Datasheet

8246A–AVR–11/09

ATtiny2313A/4313

5.4 I/O Memory

The I/O space definition of the ATtiny2313A/4313 is shown in “Register Summary” on page 252.

All ATtiny2313A/4313 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 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. 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.

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 registers

0x00 to 0x1F only.

The I/O and peripherals control registers are explained in later sections.

5.4.1 General Purpose I/O Registers

The ATtiny2313A/4313 contains three General Purpose I/O Registers. 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.

5.5 Register Description

5.5.1 EEAR – EEPROM Address Register

• Bit 7 – EEAR7: EEPROM Address

This is the most significant EEPROM address bit of ATtiny4313. In devices with less EEPROM,

i.e. ATtiny2313A, this bit is reserved and will always read zero. The initial value of the EEPROM

Address Register (EEAR) is undefined and a proper value must therefore be written before the

EEPROM is accessed.

• Bits 6..0 – EEAR6..0: EEPROM Address

These are the (low) bits of the EEPROM Address Register. The EEPROM data bytes are

addressed linearly in the range 0...(128/256-1). The initial value of EEAR is undefined and a

proper value must be therefore be written before the EEPROM may be accessed.

Bit 76543210

0x1E (0x3E) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEAR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value X X X X X X X X