Re: [Lcd4linux-devel] display on WRAP-Board - how fast is i2c? ho w good is the lpc connector?

[Luis.F.Correia <Lui...@se...>]

Hi! 

> -----Original Message-----
> From: Ernst Bachmann [mailto:e.b...@xe...] 
> Sent: Tuesday, July 25, 2006 4:40 PM
> To: lcd...@li...
> Subject: Re: [Lcd4linux-devel] display on WRAP-Board - how 
> fast is i2c? ho w good is the lpc connector?
> 
> On Tuesday 25 July 2006 17:11, Stefan Krister wrote:
> > Hi Luis,
> >
> > Luis.F.Correia schrieb am 25.07.2006 15:27:
> > > For the sake of completeness, here's an update to the schematics:
> > >
> > > The PCF is powered by +5V, but the I2C inputs are biased to
> > > +3.3v, which is the correct voltage level of the WRAP board.
> >
> > My datasheet says that voltage between 2.5V and 6.0V are 
> valid. Must I 
> > provide 5V also? And where to take ...
> >
> 
> The I2C chips usually connect to the bus using open-collector 
> drivers, so its basically only important your PCF and the uC 
> share the same ground.
> 
> (The chips can only drive the bus lines low, its pulled up only by the
> pullup-resistors)
> 
> Of course, you should connect your i2c-pullup resistors to 
> the lowest Vcc in the system, in your case to 3.3V.
> 
> That way your i2c bus would operate on 3.3V, your PCF would 
> work on 5V, but will still recognize the logic levels on the 
> bus... As always, check the Datasheet to be sure.
> 

Ernst, you are right with everything you say.

Electrically speaking it can be done in several ways without
needing any special and complex voltage conversion devices.


What I described works for me using the PCF8574 and the WRAP board.

> 
> HTH
> /Ernst


Luis Correia   
Bering uClibc Team Member

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