[sdiy] unused pins ?

[Magnus Danielson]

From: harry <harrybissell at prodigy.net>
Subject: Re: [sdiy] unused pins ?
Date: Thu, 05 Apr 2001 15:22:41 -0500

> Harry will throw in a related story...
> 
> When you tie CMOS inputs high, or low... be sure you do
> no tie any OUTPUTS by mistake.
> 
> Outputs should be left floating....

Indeed they should!

Also, please don't leave the VCC of a CMOS chip unconnected, it CAN
get through undetected since the ESD protection diodes can take an '1'
and use that to power the rest of the chip!
I've seen outputs burn to hell when they where loaded with an
unpowered board full of CMOS.
That's learning about hot-swap the hard way :-(

Let's just say that the digital fellows learned electronics quickly.

> Finding a shorted unused output is very hard. The last place you look
> in a non-functional circuit is the gates that are not used. You will be lucky
> 
> if you probe there. I found one because the chip was slightly warmer than the
> rest...

Right.

Leaving an CMOS input floating around, it will self bias into the
linear reagion and then toss some parasitic capacitance between the
input and output and voila! you got yourself some highfreq
selfoscillation just for free. Yeap, you DO feel the difference as a
heat sensation. Tossing a scope probe on the input may load the input
out of oscillation.

Keep these inputs tied, keep the outputs untied.

Cheers,
Magnus