[sdiy] Tri-state outputs and CMOS inputs.

[Dave Manley]

Dave Kendall wrote:
> 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....
> 

The answer is: it depends.

When the 3-state output goes HiZ, the input is going to float at the 
last driven value of the output...at least it will for a while, 
depending on leakage, crosstalk, etc that could disturb that input and 
cause the value to change.  Think of it as a digital sample/hold.

If the 3-state output is regularly enabled at some high rate, then it 
doesn't really need a resistor.

The worst case scenario is the input doesn't get regularly refreshed and 
there is a slow leakage path that causes the input to drift towards the 
switching threshold of the input.   At that point, the output of the 
CMOS gate could oscillate, causing noise on your power supply, and 
unknown effects to the downstream logic.

-Dave