You're reading an old version of this documentation.
For the latest stable release version, please have a look at master.

Dismiss

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).