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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
  }
}