When/Switch/Mux

When

As VHDL and Verilog, signals can be conditionally assigned when a special condition is met.

when(cond1){
  //execute when      cond1 is true
}.elsewhen(cond2){
  //execute when (not cond1) and cond2
}.otherwise{
  //execute when (not cond1) and (not cond2)
}

Switch

As VHDL and Verilog, signals can be conditionally assigned when a signal has a defined value.

switch(x){
  is(value1){
    //execute when x === value1
  }
  is(value2){
    //execute when x === value2
  }
  default{
    //execute if none of precedent condition meet
  }
}

Local declaration

It’s possible to define new signals into a when/switch statement.

val x,y = UInt(4 bits)
val a,b = UInt(4 bits)

when(cond){
  val tmp = a + b
  x := tmp
  y := tmp + 1
} otherwise {
  x := 0
  y := 0
}

Note

SpinalHDL check that signals defined into a scope are only assigned inside this one.

Mux

If you just need a Mux with a Bool selection signal, there is two equivalent syntaxes :

Syntax

Return

Description

Mux(cond, whenTrue, whenFalse)

T

Return whenTrue when cond is True, whenFalse otherwise

cond ? whenTrue | whenFalse

T

Return whenTrue when cond is True, whenFalse otherwise

val cond = Bool
val whenTrue, whenFalse = UInt(8 bits)
val muxOutput  = Mux(cond, whenTrue, whenFalse)
val muxOutput2 = cond ? whenTrue | whenFalse

Bitwise selection

A bitwise selection looks like the VHDL when syntax.

Example

val bitwiseSelect = UInt(2 bits)
val bitwiseResult = bitwiseSelect.mux(
  0 -> (io.src0 & io.src1),
  1 -> (io.src0 | io.src1),
  2 -> (io.src0 ^ io.src1),
  default -> (io.src0)
)

Also, if all possible values are covered in your mux, you can omit the default value :

val bitwiseSelect = UInt(2 bits)
val bitwiseResult = bitwiseSelect.mux(
  0 -> (io.src0 & io.src1),
  1 -> (io.src0 | io.src1),
  2 -> (io.src0 ^ io.src1),
  3 -> (io.src0)
)

muxLists(...) is another bitwise selection which take as input a sequence of tuple. Below an example of dividing a Bits of 128 bits into 32 bits :

../../_images/MuxList.png
val sel  = UInt(2 bits)
val data = Bits(128 bits)

val dataWord = sel.muxList(for(index <- 0 until 4) yield (index, data(index*32+32-1 downto index*32)))

// This example can be written shorter.
val dataWord = data.subdivideIn(32 bits)(sel)