> i2c is not famous for distance, 3' is longer than anything I 
> ever did, 
> and i2c was designed for single-board TV designs- nothing 
> outside of the 
> same PCB.

I have worked with an I2C bus containing approximately 18 slaves and close
to 3' after spanning 4 seperate boards...in a TV! Keeping the clock rate
slow was a very wise decision. We used 50KHz, but 100KHz is not out of the
question for a long bus.

Designers tend to worry a great deal about the rise time, but I have seen
I2C work flawlessly with as large as 5us rise times. It helps that most IC's
have some amount of glitch rejection on SCL. Actually, for a long bus, the
fast fall times may cause more grief than the slow rise times due to
transmission line effects.


David
 

> -----Original Message-----
> From: Bob Axtell [mailto:engineer@neomailbox.com] 
> Sent: Wednesday, January 17, 2007 4:10 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] I2C dropping data in PIC slave
> 
> All was well until I noticed the 3' cable length and the second PCB.
> 
> i2c is not famous for distance, 3' is longer than anything I 
> ever did, 
> and i2c was designed for single-board TV designs- nothing 
> outside of the 
> same PCB.
> So I think you have a noise problem, caused by poor GND, long 
> cable, and 
> probably the multiple 2K pullups (much too heavy, 3.3K works 
> for me, near
> the master).
> 
> --Bob
> 

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