ATMEGA164P_14 Datasheet, PDF (22/377 Pages) ATMEL Corporation – High-performance, Low-power Atmel

6.5 I/O Memory
The I/O space definition of the ATmega164P/324P/644P is shown in “Register Summary” on
page 356.
All ATmega164P/324P/644P I/Os and peripherals are placed in the I/O space. All I/O locations
may be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between
the 32 general purpose working registers and the I/O space. I/O Registers within the address
range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these regis-
ters, the value of single bits can be checked by using the SBIS and SBIC instructions. Refer to
the instruction set section for more details. When using the I/O specific commands IN and OUT,
the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space
using LD and ST instructions, 0x20 must be added to these addresses. The
ATmega164P/324P/644P is a complex microcontroller with more peripheral units than can be
supported within the 64 location reserved in 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 instruc-
tions can be used.
For compatibility with future devices, reserved bits should be written to zero if accessed.
Reserved I/O memory addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most
other AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore
be used on registers containing such Status Flags. The CBI and SBI instructions work with reg-
isters 0x00 to 0x1F only.
The I/O and peripherals control registers are explained in later sections.
The ATmega164P/324P/644P contains three General Purpose I/O Registers, see “Register
Description” on page 23. These registers can be used for storing any information, and they are
particularly useful for storing global variables and Status Flags. General Purpose I/O Registers
within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI, SBIS, and
SBIC instructions.
22 ATmega164P/324P/644P
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