"Paul Burridge" wrote in message
...
On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite
certain
that the ringing is really there. Ditch the ground lead from the
scope
probe and make a very short one by wrapping stiff wire round the
earth
ring at the end of the probe and leaving a piece as long as the
probe
tip sticking out. Get up close to the chip signal and ground pins
and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over
the
top of the IC. For some reason, on this chip, the supply and gnd
pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen
bit patch
to an eight bit operating system originally coded for
a four bit
microprocessor and produced by a two bit company."

The 'HC and 'AC families (at least the simpler parts) can be made to
give
_extremely_ clean square wave outputs, but as Don and Mike say, you
have to be careful.

The decoupling cap you mention above sounds like a leaded part, with a
very substantial amount of lead inductance when mounted like that. For
good results, ideally you would have planes for power and ground on
adjacent
layers in the PCB under the chip. That allows you to put the decoupler
at
either end of the chip, and still retain low inductance. Adjacent
planes also
acts as a small but very good at HF capacitor.

Boris makes a good point about the size of the loop between probe tip
and
ground. You can get little socket adapters which you solder to the
board to
minimise this.

Regards
Ian