What is inferred latch and how it is created when it is missing else statement in if condition. Can anybody explain briefly?

Question

I tried to figure out the inferred latch and why it is needed internally, but I couldn\'t find any resources with enough detail.

Answer 1

A latch is inferred when the output of combinatorial logic has undefined states, that is it must hold its previous value.

Combinatorial logic does not have any flip-flop to hold state therefore the output should always be defined by the inputs.

A short example might be:

always @* begin
  if (a == 1'b1) begin
    b =  x|y|z;
  end
end

What is b when a == 1'b0. b is not being overridden so it would hold its value. How can something hold its value when it does not have the concept of state. You have to introduce state by inferring a latch. This is normally a really bad thing.

You can imply latches and be carefull about the timing etc but inferred latches are nominally from buggy code.