Clarification of question -- RE: [En-Nut-Discussion] Hardware Question -- PCB Layout help?

[Harald Kipp]

Alex,

Routing PCBs is something special and requires a lot
of experience. IMHO, there is no shortcut. In the first
place it's learning by doing. If I remember correctly,
the first Ethernut was version 0.9 and it was sooouuu
ugly, but it worked.

You need to read a lot, its a very complex process. Like
with other topics you learn, many things you read will
not make sense first.

Before sending the design to the boardhouse: Check it,
check it again, and check it once more the next day.

At 14:14 12.01.2004 -0800, you wrote:
>Thanks for the responses!  Now let me clarify. . .
>
>I downloaded evaluation version of lots of different software packages -
>Protel, Orcad, Multisim, etc.  I found Eagle the easiset to use, and of
>course it is nice that the Ethernut schematic is already done in Eagle.  So
>I'm going to buy a copy of Eagle, but that's not where I need help.

In the meantime I'd prefer Protel. Compared to Eagle, it's
very expensive and IMO too much for newbies. Yes, I'd
recommend Eagle to start with, but I do not know many CAD
programs really.



>What I'm looking for is information about the art of actually placing and
>wiring components on a board.  For example, I know that it is a good idea to
>use decoupling capacitors near digital ICs, but how big and how many?  When

Placement and wiring _is_ the tricky part, which requires
the experience I talked about. There are many advices,
but every special situation requires specific decisions.
Of course, I'm still learning too. I started with auto
routing (there's no auto placer for Eagle) and didn't
understand, why professionals hate auto routing.
Ethernut 2 was manually routed, because in the meantime
I began to understand more about signal levels, inductivity,
capacity, EMC, ESD etc.

The specific question about decoupling is easier to answer.
The problem is, that digital circuits tend to consume power
in sharp peaks while switching. These peaks cause drops on
power lines, which may influence other circuits and produce EM.
Placing the cap near the pins of the consuming chip is
one thing. Also make sure, that power is routed pass the
capacitor. If the chip gets an opportunity to draw power
from a different trace, it will take it and the cap is
useless. The bigger the better, if the ESR is low. But
low ESR 100 uF caps are expensive and big. 100 nF ceramics
are a good choice. SMDs are much better than through hole
parts.


>do I need a ground plane and when can I just use separate analog and digital

A ground plane is not needed, but simplify one task:
Keeping the reference signal (better word for ground)
at a constant level throughout the whole board.


>grounds connected at the power input?  How wide do my traces need to be for
>high-current drivers?  When a new PCB has been made by a clueless person

What current is high? 100 mA, 1 A or 100 A? :-)
Again, in most cases the bigger the better. But
you can google the Web for minimum requirements.


>(like me), what are the most common problems?  I know that PCB design is
>more than just placing all the components on the board and hitting the
>"autoroute" button -- what more do I need to know?

Much more than I know. :-) But again my advice is,
just start, make mistakes and learn.

There are many good books. I recommend

High-Speed Digital Design
A Handbook of Black Magic
by Howard Johnson and Martin Graham
Prentince Hall

http://www.phptr.com/content/images/0133957241/0133957241.jpg

Kind regards,

Harald