[sdiy] Tri-state outputs and CMOS inputs.

[Ray Wilson]

CMOS inputs should be connected to a pull up (or down) if connected to the 
output of a tristate output. Otherwise the input can be left floating when 
the tri-state output is floating. This can lead to oscillation appearing at 
the CMOS gate's output. This can clock succeeding circuitry and cause you to 
pull your hair out. This can lead to mysterious non-deterministic behavior 
that can be hard to trouble shoot because when you probe the input you are 
essentially pulling it down and stopping it's erratic behavior. You can use 
1M or more to pull CMOS inputs down (or up as the case may be).




----- Original Message ----- 
From: "Dave Kendall" <davekendall at ntlworld.com>
To: "synth DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Friday, February 20, 2009 8:56 AM
Subject: [sdiy] Tri-state outputs and CMOS inputs.


> Hi all.
>
> I can't find a definite answer anywhere to this one - Do CMOS inputs need 
> pull-up/down resistors if connected directly to the outputs of ,say, a 
> 3-state latch - which is not connected to anything else)
> In my experiments, it *seems* to work OK, but there are too many variables 
> in the rest of the circuit to be sure....
>
> My gut feeling is that CMOS inputs *should* need resistors, but it'd be 
> good to know for definite.
>
> cheers,
> Dave
>
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