User Guide

When choosing the grid, please also ensure that each pad covers at least
one grid point. Otherwise it can happen that the Autorouter is unable to
route a signal, even though there is enough space to route it. In this case
the Autorouter issues the message Unreachable SMD at x y as it starts.
Theparameters x and y specifythepositionoftheSMDpad.
The default value for the routing grid is 50 mil. This value is sufficient
for simple through-hole layouts. Working with SMD components de
-
mandsafinerroutinggrid.Usualvaluesare25,12.5,10,or5mil.
Please remember that finer values require significantly more routing
memory.
MemoryRequirement
The amount of routing memory required depends in the first place on
the selected routing grid, the area of the board and the number of signal
layersinwhichtracksarerouted.
The static memory requirement (in bytes) for a board can be calculated
asfollows:
number of grid points x number of signal layers x 2
Note that a supply layer generated by giving the appropriate name
($name) does not occupy any autorouting memory. On the other hand,
a supply layer that has been created with the aid of one or more poly
-
gonsconsumesasmuchroutingmemoryasanyothersignallayer.
Space is also required for dynamic data, in addition to the static memory
requirement. The dynamic data require in a very rough estimate about
10% up to 100 % (in some cases even more!) of the static value. This de
-
pendsheavilyonthelayout.
Totalmemoryrequirement(roughapproximation):
staticmemoryx(1.1..2,0)[bytes]
This much RAM should be free before starting the Autorouter. If this is
insufficient, the Autorouter must store data on the hard disk. This
lengthens the routing time enormously, and should be avoided at all
costs. Short accesses to the hard disk are normal, since the job file on
theharddiskisregularlyupdated.
Try to choose the coarsest possible routing grid. This saves memory space
androutingtime!
Layer
If you want to design a double-sided board, then select Top and Bottom
as route layers. You should only use the Bottom layer for a single-sided
board. In the case of inner layers, it is helpful to use the layers from the
outsidetotheinside,i.e.first2and15andsoon.
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EAGLEManual