Re: gEDA-user: OT: I2C pullup resistor location

[John Doty <jpd@xxxxxxxxx>]

On Apr 2, 2010, at 5:10 PM, DJ Delorie wrote:

> 
> I2C is driven from both "ends" of the bus, so if you want to use the
> pull-ups as terminators, use two resistors, each twice the resistance,
> one at each end.

I2C pullups are generally a few kohms, far above any practical characteristic impedance, so they will have essentially no effect on ringing due to drive. They don't have enough drive capacity to produce ringing on upward transitions. On downward transitions the driver output resistance will be more important for damping. 

> 
> I've never heard of anyone worrying about ringing on I2C though...

I presume that I2C device makers choose drivers weak enough and receivers slow enough to avoid problems on reasonably sized boards.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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