Homebrew PCBs

Lots of good ideas, Leon.  I use most of them as well.  Wish eagle 
allowed the colored "air wire" thing as i spend too much time 
checking to see it is a supply connection.  

My wife did PCB design for many years and I asked her the advanced 
routing question.  She just laughed, shook her head and 
said "beginners. there is no magic book".  No suprise to me.  It just 
boils down to a set of design rules and techniques.  There is no 
substitution for experience.

A couple more thoughts:

I spend a lot of time looking at the schematic trying to get a sense 
of what goes best with what.  This helps me to place components in 
their best position.  I have a dual monitor set up so I can keep the 
board on one screen and the schematic on the other.  That really 
speeds up placement.

For pins that are swappable, I will change the schematic to simplify 
the layout.  By swappable, I mean any pins that can be exchanged for 
same function.  For examnple, a hex not gate has 6 inputs and 6 
outputs that can be used for the same purpose.  swapping gate 1 for 2 
may lead to simpler layout.  Same thing for microprocessor register 
pins (non-dedicated ones, of course).  Headers are another place 
where you might be able to move things around to simplify layout.  I 
guess PLDs are the ultimate case.  I bop back and forth between the 
schematic and board swapping things around, often to significant 
improvement.  Clearly, there are times when you dont want to 
complicate programming but who cares if a function is on port B pin 1 
or 2, for example.  Its kind of a pet peeve of mine that chip 
designers don't always consider layout complexity when they assign 
pins.  Compare AVR vs midrange PIC in system programming pins, for 
example (AVR got it right).

By the way, I also put SMDs on the "solder" side when doing mixed 
TH/SM layouts to avoid vias for boards I'm making myself.  If it 
prevents a drill hole, it is good.  But then if I'm using a board 
house, I dont care, so viva las vias!

Phil

--- In Homebrew_PCBs@yahoogroups.com, "Leon Heller" 
<leon_heller@h...> wrote:
> ----- Original Message ----- 
> From: "Stefan Trethan" <stefan_trethan@g...>
> To: <Homebrew_PCBs@yahoogroups.com>
> Sent: Wednesday, August 04, 2004 9:35 AM
> Subject: [Homebrew_PCBs] advanced routing
> 
> 
> > Hi,
> >
> > i was wondering if there is a tutorial for advanced routing.
> > I mean as soon as you have two layers and more than 100components
> > it starts to get more difficult.
> > If there is no tutorial we could maybe exchange some tricks.
> 
> I've looked and I don't think there is one.
> 
> Here are a few 'rules of thumb' I've come across which I find very 
useful:
> 
> >
> > How do you start?
> 
> Start placement with the largest parts.
> 
> > How do you decide which parts are on which side?
> 
> > (i put the SMD ones on top which can be connected to throughhole 
without
> > adding a via, and i aim for the same population density top and 
bottom.)
> 
> For low-cost manufacture it's best to put all the SMDs on the top. 
If you
> put them on top and bottom your technique seems OK.
> 
> >
> > Often it is required to rotate and rearrange parts to get a good 
layout,
> > but at a certain number of parts it is very hard to keep track of 
things.
> 
> Try to keep the components in functional blocks, like in the 
schematic. An
> autorouter can be useful, even if you are routing manually, to show 
where
> there are going to be problems. If you are using an autorouter, 
here are
> some tips:
> 
> http://www.connecteda.com/doc/Autorouting%20Techniques.pdf
> 
> They are applicable to any autorouter. BTW, that Electra autorouter 
is
> *very* good, it's now supplied with the Pulsonix software I use.
> 
> >
> > Do you start routing with special nets, like supply?
> 
> Critical nets like power and ground, and clocks, etc. should always 
be
> routed first.
> 
> >
> > You see, the methods i used for years on smaller circuits seem 
very, very
> > hard
> > to apply to bigger circuits, and i believe there must be some 
tricks i
> > don't
> > know yet.
> > Maybe if everyone throws in a few ideas we can all learn 
something.
> 
> Placement is the important thing, once that is optimised the actual 
routing
> should be quite easy, until you get to the last few tracks - they 
will take
> as long as all the others put together. 8-)
> 
> Always start by routing the shortest tracks, working up to the 
longest. That
> is probably the most important rule.
> 
> 
> > I always use different color "rubber bands" for GND and VCC which 
helps a
> > bit.
> 
> That can help a lot. The software I use highlights an entire net 
when it is
> selected. I can use different colours for different nets, as well.
> 
> Leon
> --
> Leon Heller, G1HSM
> http://www.geocities.com/leon_heller