User Guide

Autorouters whatsoever. However, in practice, the required amount of
time is not always available, and therefore certain boards will not be
completedevenbya100%Autorouter.
The EAGLE Autorouter is based on the ripup/retry algorithm. As soon
as it cannot route a track, it removes prerouted tracks (ripup) and tries it
again (retry). The number of tracks it may remove is called ripup depth
which is decisive for the speed and the routing result. This is, in princi
-
ple,thepreviouslymentionedrestriction.
Those who expect an Autorouter to supply a perfect board without
some manual help will be disappointed. The user must contribute his
ideas and invest some energy. If he does, the Autorouter will be a valu
-
abletoolwhichwillgreatlyreduceroutinework.
Working with the EAGLE Autorouter requires that the user places the
components and sets control parameters which influence the routing
strategy. These parameters must be set carefully if the best results are to
beachieved.Theyarethereforedescribedindetailinthissection.
7.3 ControllingtheAutorouter
The Autorouter is controlled by a number of parameters. The values in
the current Design Rules, the net classes and special Autorouter control
parametersallhaveaneffect.
The Design Rules specify the minimum clearances (DRC commands for
setting Clearance and Distance), the via diameter (Restring setting) and
the hole diameter of the vias (Sizes setting). The minimum track width
isalsospecified.
The net classes - if any are defined - specify special minimum clearances,
trackwidthsandtheholediametersforviascarryingparticularsignals.
There is also a range of special cost factors and control parameters that
can be changed via the Autorouter menu. They affect the route given to
tracks during automatic routing. Default values are provided by the pro
-
gram. The control parameters are saved in the BRD file when the layout
is saved. You can also save these values in an Autorouter control file
(*.ctl). This allows a particular set of parameters to be used for different
layouts. Neither Design Rules nor the data for various net classes are
partofthecontrolfile.
Aroutingprocessinvolvesanumberofseparatebasicsteps:
BusRouter
Normally the bus router (whose parameters are set such that the bus
structures can be optimally routed) is the first pass. This step may be
omitted.
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EAGLEManual