> -----Original Message-----
> From: H.P. de Vries [SMTP:H.P.d.Vries@STUD.TUE.NL]
> Sent: Wednesday, July 05, 2000 10:45 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: Interfacing to the ATA-interface
>
> Ok, that would be my first thought also. This also seems like the least
> work, as I need to convert only about 5 lines, as opposed to the 24 5V
> signals I had otherwise to convert.
>
> But now, how do I convert the
> voltages down in an efficient way? I don't like the idea of putting zeners
> /w resistors on the outputs, since in that way I lose my low-impedance
> output of the pic.
> Did I mention I also have to drive an i2c-bus also at 3.3v? Now how do I
> go
> about that? I'm no i2c expert at all, so this is one part I for sure won't
> find an answer to myself easily.
>
The Philips book "The I2C bus and how to use it" has a section on dealing
with multi-voltage bus systems.  The answer is amazingly simple and very
elegant.  A discreet mosfet is simply inserted in each line between the
different voltage sections.  The source connects to the three volt bus,
drain to the 5 volt bus and gate to the 3.3v supply (Vdd).

The operation isn't immediatley obvious (at least it wasn't to me) as part
of the system relies on the internal diode accross a mosfet's drain/source,
but I can report that it definately works.

Cheers

Mike




> <sigh>
> Why don't all devices just run at 5V :-)
>
> Hans
> On Wed, Jul 05, 2000 at 11:26:48PM +0200, Quentin wrote:
> > IMO, since your PIC is the main part here (comm with PC and LCD at 5V),
> > I would say run it at 5V and convert down.The PIC will also still read a
> > 3.3V input as Logic High.
> >
> > Quentin
> >
> > "H.P. de Vries" wrote:
> > >
> > > Hi.
> > >
> > > Does anyone in here know how to interface 3.3V logic to 5V logic? I
> have a
> > > PIC and some other stuff running at 3.3V , but need to interface with
> the
> > > ATA-interface from my PC, and a LCD-display, which both need 5V logic
> > > levels. Is there an easy way to do this? Or is it easier to run the
> PIC at
> > > 5V, and then convert the signals down to 3.3V?
> > > Or is it possible (yes, I say possible, not allowed) to just put it
> all
> > > together and operate the 5V logic devices at 3.3V ?
> > >
> > > Hans
> > >
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