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It seems that, to use I2C, we need to:

  • un-blacklist I2C with sudo nano /etc/modprobe.d/raspi-blacklist.conf

  • enable I2C kernel module with sudo nano /etc/modules

Why is it disabled + blacklisted by default? This sounds like a high-level protection against I2C!
If I enable it, will it cause higher power consumption?

This question is important because my RPi will be battery powered.

(I can avoid I2C and use SPI instead if it is better for power consumption).

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    It's not power consumption -- turning it on with nothing attached won't make any difference to that. As joan says, it is presumably because if you want to use it, needing to enable it makes more sense than needing to disable it if you don't want to use it and it has gotten in your way. I have yet to hear of someone accidentally attaching an I2C device without understanding that's what they were doing. – goldilocks♦ Apr 23 '15 at 5:38
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You are probably referring to an out of date web-site.

I thought that raspi-blacklist.conf was deprecated and no longer used.

I2C/SPI are not enabled by default because not many people use them. If they were enabled by default it might confuse users who wanted to use the I2C/SPI gpios as ordinary inputs/outputs.

There is unlikely to be any power difference in using I2C versus SPI. SPI is typically faster.

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