Datasheet
16
2586D–AVR–02/06
ATtiny25/45/85
6.3.1 EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access times for the EEPROM are given in Table 6-1. 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 ”Preventing EEPROM Corruption” on page 20 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 ”Atomic Byte Programming” on page 18 and ”Split Byte Programming” on page 18 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 – EEPROM Address Register
• Bit 7:1 – Res6:0: Reserved Bits
These bits are reserved for future use and will always read as 0 in ATtiny25/45/85.
• Bits 8:0 – EEAR8:0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL – specifies the high EEPROM address
in the 128/256/512 bytes EEPROM space. The EEPROM data bytes are addressed linearly
between 0 and 127/255/511. The initial value of EEAR is undefined. A proper value must be writ-
ten before the EEPROM may be accessed.
6.3.3 EEDR – 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 76543210
0x1F -------EEAR8EEARH
0x1E EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL
Bit 76543210
Read/Write RRRRRRRR/W
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value0000000X
Initial ValueXXXXXXXX
Bit 76543210
0x1D EEDR7 EEDR6 EEDR5 EEDR4 EEDR3 EEDR2 EEDR1 EEDR0 EEDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000