Paralleling CMOS gates is a common practice because the FETS are self
adjusting.  The device delivering the most current heats up, which increases
the channel resistance in the internal MOSFET's, limiting its current so the
other device must supply more current.  The process continues until
equilibrium is reached. The voltage drop on transistors decreases with
increasing temperature so more current is drawn, which decreases the drop so
more current is drawn, etc. ... positive feedback.  Moral: use resistors
with bipolar parts.  If the CMOS gates are switched together, you should be
OK without.  I think uchip says to add resistors to limit their liability
when somebody switches one output at a different time than another - POW!

If you're using the outputs for high frequency switching, then the slight
dynamic differences from one gate to another may cause a local heating
problem.  To be safe, I wouldn't try to source or sink current to the
maximum sum of the individual pin's current rating.  That is, for two pins
connected you can get more current than one pin can supply, but less than
the sum of the two.

A lot depends upon your application.  For example, I might use this trick if
it were for an inexpensive consumer product, but would not on a system where
I was driving a relay for a safety shutdown on a nuclear power plant. ;-)

Ed

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Marcel van Lieshout
Sent: Saturday, December 13, 2003 9:18 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Parallel PIC-pins


Hello group,

I saw some examples on the web paralleling PIC-pins to increase maximum
source/sink current. Is this allowed? Is it necessary to include a small
resistor on each pin to compensate differences in pin-specifications?

Marcel

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