Hello,

On Wed, 13 Jan 1999, James Cameron wrote:

> > What are signs that decoupling capacitors are needed?
>
> Spontaneous resets.  Resets or unexpected behaviour when you stick a
> transmitting mobile phone near the circuit.  Unexpected input data when
> an output is driven.

Actually 'scoping the Vdd line with a high frequency scope properly
attached (RF class grounding - not so easy sometimes) is the only way to
find out. If there are high frequency circuits involved, with
non-repetitive waveforms (such as HC gates, other logic, and
microcontrollers) you'll need a DSO to make sense of what is happening.

Lacking this, the decoupling rules outlined in design manuals and data
sheets should be obeyed, until proven wrong.

hope this helps,

        Peter