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PICList Thread
'Tie outputs together? ... and more'
1997\04\07@162736 by gtham

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Hi everybody,

A few basic PIC hardware questions for you:

1.  I need more than 25mA.  I know it works to tie outputs
   together, but is it safe, as long as the specified max
   current of the port is not exceeded?  I have a feeling
   it's frowned upon.  I measured the short-circuit current
   between outputs, and it is less than 1 microamp, hi and
   low, loaded and unloaded.  I don't have a scope here,
   so I can't see what happens during switching.

2.  Isn't sinking slightly better/more efficient than
   sourcing?
   I'm talking of ordinary loads here, say 5mA.

3.  Is a PIC likely to die from things like a momentary
   short-circuit of an output, like during measuring?


Thanks.  I don't have a formal education in this field,
so excuse the amateurish angle.

Gustaf

1997\04\07@164359 by Clyde Smith-Stubbs

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Thus spake Gustaf Tham (spam_OUTgthamTakeThisOuTspamLIDKOPING.MAIL.TELIA.COM):

> 1.  I need more than 25mA.  I know it works to tie outputs
>     together, but is it safe, as long as the specified max
>     current of the port is not exceeded?  I have a feeling
>     it's frowned upon.  I measured the short-circuit current

It's safe. Indeed, providing you respect all limits, it's recommended
in some circumstances.

> 2.  Isn't sinking slightly better/more efficient than
>     sourcing?

It used to be :-) With TTL and NMOS logic, the output drivers are not
symmetrical, and pull-down capabilities are much greater than pull-up.
With CMOS this is not true unless specifically designed that way. Check
the specs for the chip - look at the maximum sink and source currents for
a pin, and the output voltage at a specific current. Generally speaking
you will find no significant difference for CMOS chips like the PIC. There
are some outputs that are open-drain, and therefore can't pull up at all.
There are also some CMOS chips around that are designed with better pull-down
capability but that's unusual.

> 3.  Is a PIC likely to die from things like a momentary
>     short-circuit of an output, like during measuring?

No, not likely. There will be some limiting time after which you will damage
the chip if excessive current is being sourced or sunk - what it is is hard
to say, but a few seconds won't usually kill the chip - but if it does, don't
blame me!

> Thanks.  I don't have a formal education in this field,
> so excuse the amateurish angle.

Don't apologize. You asked clear, sensible questions, which is something
to be valued.

Cheers, Clyde

--
Clyde Smith-Stubbs    | HI-TECH Software,       | Voice: +61 7 3354 2411
.....clydeKILLspamspam@spam@htsoft.com      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.htsoft.com | QLD, 4051, AUSTRALIA.   |
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1997\04\15@081818 by gtham

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Thank you, Clyde and Kurt, for very informative answers.

I tied together six outputs of a 16c84, and let them both
source and sink 75mA.
I could not with certainty measure any difference in current
consumtion, so I guess that for most practical purposes below
20mA/output any difference in efficiency is negligible.
I'll stick to sinkin', though.  It feels better.

Gustaf

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