Rainbow Electronics ATmega162V - Manual (Page 16)

16
ATmega162/V
2513E–AVR–09/03
SRAM Data Memory Figure 9 shows how the ATmega162 SRAM Memory is organized. Memory configura-
tion B refers to the ATmega161 compatibility mode, configuration A to the non-
compatible mode.
The ATmega162 is a complex microcontroller with more peripheral units than can be
supported within the 64 location reserved in the Opcode for the IN and OUT instructions.
For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and
LD/LDS/LDD instructions can be used. The Extended I/O space does not exist when the
ATmega162 is in the ATmega161 compatibility mode.
In Normal mode, the first 1280 Data Memory locations address both the Register File,
the I/O Memory, Extended I/O Memory, and the internal data SRAM. The first 32 loca-
tions address the Register File, the next 64 location the standard I/O memory, then 160
locations of Extended I/O memory, and the next 1024 locations address the internal
data SRAM.
In ATmega161 compatibility mode, the lower 1120 Data Memory locations address the
Register File, the I/O Memory, and the internal data SRAM. The first 96 locations
address the Register File and I/O Memory, and the next 1024 locations address the
internal data SRAM.
An optional external data SRAM can be used with the ATmega162. 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 uses the occupies the lowest 1280 bytes in Normal mode, and the
lowest 1120 bytes in the ATmega161 compatibility mode (Extended I/O not present), so
when using 64KB (65,536 bytes) of External Memory, 64,256 Bytes of External Memory
are available in Normal mode, and 64,416 Bytes in ATmega161 compatibility mode. See
“External Memory Interface” on page 24 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 (PD7 and PD6) are inactive during the whole access
cycle. External SRAM operation is enabled by setting the SRE bit in the MCUCR
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
2-byte Program Counter is pushed and popped, and external memory access does not
take advantage of the internal pipeline 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. Interrupt, subroutine calls
and returns will need five, seven, or nine clock cycles more than specified in the instruc-
tion set manual for one, two, and three wait-states.
The five different addressing modes for the data memory cover: Direct, Indirect with Dis-
placement, 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.