Rainbow Electronics ATtiny15L - Manual (Page 36)

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ATtiny15L
1187E–AVR–06/02
EEPROM Read/Write
Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time is in the range of 4.6 - 8.2 ms, depending on the frequency of the
calibrated RC Oscillator. See Table 16 for details. A self-timing function however, lets
the user software detect when the next byte can be written. If the user code contains
code that writes 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. CPU operation under these conditions is likely to cause the
Program Counter to perform unintentional jumps and eventually execute the EEPROM
write code. To secure EEPROM integrity, the user is advised to use an external under-
voltage reset circuit in this case.
In order to prevent unintentional EEPROM writes, a two-state write procedure must be
followed. Refer to the description of the EEPROM Control Register for details of this.
When the EEPROM is read or written, the CPU is halted for two clock cycles before the
next instruction is executed.
The EEPROM Address
Register – EEAR
• Bit 7, 6 – Res: Reserved Bits
These bits are reserved bits in the ATtiny15L and will always read as zero.
• Bit 5..0 – EEAR5..0: EEPROM Address
The EEPROM Address Register (EEAR) specifies the EEPROM address in the 64 bytes
EEPROM space. The EEPROM data bytes are addresses linearly between 0 and 63.
The initial value of EEAR is undefined. A proper value must be written before the
EEPROM may be accessed.
The EEPROM Data Register –
EEDR
• Bit 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 oper-
ation, the EEDR contains the data read out from the EEPROM at the address given by
EEAR.
Bit 7 6 5 4 3 2 1 0
$1E – – EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEAR
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial vAlue 0 0 X X X X X X
Bit 7 6 5 4 3 2 1 0
$1D MSB LSB EEDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0