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Thread: I2C pullups
www.piclist.com/techref/i2cs.htm?key=i2c
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face BY : Mike Mansheim email (remove spam text)



>> Hello.  I am looking at the datasheets for 24LC32 and 24LC256 EEPROMS.
>> The datasheets state the SDA line needs to be pulled up.  No mention of
>> using a pullup on SCL.  Then in Serial PIC'n it says pullups are
>> required on both lines.  Which is correct?  Thanks for any help.

> If the master (which always drives SCL) tri-states the SCL line
> then you do need the pullup on SCL. If you only have one master
> in your setup and it actually swings hi-lo then you don't need it.

> It depends on you're masters. If you only have one master and that
> master can only be used in a one master environment (ie. a PIC which
> never selects it's SCL as an input) then it is possible that you will
> never need a pullup on SCL.

I must disagree.  A slave on the i2c bus can also drive SCL low (clock
stretching) - so if you actually drive SCL high with the master, and the
slave drives SCL low, you've got a short.  Perhaps these particular
devices never do this - I don't know - but they are allowed to by the i2c
spec.  So to have a standard i2c interface, you need pullups on both SDA
and SCL, and the only way you make either high is to set it as an input
and let the pullup float it high.

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