# Combinational loop¶

## Introduction¶

SpinalHDL will check that there is no combinatorial loop across all the design.

## Example¶

The following code :

```class TopLevel extends Component {
val a = UInt(8 bits) //PlayDev.scala line 831
val b = UInt(8 bits) //PlayDev.scala line 832
val c = UInt(8 bits)
val d = UInt(8 bits)

a := b
b := c | d
d := a
c := 0
}
```

will throw :

```COMBINATORIAL LOOP :
Partial chain :
>>> (toplevel/a :  UInt[8 bits]) at ***(PlayDev.scala:831) >>>
>>> (toplevel/d :  UInt[8 bits]) at ***(PlayDev.scala:834) >>>
>>> (toplevel/b :  UInt[8 bits]) at ***(PlayDev.scala:832) >>>
>>> (toplevel/a :  UInt[8 bits]) at ***(PlayDev.scala:831) >>>

Full chain :
(toplevel/a :  UInt[8 bits])
(toplevel/d :  UInt[8 bits])
(UInt | UInt)[8 bits]
(toplevel/b :  UInt[8 bits])
(toplevel/a :  UInt[8 bits])
```

A fix could be :

```class TopLevel extends Component {
val a = UInt(8 bits) //PlayDev.scala line 831
val b = UInt(8 bits) //PlayDev.scala line 832
val c = UInt(8 bits)
val d = UInt(8 bits)

a := b
b := c | d
d := 42
c := 0
}
```

## False-positive¶

It should be known that currently SpinalHDL is tracking combinatorial loop in a pessimistic way. If it give you a false positive combinatorial loop, you can manualy disable this checks on one signal of the loop.

```class TopLevel extends Component {
val a = UInt(8 bits)
a := 0
a(1) := a(0) //False positive because of this line
}
```

could be fixed by :

```class TopLevel extends Component {
val a = UInt(8 bits).noCombLoopCheck
a := 0
a(1) := a(0)
}
```

It should also be known that this kind of assignements (a(1) := a(0)) could make some tools unhappy (Verilator). It could be preferable to write your logics using an Vec(Bool, 8).