"Patrick M" <bono@step.polymtl.ca> wrote in message
news:42866e3f$1_3@aeinews....
> Greetings,
>
> I'm currently facing a weird problem trying to have a i2c master and
> several slaves communicate...
>
> The slave will only send an acknowledge back to the master if the data
> and clock signals have probes on them. The probes have a 1 M-ohm
> resistance... As soon as I take the probes off, the master never
> receives an acknowledge...

I think you should the datasheets. Pretty much every I2c bus I have seen
will require pull-up resistors in the order of 10k or up.

Patrick M wrote:

> Greetings,
> 
> I'm currently facing a weird problem trying to have a i2c master and
> several slaves communicate...
> 
> The slave will only send an acknowledge back to the master if the data
> and clock signals have probes on them. The probes have a 1 M-ohm
> resistance... As soon as I take the probes off, the master never
> receives an acknowledge...
> 
> Any pointer or tip on how to debug this would be greatly appreciated.

  You mean besides the obvious solution of "include free probes with 
every system ?" :)

  Also try unplug of the probes from the CRO end, and see if that
changes, and also flick the IP coupling from DC to AC.
  On most CROs that removes the grounding load, as it adds a CAP in
series.
  Those steps change the capacitance, and parallel resistance, so
you can see what element matters most.

-jg

Patrick M wrote:
> 
> I'm currently facing a weird problem trying to have a i2c master
> and several slaves communicate...
> 
> The slave will only send an acknowledge back to the master if the
> data and clock signals have probes on them. The probes have a 1
> M-ohm resistance... As soon as I take the probes off, the master
> never receives an acknowledge...
> 
> Any pointer or tip on how to debug this would be greatly
> appreciated.

The probe is acting like a pull-down resistor (my guess).  Look at
the rise and fall times on the signal lines.  My guess is that if
you add a 10k or so pull-down resistor to the lines the fall time
will be greatly improved, and the problem will go away.

-- 
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   http://www.caliburn.nl/topposting.html
   http://www.netmeister.org/news/learn2quote.html

Patrick M wrote:
> Greetings,
> 
> I'm currently facing a weird problem trying to have a i2c master and
> several slaves communicate...
> 
> The slave will only send an acknowledge back to the master if the data
> and clock signals have probes on them. The probes have a 1 M-ohm
> resistance... As soon as I take the probes off, the master never
> receives an acknowledge...
> 
> Any pointer or tip on how to debug this would be greatly appreciated.
> 
> Best regards,
> 
> 
> Patrick M.
   maybe add a pull-up resistor? 100k or even 1meg
rw

Greetings,

I'm currently facing a weird problem trying to have a i2c master and
several slaves communicate...

The slave will only send an acknowledge back to the master if the data
and clock signals have probes on them. The probes have a 1 M-ohm
resistance... As soon as I take the probes off, the master never
receives an acknowledge...

Any pointer or tip on how to debug this would be greatly appreciated.

Best regards,


Patrick M.