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how do use i2c on arduino with out pull-up ? Answered

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You're using the onboard TWI aren't you ?

Good. In that case, you can usually get away with the default port settings, but I recommend that you add an external pullup as well. The internal one is pretty weak.

External pull up ? What frequency are you intending to work at ?

Yes I know, BUT the principles are the same - This is a fairly good explanation of what it is all about.

You don't set the pull up/downs in the output mode on an AVR, you can only do it for input mode.

+1
Output means a pin on a digital IC like a Pic, Arduino or AVR
is FORCED High or low by an internal process and a "pull up"
is non sequitur  when compared to a Real output pin.

Of course you could be thinking of an open collector output like some
sensors that need a pull up resistor.

That does not occur ever in the land of micro-processors :-)

A

Ermm. Some of the LPC series have that very functionality - configurably

Didn't know that.......... Really...,  open collectors..........

Now I have to apologize to .act casual.

The pull up resistor is a passive device. In general outputs don't have a pull up they are active pins and will be high low or high impedance.

Can you explain in more detail what your trying to do - Circuit diagrams help.