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PICList Thread
'5v PIC <> 3v3 logic'
2000\05\17@032203 by David Duffy

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<x-flowed>Hi All,
This is my 1st time using 3v3 devices (ISD5008 voice chip) and was
wondering about the best way to interface the PIC to it. (5v supply)
I could power the PIC from the same (3v3) rail but then I still need to
interface to a MAX483 transceiver and possible other bits too.
Any ideas on the best way (low cost/component count) to do this?
I was thinking of a resistive divider for 5v output to 3v3 input?
Maybe an NPN transistor & resistors as a level shifter up to 5v from 3v3 ?
No matter which rail I put the PIC on, I still need the 5v rail for MAX483.
The interface between the PIC & the MAX483 uses less i/o but the
transistor level shifter will invert the 485 RX/TX lines which I can't have.
Any suggestions appreciated.  :-)
Regards...

</x-flowed>

2000\05\17@045707 by Vasile Surducan

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On 17 May 00, at 17:18, David Duffy wrote:

Question for PIClist administrator: this is a [PIC] , [OT] or [EE]
subject ?

David,
 If your ISD is sourcing less than 100mA a cheapest solution is to
use 3 diodes 6x0.6 =~ 1.8V  5v - 1.8v = 3.2V. But if you planning a
large production, then a  standard 3 terminal power supply circuit is
recommended...( also if ISD chip is more expensive than 20$ and
have absolute maximum ratings supply parameters under 5V ...)

{Quote hidden}

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Surducan Vasile, engineer
mail: spam_OUTvasileTakeThisOuTspaml30.itim-cj.ro
URL: http://www.geocities.com/vsurducan
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2000\05\17@065646 by briang

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In-Reply-To: <4.3.1.2.20000517171853.00a79180@192.168.0.100>

David Duffy <.....piclistKILLspamspam@spam@AUDIOVISUALDEVICES.COM.AU> wrote:
> Hi All,
> This is my 1st time using 3v3 devices (ISD5008 voice chip) and was
> wondering about the best way to interface the PIC to it. (5v supply)

Use an open drain output type idea.

When you have an output from the PIC (5V) to the voice chip (3V) put a
pull up resistor to 3V and instead of driving the output high for logic
one just set it to high impedance with the TRIS register.

The other way round an output from the voice chip into a PIC input
should work as is because the PIC will take 3V as logic one anyway.

Brian Gregory.
briangspamKILLspamcix.co.uk

2000\05\17@145149 by Octavio Nogueira

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I'm using a 74LVC14 to do that. 3.3V to it
and 5v to PIC.

Friendly Regards

Octavio Nogueira
===================================================
.....nogueiraKILLspamspam.....propic2.com                  ICQ# 19841898
ProPic tools - low cost PIC programmer and emulator
http://www.propic2.com
===================================================

{Original Message removed}

2000\05\17@160851 by Mark Willis

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I'd call it [PIC];  I think I'd get lots of nasty e-mail, if I called
something wired directly TO a PIC chip, anything BUT that topic <G>

I'd call it [EE] if you were discussing a signal wired from the ISD5008
to the MAX483 directly, or how best to create both the 5V and 3.3V power
bus powers from a single AA battery, or ...

 Mark

Vasile Surducan wrote:
{Quote hidden}

--
I re-ship for small US & overseas businesses, world-wide.
(For private individuals at cost; ask.)

2000\05\18@012608 by David Duffy

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<x-flowed>At 11:55 5/17/00 +0100, you wrote:
>In-Reply-To: <4.3.1.2.20000517171853.00a79180@192.168.0.100>
>
>David Duffy <piclistspamspam_OUTAUDIOVISUALDEVICES.COM.AU> wrote:
> > Hi All,
> > This is my 1st time using 3v3 devices (ISD5008 voice chip) and was
> > wondering about the best way to interface the PIC to it. (5v supply)

Brian Gregory wrote;
>Use an open drain output type idea.
>
>When you have an output from the PIC (5V) to the voice chip (3V) put a
>pull up resistor to 3V and instead of driving the output high for logic
>one just set it to high impedance with the TRIS register.
>The other way round an output from the voice chip into a PIC input
>should work as is because the PIC will take 3V as logic one anyway.

Hmmm... Seems like a good idea but...
Does the PIC input being held at 3V (near 5V mid-point) pose a problem?
If it's not a schmitt trigger input, I thought CMOS inputs are unhappy
about this.
Regards...

</x-flowed>

2000\05\18@145141 by briang

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In-Reply-To: <@spam@00May17.221637cest.12869KILLspamspamverleihnix.elektronik21.de>

=?iso-8859-1?Q?Martin_Sch=E4fer?= <KILLspamschaeferKILLspamspamelektronik21.de> wrote:
>
> Brian Gregory wrote:
>
> > The other way round an output from the voice chip into a PIC input
> > should work as is because the PIC will take 3V as logic one anyway.
>
> But what about the power consumption? You should in any case put a resistor
> (lets say 4K7) between 3V3 out and 5V in.
>
> You could do the same in the other direction (resistor between 5V out and
> 3V3 in) if the 3V3 device has internal diodes to VCC. Its quite usual to
> connect even 110 VAC with a resistor (more than 4K7 :-) ) directly to an
> logic input.
>
> Martin


> But what about the power consumption?
A 10k pull up would only use 500uA that doesn't sound too bad.

> You should in any case put a resistor
> (lets say 4K7) between 3V3 out and 5V in.
Why? Do you mean just in case the input ever gets set as an output?


> You could do the same in the other direction (resistor between 5V out and
> 3V3 in) if the 3V3 device has internal diodes to VCC. Its quite usual to
> connect even 110 VAC with a resistor (more than 4K7 :-) ) directly to an
> logic input.

I regard it as bad practice to use those internal diodes unless the data sheet
of the device says you are allowed to do so. I have known CMOS chips that self
destruct at the slightest hint of any current flowing in those diodes.

In summary resistors in series with the lines in both directions sounds okay
to me and would help protect the 3V chip from possible 5V signals. I don't
like to idea of using the internal diodes to limit the input voltage of the 3V
chip though.

Brian Gregory.
RemoveMEbriangTakeThisOuTspamcix.co.uk

2000\05\18@145145 by briang

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In-Reply-To: <4.3.1.2.20000518151218.00a73180@192.168.0.100>

David Duffy <spamBeGonepiclistspamBeGonespamaudiovisualdevices.com.au> wrote:
{Quote hidden}

One of the TTL compatible inputs on the PIC will accept 3V as logic 1 with
no problem at all however come to think of it the schimitt trigger input
pins might not register it as logic 1.

Brian Gregory.
RemoveMEbriangspamTakeThisOuTcix.co.uk

2000\05\18@183259 by David Duffy

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<x-flowed>Thanks for all the suggestions for this problem.
I looked through the ISD site's application notes & there is one
about interfacing to 5V micro's. It says that the ISD4000 series
can accept 5V no problem but the ISD5008 data sheet says in
the specs that voltage on ANY input is Vcc + 0.3V max!
I emailed ISD & Bruce Jordan replied very promptly with;

Quote
  Yep, the ISD5008 will accept 5 volt logic signals without difficulty.  You
  still need to do the same thing as for the ISD4000 with the MISO line,
  though.  Sorry it is not clear from the Data Sheet; I will see what we can
  do for the next revision.
Unquote

Isn't it nice to get questions answered from semi manufacturers ?!
Regards...

At 09:14 5/18/00 +0300, you wrote:
{Quote hidden}

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