What makes an inferred latch? How to avoid creating inferred latches? When does know you need latches?

What makes an inferred latch?
For combinatorial logic, the output of the circuit are a function of input only and should not contain any memory or intern Al State (latch).

In Verilog, a variable would keep its previous value if it's not assigned a value of an always block. A latch must is created to store this present value.

An incomplete If-else statement would generate latches. An if-else statement was considered "incomplete" if the output state was not a defined for all possible input conditions. The same goes for an incomplete case statement, or a case statement that does does have a default:item.

Why is inferred latches bad?
Inferred latches can serve as a ' warning sign ', the logic design might not being implemented as intended. A crucial If-else or case statement might is missing from the design.

Latches can leads to timing issues and race conditions. They may leads to combinatorial feedback-routing of the output back to the Input-which can be unpredictable.

To avoid creating inferred latches:

Include all the branches of a if or case statement
Assign a value to every output signal in every branch
Use default assignments at the start of the procedure, so every signal would be assigned.

Some parts paraphrased from ' FPGA prototyping by Verilog Examples ' by P. Chu

When does know you need latches?

Which, as you implied, is a subjective question. Expect more opinion than fact as answers. That being said, this is my opinion:

I, like your, often find better ways to use flip-flops thus avoiding latches. The resulting logic is often more elegant and robust. But there is times where I I don ' t have enough control over the logic to avoid latches. For example, I might is interfacing to a processor bus that requires latches to meet the desired specifications. Since I can ' t redesign the CPU or the bus, I ' m stuck with the latch.

In the past 13+ years, that's the only time I had needed latches.

What makes an inferred latch? How to avoid creating inferred latches? When does know you need latches?