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Muxes-and-Input-Selection.md

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Selecting inputs is very useful in hardware description, and therefore Chisel provides several built-in generic input-selection implementations. The first one is Mux. This is a 2-input selector. Unlike the Mux2 example which was presented previously, the built-in Mux allows the inputs (in0 and in1) to be any datatype as long as they are the same subclass of Data.

by using the functional module creation feature presented in the previous section, we can create multi-input selector in a simple way:

Mux(c1, a, Mux(c2, b, Mux(..., default)))

However, this is not necessary since Chisel also provides the built-in MuxCase, which implements that exact feature. MuxCase is an n-way Mux, which can be used as follows:

MuxCase(default, Array(c1 -> a, c2 -> b, ...))

Where each selection dependency is represented as a tuple in a Scala array [ condition -> selected_input_port ].

Chisel also provides MuxLookup which is an n-way indexed multiplexer:

MuxLookup(idx, default, 
          Array(0.U -> a, 1.U -> b, ...))

This is the same as a MuxCase, where the conditions are all index based selection:

MuxCase(default, 
        Array((idx === 0.U) -> a,
              (idx === 1.U) -> b, ...))

Note that the conditions/cases/selectors (eg. c1, c2) must be in parentheses.

[Prev(Functional Module Creation)](Functional Module Creation) [Next(Polymorphism and Parameterization)](Polymorphism and Parameterization)