>> Uhhh, "PARALLEL with existing pullups".

Oops! I was thinking parallel and typed series. Sorry for the confusion.

David


-----Original Message-----
From: Robert Rolf [mailto:Robert.Rolf@ualberta.ca] 
Sent: Wednesday, January 17, 2007 11:19 AM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] I2C dropping data in PIC slave

David Novak wrote:

> 1.) What is your clock rate? Have you tried slowing it to see if the
> situation improves?
> 
> 2.) Are you pulling the I2C lines to 5V? If so, 2K seems too stiff. Check
> the iOL specs. for the IC's on the bus. I would recomment pullups only at
> the master. Remember, additional pullups will be in series with existing
> pullups.

Uhhh, "PARALLEL with existing pullups".

I recommend splitting the pullup value between the master and the farthest
slave.
I cuts down on ringing. e.g. 4.7K local and 4.7K distant giving 2.4K
effective value.
You'll want the low value (2.2K effective) if you are using high speed
(400kHz).
10K is typical for low speed (100kHz).

You should look at the signals with a scope to see if there is ground
bounce,
ringing, or high frequency noise on the lines that could be corrupting the
data.
The rising edges should be, at most, 20% of the bit time. If worse, you have
too
much cable capacitance.

> 3.) You may want to try a series resistor to dampen the noise.

A small value like 100R works well on long cables.

Robert

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