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PICList Thread
'[PIC] 3V3 to 5V LCD interface'
2005\06\28@034129 by Groups

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I need to interface a micro driven by 3.3V to an LCD that is powered from
5V.

Would series resistors in the signal lines be sufficient to prevent the
internal
feedthrough of 5V in the LCD effecting the micro port pins ?

Last time I did this, I inadvertently connected the micro pins direct to LCD
and noticed
distortion during the signal transitions (though the display seemed to work
ok and micro was
doing all it should have).

I suspect if the signal is current limited it would prevent this ?

Thanks in advance.



2005\06\28@090234 by alan smith

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Would it be easier to run the PIC at 5V?  Essentially if you are driving OUT from the PIC, your levels are going to be lower than a 5V TTL but should still be sufficient to drive it.  Putting series resistors in would be for a termination issue, not to drop the voltage.  If you are really worried about it, there are several voltage translators out there where you can drive I/O from one to another.  784LVX series I think....?  Try a search on Fairchilds site for them

Groups <spam_OUTgroupsTakeThisOuTspamorbitcoms.com> wrote:I need to interface a micro driven by 3.3V to an LCD that is powered from
5V.

Would series resistors in the signal lines be sufficient to prevent the
internal
feedthrough of 5V in the LCD effecting the micro port pins ?

Last time I did this, I inadvertently connected the micro pins direct to LCD
and noticed
distortion during the signal transitions (though the display seemed to work
ok and micro was
doing all it should have).

I suspect if the signal is current limited it would prevent this ?

Thanks in advance.



2005\06\28@091944 by Alan B. Pearce

face picon face
>I need to interface a micro driven by 3.3V to an LCD
>that is powered from 5V.

Maxim make some translator chips. IIRC there is also a version of the HC245
chip which is TTL levels, but runs from a 5V supply, which would do it if
run from the LCD supply.

2005\06\28@092530 by Groups

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The reason the micro is running on 3.3V is because the rest of the circuit
is running 3.3V (memory, Radio circuitry, coms interface etc).

The LCD is the only 5V device in the project. The LCDs are Plasma displays
and I am unable to obtain 3V supply versions of them yet readily
and at reasonable cost and low MOQ.

The idea of the resistors was to act as a current limit. (As long as they
were not so
high in value that they could not pull the Data lines of the LCD to 0V).

The LCD inputs can handle being driven at 3V no problem. My main concern is
that 5V internally in the LCD seems to be feeding back through the signal
pins.

Kind Regards

David Huisman
-----------------------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com

{Original Message removed}

2005\06\28@093819 by Peter Onion

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On Tue, 2005-06-28 at 23:21 +1000, Groups wrote:

> The LCD inputs can handle being driven at 3V no problem. My main concern is
> that 5V internally in the LCD seems to be feeding back through the signal
> pins.

I think this because the PIC pins have protection diodes to Gnd and Vcc.
The 5v logic signals from the LCD will try and pull the PIC Vcc up to 5v
via these diodes.  

A series resistor on each line will allow the diodes to clamp the levels
on the PIC pins to 3v3 without drawing excessive current from the LCD
signal drivers.

Peter

2005\06\28@095737 by Jan-Erik Soderholm

face picon face
> A series resistor on each line will allow the diodes to clamp
> the
levels
> on the PIC pins to 3v3 without drawing excessive current from
the LCD
> signal drivers.

But, designing in the use of the
clamping/protecton diods,
is a bad idea, isn't it ?

Maybe just a
resistor based voltage divider 5V -> 3.3 V ?

Jan-Erik

>
> Peter
>
>
--

2005\06\28@100937 by Peter Onion

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On Tue, 2005-06-28 at 15:57 +0200, Jan-Erik Soderholm wrote:

> But, designing in the use of the
> clamping/protecton diods,
> is a bad idea, isn't it ?
>
> Maybe just a
> resistor based voltage divider 5V -> 3.3 V ?

I take your point.  I was assuming the pins were bidirectional so I
dismissed the idea of a divider, but on reflection it wouldn't matter
anyway.

Peter



2005\06\28@192710 by Groups

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face
Thanks for the feedback on this issue.



Kind Regards

David Huisman
-----------------------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com
PO Box 4474 Lakehaven
NSW 2263, AUSTRALIA
Phone: 61-2-4393-3627
Fax  : 61-2-4393-3685
Mobile: 61-413-715-986

{Original Message removed}

2005\06\29@090334 by alan smith

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To make the design as bullet proof as possible.....best to use a translator of some sort.  I am currently using  some 74LCX07 parts to do much the same.  PIC drives out to these OD buffers, so just adding the pullup on the output does the job.  When you assert a high on the input, it drives them to ground.  I'm interfacing the PIC that is running on 3.3V to logic running on 1.2V, 1.8V,2.5V.  This part should work fine for you since your not doing any bi-directional stuff.

Groups <.....groupsKILLspamspam@spam@orbitcoms.com> wrote:Thanks for the feedback on this issue.



Kind Regards

David Huisman
-----------------------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com
PO Box 4474 Lakehaven
NSW 2263, AUSTRALIA
Phone: 61-2-4393-3627
Fax : 61-2-4393-3685
Mobile: 61-413-715-986

{Original Message removed}

2005\06\29@104040 by Gerhard Fiedler

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Groups wrote:

> Thanks for the feedback on this issue.

I always find it very interesting when the OP posts his conclusions, and
possibly the solution finally used (where applicable).

Gerhard

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