On May 29, 2005, at 9:08 AM, jrem wrote:

> I am looking for Eagle Light clues for the best way to layout a single
> sided board with the minimum number of top side routings and vias.

The Eagle autorouter is not very good at single sided boards.  It's big
"trick" for routing seems to involve moving tracks to another layer, and
when it can't do that...  Still, attempting a single sided route can be
a good way of evaluating your placement, orientation, and packaging  
choices.
There is a 1layer.ctl file at

http://www.cadsoft.de/cgi-bin/download.pl?page=/home/cadsoft/ 
html_public/download.htm.en&dir=pub/userfiles/misc

that sets up the autorouter somewhat appropriately.

Density is the enemy of one-sided routing (and vis versa.)  If you're  
doing
homemade boards, you may find it worthwhile to make modified libraries  
with
extra space between pads (transistors with .15 inch lead spacing instead
of .1, for instance.)  Resistors are your friends; you can fit several
traces underneath a typical 1/4W resistor, and it's a good trick for  
getting
signals to where you want them (although I'm not sure I can explain  
exactly
how that works; you just have to get a feel for it...)

Leave a lot of room between your components for your initial attempts at
routing; you can always move them closer together after you have a good  
idea
where there is room for them.

Be willing to swap pins around, even pins that would not traditionally  
be
considered swappable (like the IO pins on a microcontroller.)  It's  
pretty
easy to change which pin does what in software, and it can be worth it  
if
it makes your board layout easier.  You will need to learn just which  
pins
can and can't be switched: on a 12f675, GP0,1,2,4,5 are swappable if  
you're
doing digital IO, but not if you're using some of their alternate  
functions
(comparator, timer, etc), and GP3 is input only, for instance.

Some chips just seem better designed for single sided PCB layouts.
74x540 vs 74x240 is obvious, but also I find the "end" power pins of a
12f PIC more convenient than the corner pins of an ATtiny11, for  
instance.
Some of the modern chips with multiple GND and Power pins are pretty  
impossible.

People have recommended putting the power traces in first, which makes  
a fair
amount of sense.  However, typical power routing can be very disruptive  
to the
layout of other signals, and you may want to think of this as laying  
out the
power BUSSES first, and worrying about the actual chip connections to  
the
power later; you may get your most "bang for the buck" using jumpers on  
the
power net (and jumpers are nice and heavy compared to the tiny traces  
you can
run signals through.)

BillW

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