DOC7707 datasheet(19/307 Pages) ETC1

19 / 307 page

7707F–AVR–11/10

AT90USB82/162

For a detailed description of SPI, debugWIRE and Parallel data downloading to the EEPROM,

see page 257, page 241, and page 246 respectively.

5.3.1

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 5-2. 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 “Preventing EEPROM Corruption” on page 23. 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.

5.3.2

The EEPROM Address Register – EEARH and EEARL

• Bits 15..12 – Res: Reserved Bits

These bits are reserved bits in the AT90USB82/162 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

512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and

512. The initial value of EEAR is undefined. A proper value must be written before the EEPROM

may be accessed.

5.3.3

The EEPROM Data Register – EEDR

• 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

151413

1211

–

EEAR11

EEAR10

EEAR9

EEAR8

EEARH

EEAR7

EEAR6

EEAR5

EEAR4

EEAR3

EEAR2

EEAR1

EEAR0

EEARL

Read/Write

R/W

Initial Value

Bit

765432

MSB

LSB

EEDR

Read/Write

R/W

Initial Value

000000

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