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That is to say, can I just connect the SCL and SDA lines from an externally powered EEPROM to an arduino to write to it? The EEPROM in question is AT24C16A.

I am trying to make a circuit which relies on EEPROM data that I might have to change later and I was wondering if I could get away with just a 2 wire header instead of four.

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As long as:

  1. The ground of the Arduino and the AT24C16A are tied together.
  2. The GND connection would dictate a 3-pin connector. (Or a three wire connection from Arduino to AT24C16A assembly).
  3. The Arduino voltage swings are compatible with the EEPROM levels.
  4. The Arduino software generates correct protocol for the I2C interface.
  5. The software generates the correct SCL frequency of the AT24C16A.

You should be able to make it work just fine.

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  • \$\begingroup\$ What do you mean by "The GND connection would dictate a 3-pin connector"? \$\endgroup\$ – Wouter van Ooijen May 25 '18 at 6:01
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    \$\begingroup\$ SDA,SCL + Vss(gnd) \$\endgroup\$ – Sunnyskyguy EE75 May 25 '18 at 6:12
  • \$\begingroup\$ Conditions 3, 4, 5 are met. To be clear: Do you mean that the ground of the battery needs to be connected directly to the ground of the Arduino? Would it be safe? \$\endgroup\$ – Nirav May 25 '18 at 8:38
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    \$\begingroup\$ @KittyHawk - If the battery is what is being used to power the AT24C16A then yes. But the more general statement is that no matter what powers the EEPROM the ground of the AT24C16A assembly must be connected to the ground of the Arduino. \$\endgroup\$ – Michael Karas May 25 '18 at 11:16

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