Datasheet

7799D–AVR–11/10

ATmega8U2/16U2/32U2

Extended I/O space from 0x60 - 0x1FF in SRAM, only the ST/STS/STD and LD/LDS/LDD

instructions 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.

7.4.1 General Purpose I/O Registers

The ATmega8U2/16U2/32U2 contains three General Purpose I/O Registers. These registers

can be used for storing any information, and they are particularly useful for storing global vari-

ables 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.

7.5 Register Description

7.5.1 EEARH and EEARL – The EEPROM Address Register

• Bits 15:12 – Res: Reserved Bits

These bits are reserved and will always read as zero.

• Bits 11:0 – EEAR[8:0]: EEPROM Address

The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the

512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and

512. The initial value of EEAR is undefined. A proper value must be written before the EEPROM

may be accessed.

7.5.2 EEDR – The EEPROM Data Register

• Bits 7:0 – EEDR[7:0]: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to the

EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the

EEDR contains the data read out from the EEPROM at the address given by EEAR.

Bit 15 14 13 12 11 10 9 8

0x22 (0x42) – – – – EEAR11 EEAR10 EEAR9 EEAR8 EEARH

0x21 (0x41) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL

76543 210

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

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 X X X X

XXXX X X XX

Bit 76543210

0x20 (0x40) MSB LSB EEDR

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

Initial Value 0 0 0 0 0 0 0 0