Hi Bill! Thanks! I
use KiCAD for schematic capture and PCB layout. I use FreeRouting.net
for the PCB trace routing and optimization. All of the PCB design
information *including the tool chain* is completely open source/free
software except for FreeRouting.net which is *free to use* but not open
source/free software.
The general PCB design process
is to design a schematic for your design (CPU board, IO board, etc) and
capture it in KiCAD. Make sure it has all the compatible connections and
passes Electrical Rules Check. Then generate a network file of all the
connections, assign footprints to the parts, and then layout the PCB.
I layout the PCBs to minimize
the overall length of connections and minimize the number of vias.
Basically that?s just grouping like components in a circuit together.
For example, start with the bus interface and place what components are
directly connected to it, then which are connected to those, and so on.
Sometimes I start with the CPU and work outwards but generally speaking it is
a balance between the core component (IO device or CPU) and the bus
interface.
The trace route optimization
can take days to months depending on the design and how rigorously it is
optimized. The time required scales exponentially with circuit
complexity. For example a simple circuit like the SCSI-1 to IDE/SD
bridge will only take days for robust optimization. However the S-100
68K V3 CPU board has been redesigned multiple times and has been in trace
route optimization for more than two years.
Trace route optimization is a
different process than hand routing or autorouting the PCB traces.
Optimization starts with a completed PCB trace route and seeks new alternative
routes to shorten overall trace length and the number of vias. I can
usually get an unoptimized PCB trace routed by hand or by autorouter in a day
or so.
The problem with unoptimized
PCB trace layouts, even manual ones, is they generate some very long and
complex trace lengths that can screw up timing. Trying to debug an
unoptimized PCB layout is much more difficult since it is so hard to separate
logical problems from trace related anomalies. I usually do a mix of
manual and autorouted PCB trace layouts.
As a related side issue, there
is a major limitation IMO to free/open source hobbyist PCB design because
FreeRouting.net is free to use but not Free/Open Source software.
Without a PCB autorouter we are left with only manual PCB routing and manual
optimization. There have been multiple requests to the FreeRouting.net
author to open up his code but due to pre-existing commercial licensing
relationships that is just not possible. That?s not a criticism of
FreeRouting.net and I appreciate all they?ve done but it is still a huge issue
for hobbyist PCB design projects.
If you or anyone else is
interested in helping the hobbyist PCB design community, please consider
contributing effort to the QAutorouter project. It is porting the gEDA
autorouter code into a standalone tool set builders can use to manually trace
route, autoroute, and optimize PCB traces. Mike Sharkey (on this list)
is the main author. Strategically, this is the most important hobbyist
project any builder could be working on since it lays the foundation for a
completely free/open source EDA toolset which would benefit *all*
projects. This is a pure software development issue and a great
opportunity to make a lasting contribution.
http://sourceforge.net/projects/qautorouter/
Thanks and have a nice
day!
Andrew Lynch