user30472: I will implements an emergency backup feature before power off, on Arduino Mega 2650.
Arduino is powered by capacitor, for the time (I hope) of the EEPROM write.
On AVR2650 Datasheet is reported that the typical EEPROM writing time is 3.3ms.
But 3.3ms for what? a byte? a word? the whole EEPROM?
I need the data for calculate the capacitors.

user13432: 
On AVR2650 Datasheet is reported that the typical EEPROM writing time is 3.3ms. But 3.3ms for what? a byte? a word? the whole EEPROM?

That is per byte (erase and write 3.4 ms, only write is 1.8 ms). When writing multiple bytes there are a few clock cycles to be gained by preparing for the next byte during an ongoing EEPROM write. Or even better use an ISR for the EEPROM feed. Not faster but allows some additional processing while a block is written. The largest gain is to have the EEPROM erased before the write (1.8 ms per byte instead of 3.4 ms). 
Please see http://www.nongnu.org/avr-libc/user-manual/group__avr__eeprom.html,  http://www.atmel.com/images/doc2578.pdf and table 9-1, pp. 35, http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf.
Cheers!

30472: <p>I will implements an emergency backup feature before power off, on Arduino Mega 2650.
Arduino is powered by capacitor, for the time (I hope) of the EEPROM write.</p>

<p>On AVR2650 Datasheet is reported that the typical EEPROM writing time is 3.3ms.
But 3.3ms for what? a byte? a word? the whole EEPROM?</p>

<p>I need the data for calculate the capacitors.</p>
 13432: <blockquote>
  <p>On AVR2650 Datasheet is reported that the typical EEPROM writing time is 3.3ms. But 3.3ms for what? a byte? a word? the whole EEPROM?</p>
</blockquote>

<p>That is per byte (erase and write 3.4 ms, only write is 1.8 ms). When writing multiple bytes there are a few clock cycles to be gained by preparing for the next byte during an ongoing EEPROM write. Or even better use an ISR for the EEPROM feed. Not faster but allows some additional processing while a block is written. The largest gain is to have the EEPROM erased before the write (1.8 ms per byte instead of 3.4 ms). </p>

<p>Please see <a href="http://www.nongnu.org/avr-libc/user-manual/group__avr__eeprom.html" rel="nofollow noreferrer">http://www.nongnu.org/avr-libc/user-manual/group__avr__eeprom.html</a>,  <a href="http://www.atmel.com/images/doc2578.pdf" rel="nofollow noreferrer">http://www.atmel.com/images/doc2578.pdf</a> and table 9-1, pp. 35, <a href="http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf" rel="nofollow noreferrer">http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf</a>.</p>

<p>Cheers!</p>
