Muxes and Input Selection
Selecting inputs is very useful in hardware description, and therefore Chisel provides several built-in generic input-selection implementations.
Mux
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)))
MuxCase
The nested Mux
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 ].
MuxLookup
Chisel also provides MuxLookup
which is an n-way indexed multiplexer:
MuxLookup(idx, default)(Seq(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.