I2C device addressing - why 4 MSB are fixed?

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nareshgtr

Joined: 17 Feb 2007
Posts: 48
Helped: 2
Location: Bangalore

31 Oct 2009 7:27 I2C device addressing

Hi,
In I2C compliant devices why 4 Most significant bits are fixed. Example All EEPROMs address start with 1010 and next three bits are A2A1A0. I am assuming to reduce the address decoding logic. Is there is any specific reason.
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V. Naresh Kumar

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ChrisC

Joined: 30 Sep 2009
Posts: 34
Helped: 8
Location: London, UK

31 Oct 2009 12:43 I2C device addressing

More likely to keep the package size to a minimum - on an 8-pin EEPROM you need 2 pins for power, 2 for the I2C bus, and usually 1 for write protection, which leaves 3 for addressing. Giving up write protection just to gain an extra address bit probably isn't a good idea, so realistically the only way to add more user-definable address bits would be to go up to the next largest package type.


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