Internal SRAM occupies the lowest 4,608/8,704 bytes, so when using 64KB (65,536 bytes) of
External Memory, 60,478/56,832 Bytes of External Memory are available. See “External Mem-
ory Interface” on page 27 for details on how to take advantage of the external memory map.
When the addresses accessing the SRAM memory space exceeds the internal data memory
locations, the external data SRAM is accessed using the same instructions as for the internal
data memory access. When the internal data memories are accessed, the read and write strobe
pins (PG0 and PG1) are inactive during the whole access cycle. External SRAM operation is
enabled by setting the SRE bit in the XMCRA Register.
Accessing external SRAM takes one additional clock cycle per byte compared to access of the
internal SRAM. This means that the commands LD, ST, LDS, STS, LDD, STD, PUSH, and POP
take one additional clock cycle. If the Stack is placed in external SRAM, interrupts, subroutine
calls and returns take three clock cycles extra because the three-byte program counter is
pushed and popped, and external memory access does not take advantage of the internal pipe-
line memory access. When external SRAM interface is used with wait-state, one-byte external
access takes two, three, or four additional clock cycles for one, two, and three wait-states
respectively. Interrupts, subroutine calls and returns will need five, seven, or nine clock cycles
more than specified in the instruction set manual for one, two, and three wait-states.
The five different addressing modes for the data memory cover: Direct, Indirect with Displace-
ment, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register file,
registers R26 to R31 feature the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given
by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and post-incre-
ment, the address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O registers, and the 4,196/8,192 bytes of internal
data SRAM in the ATmega640/1280/1281/2560/2561 are all accessible through all these
addressing modes. The Register File is described in “General Purpose Register File” on page
14.
21
2549L–AVR–08/07
ATMEL [ ATMEL ]
