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ATMEGA644 Datasheet(PDF) 21 Page - ATMEL Corporation |
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ATMEGA644 Datasheet(HTML) 21 Page - ATMEL Corporation |
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21 / 374 page  21 2593L–AVR–02/07 ATmega644 6.3.1 EEPROM Read/Write Access The EEPROM Access Registers are accessible in the I/O space. The write access time for the EEPROM is given in Table 6-2. A self-timing function, however, lets the user software detect when the next byte can be written. If the user code contains instruc- tions that write the EEPROM, some precautions must be taken. In heavily filtered power supplies, V CC is likely to rise or fall slowly on power-up/down. This causes the device for some period of time to run at a voltage lower than specified as minimum for the clock frequency used. See Section “6.3.5” on page 25. for details on how to avoid problems in these situations. In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the description of the EEPROM Control Register for details on this. When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed. 6.3.2 EEARH and EEARL – The EEPROM Address Register • Bits 15:12 – Res: Reserved Bits These bits are reserved bits in the ATmega644 and will always read as zero. • Bits 11:0 – EEAR8:0: EEPROM Address The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 4K bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 4096. The initial value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed. 6.3.3 EEDR – The EEPROM Data Register • Bits 7:0 – EEDR7:0: 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 141312 11 10 9 8 0x22 (0x42) – – – – EEAR11 EEAR10 EEAR9 EEAR8 EEARH 0x21 (0x41) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL 76 54 3 2 10 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 X XXX 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 00000000 |
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