Latch detected

Introduction

SpinalHDL will check that no combinational signals will infer a latch during synthesis. In other words, this is a check that no combinational signals are partially assigned.

Example

The following code:

class TopLevel extends Component {
  val cond = in(Bool())
  val a = UInt(8 bits)

  when(cond) {
    a := 42
  }
}

will throw:

LATCH DETECTED from the combinatorial signal (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/io_a definition via the stack trace
  ***

A fix could be:

class TopLevel extends Component {
  val cond = in(Bool())
  val a = UInt(8 bits)

  a := 0
  when(cond) {
    a := 42
  }
}

Due to mux

Another reason for a latch being detected is often a non-exhaustive mux/muxList statement with a missing default:

val u1 = UInt(1 bit)
u1.mux(
  0 -> False,
  // case for 1 is missing
)

which can be fixed by adding the missing case (or a default case):

val u1 = UInt(1 bit)
u1.mux(
  0 -> False,
  default -> True
)

In e.g. width generic code it is often a better solution to use muxListDc as this will not generate an error for those cases were a default is not needed:

val u1 = UInt(1 bit)
// automatically adds default if needed
u1.muxListDc(Seq(0 -> True))