hardware board design - beginner... pls help

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S45

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Dec 30, 2006, 7:43:27 AM12/30/06
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Hello,

I am an electronics graduate and I wish to learn hardware board design.
I would like to learn High speed board design(>100MHz), schematic
entry, signal integrity and prototype testing.

How shall i begin? Whats the flow of board design?. Please let me know
some pointers or web resources.

Thanks

Luhan

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Dec 30, 2006, 10:47:26 AM12/30/06
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If you could tell us what your current expertise is, you may get better
advice on what you can do from here. My own collage experience
(University of Illinois, Urbanna), is that little of any practical
experience is gained taking Electrical Engineering. My expertise is
99% self taught.

Luhan

PeteS

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Dec 30, 2006, 11:12:00 AM12/30/06
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Before tackling high speed design, I strongly suggest doing basic PCB
design, schematic entry &cetera.

Volumes could, and have, been written on this subject. Google is your
friend:

The search term:
PCB design tutorial

Turns up about 783,000 hits; most aren't much good, but there's a few
decent hits on the first page.

Cheers

PeteS

D from BC

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Dec 30, 2006, 11:43:23 AM12/30/06
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.

Luhan
----------------
I like to view electronics an art. Not knowing where to begin is
a special time in a project.. It's kinda like jumping in bed
with a new girlfriend. Would you like it if somebody was
coaching you while you in bed with a girlfriend.. Probably
not..Go for you our style and explore...
Make a choice... :)
Or course...knowing what others have done can save a lot of
time.

D

Paul Mathews

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Dec 30, 2006, 11:48:22 AM12/30/06
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Look for books and articles by authors Howard Johnson and Mark
Montrose.
Paul Mathews

Luhan

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Dec 30, 2006, 1:04:19 PM12/30/06
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Electronics, like just about anything else, can be a job, a skill, or
an art - depending on the person doing it. I most enjoy designing
things with 'hardly any parts' that look like 'they shouldn't work'.

Luhan

John Larkin

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Dec 30, 2006, 1:25:02 PM12/30/06
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There is an enormous body of technique and lore that is widely
accepted in industry but virtually unknown in academia. And difficult
to pick up on your own. If you can, get a junior-level job in a small
or medium-sized company as an engineering aide or whatever, try to be
as helpful as you can, and learn from the experienced guys. If they
aren't helpful, or it looks like they're not doing good stuff, go
somewhere else.

For example, I've never heard of a college course mentioning, much
less teaching, documentation standards and control; well, maybe one
course taught by a former co-worker who spent time in the real world.

John


PeteS

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Dec 30, 2006, 4:02:40 PM12/30/06
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So true

For the OP:

Here's a typical very simple design flow and note that each and every
stage requires thorough documentation:

1. Get requirements from customer / marketing.

This is more difficult than it sounds; you can't just say 'I want to
sample some signal and then control something else based on it' - there
has to be a specific link. That a *very* simplistic example,
incidentally. There's a lot more - power requirements, battery
operation?, chargers, size, weight - there's a thousand and one things
(per the old saying) that have to be thoroughly _defined_.

2. Choose the architecture to meet the requirements. This is 90%
experience at least. A simple design can be easily found (at least
architecturally) on the web, but as things get more complex (and
customised) the chances of finding what you need already done lessen (as
they should - if it's already been done why are you doing it unless to
make something cheaper / better?)

3. Choose the parts to implement the architecture. Again, more difficult
than it sounds. Make sure the parts are actually available and not
vapour-ware, and are reasonably affordable.

4. Start schematic capture. Notice how far along we are before this even
starts? Of course, you'll need to make the parts and symbols yourself
(lots of threads on this both here and on sci.electronics.cad) so be
prepared to do a lot of paperwork and checking.

Have the parts and symbols checked thoroughly by someone other than
yourself if possible (this is a matter of not always being able to see
your own errors).

5. If you are doing anything with constraints (anything non-trivial in
other words) then make sure you enter the constraints at the schematic
level if possible - I well remember setting the constraints for a single
sub-system (DDR at 200/400) where setting the constraints took longer
than the schematic capture phase - and that was but a small part of that
board. Once more, document thoroughly what those constraints are and why
they are set that way.

6. Run the various error and warning checks on the schematic; once you
have zero errors and zero warnings (well, there are some warnings that
are ok, rather like the output of lint), run the netlist.

7. Choose a PCB stack - this is, to a great extent, experience. How many
layers, what each layer is (signal, ground, power etc) and make this
before even considering importing the netlist. Also, define the board
outline and any breakaways (necessary in manufacture) that may be needed.

8. Export the netlist and then import it into the layout tool. You'll
need to make sure you have all the device symbols and padstacks set up
prior to this. At this time, you should also generate the PCA BOM - note
that this is not the final BOM! Again, lots of documentation.

Now, and only now, do we get to layout. This is a subject in it's own
right, but here's a typical simple flow:

9. Place the components as required by the various schematic set rules
(see above) and anything else that may be relevant (power distribution,
signal distances, decoupling devices - again, there's huge numbers of
variables)

10. Hand route the power - this can be *very* difficult depending on how
many power domains exist. The highest number I've personally done on one
board was 27 separate power domains (because I was switching the power
to many devices) and set up the power and ground planes (but we'll have
to return to them)

11. Hand route any critical signals - autorouters, in general, suck for
critical signals, and it's best (usually, imo, ymmv) to route the
critical stuff and mark it as fixed and then let the autorouter do it's
thing.

12. Now go back and iterate steps 9 to 11 until it's all done and meets
the rules.

13. If any pin-swapping was done, go and back-annotate the schematic.

14. Set up the documentation layers and so forth on the layout.

15. Generate gerbers and assembly prints - this is usually known as a
fab-pack.

16. Generate the final BOM (including housings, clips, plug-in modules,
whatever) and remember to check the PCA BOM - if you are lucky, you can
ship it off to be made :)

Note I left out some pretty common steps because they aren't always
done, but every single step has to be documented, and there must always
be a master document - in my case, that's the spec - from which the
design had to flow. This is the absolute minimum.

In addition, I did not introduce the _very_ common issue of an updated
netlist in the middle of layout (may my PCB guys forgive me ;) ).

Practical design requires a lot of documentation as well as design skills ;)

As I and others (especially John) noted, there's a lot to it that is
usually never encountered in college.

Cheers

PeteS

Paul E. Schoen

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Dec 30, 2006, 5:00:15 PM12/30/06
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"S45" <satur...@gmail.com> wrote in message
news:1167482607....@v33g2000cwv.googlegroups.com...

PeteS provided an excellent flow guide to this process. PCB design is
partly Art and partly Science. I use PADS-Logic and PADS-PCB (with their
Blaze autorouter), and it is very good, although the parent company Mentor
Graphics has an unreasonable annual maintenance/support scheme (scam?) that
would have me advise anyone not to purchase their package. Cadence is
probably now a better option for high end packages of this type ($5000 to
$20,000+).

For your own use and education, you can download from (www.mentor.com)
their complete latest version which is fully usable for very small designs,
and it is a good place to start. They have some good tutorials and sample
designs. Another very valuable resource for users of this product is a very
helpful and friendly forum you can read and join to post questions:
http://listserver.pads.com/read/all_forums/

Good luck:

Paul


JeffM

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Dec 30, 2006, 7:16:40 PM12/30/06
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"S45" <saturday45@ gmail.com> wrote:
>>>I wish to learn hardware board design.

PeteS wrote:
>Here's a typical very simple design flow[...]


>1. Get requirements from customer / marketing.

>. . .
>16. Generate the final BOM[...]

Ken Smith went into even more detail when he covered this: 8-)
http://groups.google.com/group/sci.electronics.design/browse_frm/thread/85d427921d4ae936/4f65710c80ef5961?q=the-main-stepps-involved-in-circuit-board-design
news:cscid6$76e$3...@blue.rahul.net

Part 2 by George Gonzalez (Ancient_Hacker) is worthy as well:
http://groups.google.com/group/sci.electronics.design/browse_frm/thread/c50de971e779f7e8/c32c0c19ccd2080e?q=you-could-add-a-few-more
news:1138038327.2...@g43g2000cwa.googlegroups.com

PeteS

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Dec 30, 2006, 7:25:01 PM12/30/06
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I have said for many years that design is where Art meets Science.

Cheers

PeteS

PeteS

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Dec 30, 2006, 7:30:20 PM12/30/06
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That's one of the great threads - I didn't remember where they were :)

Good catch for finding them!

Cheers

PeteS

jasen

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Dec 30, 2006, 5:54:47 PM12/30/06
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On 2006-12-30, D from BC <myreal...@comic.com> wrote:


> It's kinda like jumping in bed with a new girlfriend.

> Or course...knowing what others have done can save a lot of
> time.

LOL


--

Bye.
Jasen

S45

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Dec 30, 2006, 11:05:21 PM12/30/06
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Thank you everybody for your replies. My special thanks to PeteS.

Happy New Year 2007!

David Starr

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Dec 31, 2006, 9:06:03 PM12/31/06
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Board design breaks down into electrical design and mechanical design
(PCB layout). My experience is from the electrical side of things. The
electrical designer is responsible for the electrical schematic, the net
list, the BOM and the acceptance test procedure. He has project
responsibility, in other words if the board fails to work, it's the
electrical designer's fault. Since this is so, it behooves the
electrical designer to establish a tight working relationship with the
mechanical designer and to understand as much as possible about the
mechanical design process.
Electrical design starts with the specification writing. As designer
you are faced with the "don't know squared" problem, the customer does
not know that he doesn't know what he wants. Typically the designer
writes the specification and then coordinates it with the customer. The
spec needs to be a "requirements" spec, that spells out what the board
is supposed to do, how fast it has to run, all the inputs and outputs,
the operating environment, the size, weight and power consumption, the
case work, the temperature and air pressure range, and anything else
that the customer or the designer cares about. Limitations (can't work
with USB 2.0, reprogramming requires desoldering parts, no support for
rotary dialing, what ever) should be spelled out and agreed to by the
customer. The spec MUST be written in language meaningful to the
customer, otherwise the customer's eyes glaze over and eventually he
signs off on something he doesn't really understand just to get the
project moving. This can lead to disaster later in the project when the
customer is shown the first working breadboard and says "Oh I didn't
mean for it to work that way."
The spec has a strong effect on the cost of the product. Each
feature called out in the spec raises the cost of the board. As
designer and manufacturer you want to get the cost down. On the other
hand if the board lacks the features customer's truly need then it won't
sell either.
Specs should put numbers on as many things as possible. For instance
say the product runs on 120VAC +/- 30 volts 50-60 Hz rather than "runs
on line voltage". Always spec size and weight because cost is dependant
upon size and weight. Spend a lot of time to get the spec right. The
wrong spec will kill the project before its finished. Get senior
management, both yours and the customer's, to sign off on the spec. Get
real signatures, in ink, on paper. Keep the signed copy yourself if
possible, otherwise keep a Xerox. Make sure the spec has a date and a
revision number on every page. Change both the date and revision each
time a change, no matter how small, is made in the spec. Be sure no one
can change the spec without your knowledge. Remember, at the end of the
job, you won't get paid if the product fails to meet spec.
While the spec negotiations are ongoing, consider the
microprocessor. Nowadays most boards contain one or more
microprocessors. Any digital logic running at less than a few kilohertz
can be done by the microprocessor in code which is economical to
manufacture. A lot of your development cost will go into the
programming of same. Get the software guy[s] on board early, make sure
they attend the spec reviews and understand the product. Select a
microprocessor that's easy to program AND that the software guy[s] like.
Easy to program means a C compiler, a symbolic debugger, and a
software testbed so the software guys have something to work on before
the first board is built. Don't get stuck in assembly language
programming. The Intel 80186 runs the same code as the PC's, which
means the software guys can test their code on a PC long before a test
board becomes available, which makes it my favorite choice. If the
software guys want to use this micro or that micro go along with them.
They are indicating the micro upon which they have a lot of experience ,
which means they can get the product up and running faster than if they
have to learn a new processor.
Figure out how you are going to test the product. If you are lucky
you will have the volume to afford to build bed-of-nails test fixtures
for Genrad board testers. More likely you don't have the volume to
afford either the fixturing ($25K?) or the Genrad machine ($100K?).
More cost effective is to have the on board micro processor run a
built-in-self-test upon power up. For this to work the board needs to
be able to stimulate its inputs and monitor its outputs. In some cases
provision of feedback paths from output to input will serve both
purposes. In this day of surface mount it is nigh on impossible to hand
probe chips, you have to provide test points that allow observation of
key signals with scope or logic analyser. Much of the electrical
designer's time is spent testing and verifying the prototype, don't
paint yourself into a corner for lack of test points.
Parts selection comes next. You should be able to find large scale
integration (LSI) chips to handle most general purpose tasks A/D, D/A,
serial I/O of various sorts, audio and video compression, oscillators,
voltage regulators, parity generator, filters and so on.
Any digital tasks that lack LSI, and run too fast for the micro to
handle are done in programmable chips, PALS or FPGA's. PALS are
simpler, and the design tools are cheap to free. FPGA's are larger,
more powerful, but the programming tools are pricey ($20k) and tricky.
Both parts can be programmed "schematically" or "programmably". I
prefer "programmably" because a short program listing is easier to read
and debug from than a 50 page schematic. PAL and FPGA programming tools
allow to user to specify test vectors and exercise the design in
software before burning chips. I always write test vectors and run the
chip simulator to see that the programmable chip design works in
simulation. It's much easier to debug a simulation than to debug a live
board with a scope.
Side by side with programming the PAL's and FPGA's you start doing
your schematic. There are a lot of schematic capture packages out
there. If you have a choice, go with ORCAD. It's very dependable and
fool proof. You can hit the R-for-Repeat key and get a data bus drawn
with the signal labels (D0, D1,D2..Dwhatever) automatically assigned.
Each press of "R-for-Repeat" draws another bus entrance. Saves a lot of
work and avoids the common error of labeling two bus leads with the same
label creating a short in the net list.
ORCAD supports hierarchical design. Circuits used in many places
(say rows in a memory array) can be drawn ONCE but used repeatedly.
This feature saves a lot of errors (the number of errors is proportional
to to the size of the schematic). It also permits changes to be done
once, on one sheet and be implimented thru out the design.
Schematics are easiest to read if they are all on ONE page. Offpage
connections are an abomination. Use the biggest possible paper size to
reduce the number of off page connections. ORCAD will, if asked, find
errors such as two outputs connected together or a pin not connected to
anything. Always ask, and then clean up all the errors. Then with a
clean error free schematic that looks like it will work electrically,
have ORCAD create a net list. Get an assistant and compare the net list
with the schematic run by run, yellow out the runs on the schematic and
on the net list as you go. You will probably find a few things on the
net list the do not match the schematic. Fix the schematic and re run
the net list until they match perfectly. Have ORCAD prepare the BOM.
Check same and assemble the manufacturers data sheets for each and every
part in the BOM. Many parts come in many different ppackage styles
(DIP, PLCC, various types of surface mount). Decide which package
style you are going to use and make sure the mechanical designer (PC
layout guy) understands what your choices are.
Get with the layout guy and make sure he understands about board
size (outline) and the location and size of all the mounting holes,
otherwise the board won't fit into the casework. Make sure nothing
sticks up too high, otherwise the board won't fit into its slot. Hand
check the pin out of every IC, make sure that pin 1 is where it ought to
be. Intel chips usually put pin 1 on a corner. Most other makers put
pin 1 in the center of an edge. If the layout mixes this up, the board
won't work. Make sure the surface mount passives are of the proper
size, otherwise they won't fit. Use at least four layers, two routing
layers, power, and ground. Insure that every part has a reference
designation silk screened on the board and that the ref des can be read
AFTER the parts are soldered down. Without the silkscreen the board
won't get stuffed right, and it is extremely tedious to trouble shoot it.

There is more, but this will serve as a start.


David Starr

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