Any issues with running PICs from -5V supply ?

[Peter Onion]

Resent with a TAG this time...

I'm building a PIC circuit to interface to a very old computer that uses
0v for "1" and -10V for "0".
(http://www.youtube.com/watch?v=8EKBNfm5B4U)

It will make for simpler interface circuits if I run the PIC from 0V and
-5V.  (I.E. 0V on Vcc pins and -5V on Gnd pins).  

Are there any issues with doing this ?  

PeterO


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[Jan-Erik Soderholm]

Peter Onion wrote:

> Resent with a TAG this time...
>
> I'm building a PIC circuit to interface to a very old computer that uses
> 0v for "1" and -10V for "0".
> (http://www.youtube.com/watch?v=8EKBNfm5B4U)
>
> It will make for simpler interface circuits if I run the PIC from 0V and
> -5V.  (I.E. 0V on Vcc pins and -5V on Gnd pins).  
>
> Are there any issues with doing this ?  
>
> PeterO
>
>

All voltages are rellative.

The PIC doesn't "know" that what it sees
as "5V" is called by you as "0V".

You could use 15V as "GND" and 20V as "5V" as
long as you do not do anything else "funny" with
your interfaces...
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[Gerhard Fiedler]

Peter Onion wrote:

> I'm building a PIC circuit to interface to a very old computer that
> uses 0v for "1" and -10V for "0". (http://www.youtube.com/watch?v=8EKBNfm5B4U)
>
> It will make for simpler interface circuits if I run the PIC from 0V
> and -5V.  (I.E. 0V on Vcc pins and -5V on Gnd pins).
>
> Are there any issues with doing this ?  

People already responded and told you that voltages are relative and
that it works. Which is mostly correct.

What you may need to keep an eye on are grounding issues. These are a
bit more complex when you have grounds on different levels. What you see
on your PIC circuit as ground is not a ground anywhere else, AFAICT, nor
a low impedance point anywhere else.

Depending on the power supply situation on this old computer, using the
-10V as your PIC ground may have the advantage that you can connect your
PIC ground at least to something with low impedance on that computer
(assuming that there is a -10V supply rail with a low impedance). Level
translation is just as simple.

Of course power supply isolation is imperative :)

Gerhard
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[Jo Scherpenisse]

----- Original Message -----
From: "Jan-Erik Soderholm" <jan-erik.soderholm@...>
To: "Microcontroller discussion list - Public." <piclist@...>
Sent: Sunday, November 01, 2009 6:15 PM
Subject: Re: [PIC] Any issues with running PICs from -5V supply ?

> Peter Onion wrote:
>> Resent with a TAG this time...
>>
>> I'm building a PIC circuit to interface to a very old computer that uses
>> 0v for "1" and -10V for "0".
>> (http://www.youtube.com/watch?v=8EKBNfm5B4U)
>>
>> It will make for simpler interface circuits if I run the PIC from 0V and
>> -5V.  (I.E. 0V on Vcc pins and -5V on Gnd pins).
>>
>> Are there any issues with doing this ?
>>
>> PeterO
>>
>>
>
> All voltages are rellative.
>
> The PIC doesn't "know" that what it sees
> as "5V" is called by you as "0V".
>
> You could use 15V as "GND" and 20V as "5V" as
> long as you do not do anything else "funny" with
> your interfaces...

Yes, I agree, but be very carefull with measuring equipment such as an
oscilloscope. A ground error is easy made..
I should prefer read voltage translation. It is not that difficult.

Jo

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[Thomas C Sefranek]

  ----- Original Message -----
  From: Gerhard Fiedler
  To: Microcontroller discussion list - Public.
  Sent: Sunday, November 01, 2009 2:56 PM
  Subject: Re: [PIC] Any issues with running PICs from -5V supply ?


  Peter Onion wrote:

  > I'm building a PIC circuit to interface to a very old computer that
  > uses 0v for "1" and -10V for "0". (http://www.youtube.com/watch?v=8EKBNfm5B4U)
  >
  > It will make for simpler interface circuits if I run the PIC from 0V
  > and -5V.  (I.E. 0V on Vcc pins and -5V on Gnd pins).
  >
  > Are there any issues with doing this ?  

  People already responded and told you that voltages are relative and
  that it works. Which is mostly correct.

  What you may need to keep an eye on are grounding issues. These are a
  bit more complex when you have grounds on different levels. What you see
  on your PIC circuit as ground is not a ground anywhere else, AFAICT, nor
  a low impedance point anywhere else.

  Depending on the power supply situation on this old computer, using the
  -10V as your PIC ground may have the advantage that you can connect your
  PIC ground at least to something with low impedance on that computer
  (assuming that there is a -10V supply rail with a low impedance). Level
  translation is just as simple.

  Of course power supply isolation is imperative :)

  Gerhard

I'd suggest just using level inverter/converters like the old 1488/1489 and not deal with all this.


  *
  |  __O    Thomas C. Sefranek  tcs@...
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