Re: gEDA-user: Question for DJ about his alarm clock

[der Mouse <mouse@xxxxxxxxxxxxxxxxxxxx>]

> The power plane?  That board has two planes (power and ground).  Why
> is one always the reference?

In low-voltage electronics, there is no such thing as a voltage in
isolation; voltage exists only in reference to somewhere else.

In high-voltage electrostatics work, there is an absolute reference,
that being "electrically neutral", as in "having equal amounts of
positive and negative charges".  In theory, the same measure could be
applied to low-voltage circuits too, but the imbalance in charge
carriers is so tiny compared to the number of charge carriers involved
in the kind of current flow typically involved in low-voltage circuits
that it's not a useful way to look at it in practice (whereas in
high-voltage electrostatics work, current flow is so low compared to
the potentials built up that the converse view is more useful).

In many - I'd even go so far as to say "most" - low-voltage circuits,
there is only one reference, and it's called "ground".  When mixing
analog and digital, this is sometimes split into 'analog ground" and
"digital ground", but each is still "ground" to its portion.

Which is all to say that I'd say that, _by definition_, the ground
plane is the reference: using the term "ground" for it is effectively
saying "this is the one that's my voltage reference point".

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