The next code examples show assembly and C functions for reading the EEPROM. The examples assume that interrupts
are controlled so that no interrupts will occur during execution of these functions.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic
rjmp
EECR,EEWE
EEPROM_read
; Set up address (r18:r17) in address register
out
out
EEARH, r18
EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from data register
in
r16,EEDR
ret
C Code Example
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
4.3.5 Preventing EEPROM Corruption
during periods of low VCC, 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. Secondly, 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 completed provided that the power supply voltage is sufficient.
4.4
I/O Memory
The I/O space definition of the ATmega16/32/64/M1/C1 is shown in Section 29. “Register Summary” on page 299.
All Atmel® ATmega16/32/64/M1/C1 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 Atmel ATmega16/32/64/M1/C1 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.
ATmega16/32/64/M1/C1 [DATASHEET]
23
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
