[sdiy] CMOS unused inputs...

[Scott Gravenhorst]

Unused inputs of unused gates can be grounded or set to Vcc.  Unused inputs
on gates in use should also be set to either ground or Vcc, but you need to
make sure that the level you choose makes sense.  For example, a NAND gate
the is used as an inverter means that one input is set to Vcc.  If you
ground one input of a NAND, the output will never go low.

The unused flip flop in the 4027 needs all of it's inputs either grounded or
Vcc.  This is really just to prevent noise from toggling these because more
current is drawn by gates that are switching.  This switching can also cause
noise to be presented to the power rails.  Don't ground any outputs, they
should float if they are not being used.  There are only very special cases
where you would want to ground an output - creating a stiff Vcc/2 reference
is one of them, but this isn't going on here.

"Pat Kammerer" <spivkurl at wearerecords.com> wrote:
>Hi,
>
>I ground unused CMOS inputs right?  Does that include switches, and flip
>flops?  Only inputs?
>
>I'm asking because I've finished the suboctave and LFO sections of the
>4069 VCO that I was working on, and something is wierd.  I can hear the
>Sub octave as well as the pulse and saw, but the level controls seem odd. 
>The LFO is not working.  

Did you check this with an Oscope or do you say this because the PWM doesn't
seem to work?

>My pitch and PWM CV barely works, like there is NO range at all, 

Hmm, I thought you had gotten the current sink working...  No?

>and the potentiometer direction is reversed.  

Reverse the connections on the outside lugs, this will correct it's
rotational sense.

>Finally I was
>trying to figure out what was happening, and I checked the supply voltage.
>My supply is inverted when the circuit is on.  

I'm not sure I understand the last sentence.

>This would explain the 
>issue with the CV.  Could something ungrounded and unused in a 4066 or
>4027 cause this?  I've never used either of those chips so... you know.

No, open inputs are going to cause some noise on the rails more than
anything and only if the noise voltages on those open inputs causes the
gates to toggle.  It's always a good idea to ground (or Vcc) all of the
inputs of _unused_ gates.  The problems you are having are not typical of
the symptoms of a noisy power supply rail.

>Thanks for your help.  It's definitely getting close though... and I need
>some sleep badly.
>
>Later,
>Pat
>

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