Assignments

There are multiple assignment operators:

Symbol	Description
`:=`	Standard assignment, equivalent to `<=` in VHDL/Verilog. The last assignment to a variable wins; the value is not updated until the next simulation delta cycle.
`\=`	Equivalent to `:=` in VHDL and `=` in Verilog. The value is updated instantly in-place.
`<>`	Automatic connection between 2 signals or two bundles of the same type. Direction is inferred by using signal direction (in/out). (Similar behavior to `:=`)

// Because of hardware concurrency, `a` is always read as '1' by b and c
val a, b, c = UInt(4 bits)
a := 0
b := a
a := 1  // a := 1 "wins"
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.
uartCtrl.io.uart <> io.uart

It also support Bundle assignment, Bundle multiple signals together use () to assign and assign to

val a, b, c = UInt(4 bits)
val d       = UInt(12 bits)
val e       = Bits(10 bits)
val f       = SInt(2  bits)
val g       = Bits()

(a, b, c) := B(0, 12 bits)
(a, b, c) := d.asBits
(a, b, c) := (e, f).asBits
g         := (a, b, c, e, f).asBits

It is important to understand that in SpinalHDL, the nature of a signal (combinational/sequential) is defined in its declaration, not by the way it is assigned. All datatype instances will define a combinational signal, while a datatype instance wrapped with Reg(...) will define a sequential (registered) signal.

val a = UInt(4 bits) // Define a combinational signal
val b = Reg(UInt(4 bits)) // Define a registered signal
val c = Reg(UInt(4 bits)) init(0) // Define a registered signal which is set to 0 when a reset occurs

Width checking

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

Resizing techniques	Description
x := y.resized	Assign x with 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 is one case where Spinal automatically resizes a value:

Assignment	Problem	SpinalHDL action
myUIntOf_8bit := U(3)	U(3) creates an UInt of 2 bits, which doesn’t match the left side (8 bits)	Because U(3) is a “weak” bit count inferred signal, SpinalHDL resizes it automatically

Combinatorial loops

SpinalHDL checks that there are no combinatorial loops (latches) in your design. If one is detected, it raises an error and SpinalHDL will print the path of the loop.