Datasheet

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ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It is recommended to
have the Global Interrupt Flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this
bit and wait for a zero before writing the next byte. When EEPE has been set, the CPU is halted for two cycles
before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set
up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The
EEPROM read access takes one instruction, and the requested data is available immediately. When the
EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is
neither possible to read the EEPROM, nor to change the EEAR Register.
The calibrated Oscillator is used to time the EEPROM accesses. The following table lists the typical
programming time for EEPROM access from the CPU.
The following code examples show one assembly and one C function for writing to the EEPROM. The examples
assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during
execution of these functions. The examples also assume that no Flash Boot Loader is present in the software. If
such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
Table 5-3. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time
EEPROM write
(from CPU)
26,368 3.3ms