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I'm playing with I2C on an AVR. I need the devices separated, and each device has its own battery. For the I2C I'm not clear if it is OK to have the pull up resistor within the source device, then share on a connection cable: ground, SCL and SDA?

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    \$\begingroup\$ It's probably also worth mentioning if both the devices are running at the same supply voltage (3V3, 5V etc) and if not what voltage they are running at. \$\endgroup\$
    – PeterJ
    Aug 29, 2013 at 21:54

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Should be fine,if the voltage on the pullup is at an acceptable on for the slave and the master. If not, you need a level shifter. Also, you need to pay attention to total cable capacitance if you want to keep your speed up. You might need to use smaller resistors than you expect.

http://www.edn.com/design/analog/4371297/Design-calculations-for-robust-I2C-communications seems like a good reference.

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