7.10 I/O Memory

All ATmega3208/3209 I/Os and peripherals are located in the I/O space. The I/O address range from 0x00 to 0x3F can be accessed in a single cycle using IN and OUT instructions. The extended I/O space from 0x0040 - 0x0FFF can 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 registers, the value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the Instruction Set section for more details.

For compatibility with future devices, reserved bits should be written to ‘0’ if accessed. Reserved I/O memory addresses should never be written.

Some of the interrupt flags are cleared by writing a ‘1’ to them. On ATmega3208/3209 devices, the CBI and SBI instructions will only operate on the specified bit, and can, therefore, be used on registers containing such interrupt flags. The CBI and SBI instructions work with registers 0x00 - 0x1F only.

General Purpose I/O Registers

The ATmega3208/3209 devices provide four General Purpose I/O Registers. These registers can be used for storing any information, and they are particularly useful for storing global variables and interrupt flags. General Purpose I/O Registers, which reside in the address range 0x1C - 0x1F, are directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.