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).