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Combinational loop

Introduction

SpinalHDL will check that there are no combinatorial loops in 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 possible 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 said that SpinalHDL’s algorithm to detect combinatorial loops can be pessimistic, and it may give false positives. If it is giving a false positive, you can manualy disable loop checking on one signal of the loop like so:

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 said that assignments such as (a(1) := a(0)) can make some tools like Verilator unhappy. It may be better to use a Vec(Bool, 8) in this case.