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I understand that it's a type of ROM, but is there more to it? How can it be used for?

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EEPROMS (Electrically Erasable Programmable Read-Only Memory) are a type of non-volatile memory (they maintain the info even when not powered. You can read more about EEPROMS on Wikipedia.

They are used by Pi HATS (Hardware Attached on Top) EEPROM. Per the Raspberry Pi Foundation's blog.

The EEPROM holds the board manufacturer information, GPIO setup and a thing called a ‘device tree‘ fragment – basically a description of the attached hardware that allows Linux to automatically load the required drivers.

They should not be used for anything but communicating with the HAT's EEPROM.

The blog post announcing HATs can be found here, and the HAT spec can be found here.

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    Using GPIO 0/1 on a Pi3 (ID_SD/ID_SC) is pretty much a no no because that bus is also used to control internal GPIO. On earlier Pi models the bus may be used as a normal I2C bus if a HAT isn't being used.
    – joan
    Jun 25, 2016 at 16:47
  • @joan I have also read that the Pi3 has a port expander, and it seems reasonable that this would be I²C but the pins seem to be in input mode. Are you aware of any documentation of the use?
    – Milliways
    Jun 26, 2016 at 6:32
  • @Milliways I vaguely remember reading a post where someone mentioned the model number and I do remember a post where someone had communicated with the device to switch one of the Pi3 LEDs. They had put up github code which I haven't tried. The posts were on the raspberrypi.org forums. I'll have a look later and post a link if I can find the information.
    – joan
    Jun 26, 2016 at 7:54
  • @Milliways Have a look at raspberrypi.org/forums/viewtopic.php?f=72&t=139753 which shows (indirectly via a link) how to control the activity LED on the Pi3. The best search string seems to be "port expander pi3".
    – joan
    Jun 26, 2016 at 9:23
  • Actually @joan You can use GPIO 0 & 1 with very little work. For proof of this look at the Official Dots board. Jun 28, 2016 at 19:12

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