[Coco] WAS new mylars, NOW PCB Design

[Salvador Garcia]

Thanks Ed,


I remember ordering .1uF caps for the boards that we designed. We might end up not having the LAN controller to build the prototypes, but we never had a shortage of caps! Our design slogan was "when in doubt, add a decoupler".


Since our boards sometimes had up to 25 chips, we made sure that each of these had its own dedicated cap.


Salvador




-----Original Message-----
From: Zippster <zippster278 at gmail.com>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Tue, Aug 11, 2015 4:58 pm
Subject: Re: [Coco] WAS  new mylars, NOW PCB Design


Decoupling caps are something that is often cited as being something people
forget at first,
or don’t use properly.  Which makes sense, because they aren’t
always mentioned on data
sheets, and sometimes not even shown on some of the
simple schematics, being assumed.

You probably did have a clean supply from the
USB port on your board, but a lack of decoupling
could have introduced noise
into the board’s power as whatever ICs were on board tried to draw
too much
power too quickly.  That’s where the decoupling caps come in.  

If there are
manufacturer suggested decoupling guidelines in a device data sheet, use them
of
course, but a general rule of thumb when nothing is mentioned is to use one
.01uF - .1uF ceramic
capacitor per power pin (some devices have several or
more).  In addition to this you can
use if necessary Electrolytic or Tantalum
caps of higher value at the power input to your board,
and distributed around
the board if it’s a large one.

This should be sufficient for frequencies up to
50mhz or so.  At higher frequencies interplane 
capacitance such as Mark
mentioned earlier (capacitance between ground and power planes)
become useful
(and necessary above around 500mhz).

So, basically, until you’re dealing with
higher frequencies the old standard .01uF to .1uF per
per power pin rule of
thumb should be good.  :)

- Ed



> On Aug 11, 2015, at 3:14 PM, Salvador
Garcia <ssalvadorgarcia at netscape.net> wrote:
> 
> I'll start at the end:
> "It’s
a lot of fun building things,
> isn’t it?  :)"
> 
> Oh yeah!!!
> 
> Thank you
starting this thread.
> 
> I think my gravest error with my first board is that
I did not use any decoupling caps. I knew these are important, but decided to
not include them because my board gets it power from the PC's USB port. I should
not have assumed that the 5v was clean.just because it had been "preprocessed"
by the PC.
> 
> Way back then when I managed product design I got a chance to
work with the PCB layout engineer and understand the the trails of getting
everything routed. I captured the schematic using ORCAD and the engineer
imported the nets into PADS PCB. The engineer then took up to 4 weeks routing
everything. He initially would autoroute, but then would manually work with each
one individually. Since there were 4 layer boards, two of these were reserved
for power and ground and the other two for traces. This was akin to solving a
complex puzzle, sometimes leading to a dead end. In that case the engineer would
start from scratch to find another way to route. Usually the first 90% of the
traces were no problem. It was the last 10% that provided a challenge and the
last 2% that determined whether the board could be routed or not. At times the
engineer changed the placement, taking into account any placement that was
critical.
> 
> I learned enough from this engineer to know that various factors
have to be taken into account, some of which you mention.
> 
> Fortunately, I
did include a pseudo ground plane (meaning that I have a ground plane, but it
only covers part of the PCB) so at least I can say I am headed in the right
direction (even if I am not there yet. The ground plane covers all ground
connections except one. I have traces on that layer too. As I learn more I hope
to make the boards more efficient and robust.
> 
> Great tip on building your
own component library. I had to do that to include an Arduino Nano in the
design. It came out better than I expected, but still with a few holes (For one,
I missed adding the silkscreen to define the component outline)..
> 
> Thanks
for offering to share. I am ready to learn as much as I can.
> 
> 
>
Salvador

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