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 VCC 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 ATmega48/88/168 is shown in “Register Summary” on page 343.
All ATmega48/88/168 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. The ATmega48/88/168 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 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.
7.5.1
General Purpose I/O Registers
The ATmega48/88/168 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.
22
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
