ATMEGA48P datasheet(21/420 Pages) ATMEL

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8025L–AVR–7/10

ATmega48P/88P/168P

7.4.2

Preventing EEPROM Corruption

During periods of low V

CC, the EEPROM data can be corrupted because the supply voltage is

too low for the CPU and the EEPROM to operate properly. These issues are the same as for

board level systems using EEPROM, and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First,

a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Sec-

ondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can

be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal

BOD does not match the needed detection level, an external low V

CC reset Protection circuit can

be used. If a reset occurs while a write operation is in progress, the write operation will be com-

pleted provided that the power supply voltage is sufficient.

7.5

I/O Memory

The I/O space definition of the ATmega48P/88P/168P is shown in ”Register Summary” on page

395.

All ATmega48P/88P/168P 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 instr u ction s , 0x2 0 must be adde d to t h e s e ad dre s ses. The

ATmega48P/88P/168P is a complex microcontroller with more peripheral units than can be sup-

ported 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.

7.5.1

General Purpose I/O Registers

The ATmega48P/88P/168P 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.

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