There are multiple assignment operator :




Standard assignment, equivalent to ‘<=’ in VHDL/Verilog :
last assignment win, value updated one the next simulation delta cycle


Equivalent to := in VHDL and = in Verilog
value updated instantly


Automatic connection between 2 signals or two bundles of the same type. Direction is inferred by using signal direction (in/out). (Similar behavioral than :=)

//Because of hardware concurrency, `a` is always read with the value '1' by b and c
val a,b,c = UInt(4 bits)
a := 0
b := a
a := 1  //a := 1 win
c := a

var x = UInt(4 bits)
val y,z = UInt(4 bits)
x := 0
y := x      //y read x with the value 0
x \= x + 1
z := x      //z read x with the value 1

// Automatic connection between two UART interfaces. <> io.uart

It is important to understand that in SpinalHDL, the nature of a signal (combinatorial/sequancial) is defined in its declaration and not by the way it is assigned. All datatype instances will define a cominatorial signal, while a datatype instance wrapped into a Reg(…) sementic will define a sequancial (register) signal.

val a = UInt(4 bits) //Define a combinatorial signal
val b = Reg(UInt(4 bits)) //Define a registred signal
val c = Reg(UInt(4 bits)) init(0) //Define a registred signal which is set to 0 when a reset occure

Width checking

SpinalHDL checks that bit count of left and right assignment side match. There is multiple ways to adapt the width of a given BitVector (Bits, UInt, SInt) :

Resizing ways


x := y.resized

Assign x wit a resized copy of y, resize value is automatically inferred to match x

x := y.resize(newWidth)

Assign x with a resized copy of y, size is manually calculated

There are one cases where spinal automatically resize things :



SpinalHDL action

myUIntOf_8bit := U(3)

U(3) create an UInt of 2 bits, which don’t match with left side

Because U(3) is a “weak” bits count inferred signal, SpinalHDL resizes it automatically

Combinatorial loops

SpinalHDL check that there is no combinatorial loops (latch) in your design. If one is detected, it rises an error and SpinalHDL will print you the path of the loop.