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Configurations: Part 2

Let's review default binding before we go any further. In default binding, the chip socket (component declaration) carries a chip (design entity) of the same name (say, AOI) as we've already seen. The chip is inserted into the socket courtesy of a component instantiation and a configuration declaration. If we omit the configuration or if we use a default configuration, the socket and chip must have the same name.

If we want to choose a particular die (architecture) for our chip, we must specify the architecture in the configuration.

Now. Suppose we want to create a general-purpose socket and at some later time, we want to specify which chip will be plugged into the socket. To do this requires a late-binding configuration declaration.

Late binding

The syntax is really no different than before except that we choose a different chip name for the bound design entity, it does not have to be the same as the component declaration. Let us suppose that a spec change (happens too often, doesn't it?) is required. The spec change requires a 4-input AND gate rather than a 2-input multiplexer. One way to tackle this requirement is to use late binding. This requires no change to the MUX2 at all except in the configuration. So, in a hardware sense, we're extracting the AOI gate from its socket and inserting a 4-input AND gate.

Late-binding configuration of MUX2

use WORK.all;

configuration AND4_CFG of MUX2 is
  for STRUCTURE
    for G2 : AOI
      use entity work.AND4(quick_fix);
      -- architecture quick_fix of AND4 specified for AOI component
    end for;
  end for;
end AND4_CFG;

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