ATtiny45 datasheet(15/196 Pages) ATMEL

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7598H–AVR–07/09

ATtiny25/45/85

5.2.1

Data Memory Access Times

This section describes the general access timing concepts for internal memory access. The

internal data SRAM access is performed in two clk

CPU cycles as described in Figure 5-3.

Figure 5-3.

On-chip Data SRAM Access Cycles

5.3

EEPROM Data Memory

The ATtiny25/45/85 contains 128/256/512 bytes of data EEPROM memory. It is organized as a

separate data space, in which single bytes can be read and written. The EEPROM has an

endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the

CPU is described in the following, specifying the EEPROM Address Registers, the EEPROM

Data Register, and the EEPROM Control Register. For a detailed description of Serial data

downloading to the EEPROM, see page 138.

5.3.1

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 5-1. 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.

clk

Data

Address

Address valid

Compute Address

CPU

Memory Access Instruction

Next Instruction

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