Contents
- 1 What causes latch inference?
- 2 Why is inferring latches a bad design practice and how can you accidentally infer a latch?
- 3 What you mean by inferring latches?
- 4 How to avoid unintentional latch inference in VHDL?
- 5 When to assign D to Q in VHDL?
- 6 Why does VHDL process statement warning at < location >?
What causes latch inference?
When a variable asynchronously holds its previous value under certain conditions, the variable will infer a latch. Important: The inferred latch may or may not function correctly in your target device depending on the complexity of the conditions under which the variable holds its previous value.
Why is inferring latches a bad design practice and how can you accidentally infer a latch?
Inferred latches can serve as a ‘warning sign’ that the logic design might not be implemented as intended. A crucial if-else or case statement might be missing from the design. Latches can lead to timing issues and race conditions.
What is a latch in FPGA?
The output Q only gets the value on D when Enable is 1. When Enable is 0, it doesn’t matter what the input D is doing, the output will not change. It keeps its previous value, hence why it is called a latch! The output is latched when enable is low.
What you mean by inferring latches?
A latch is inferred within a combinatorial block where the net is not assigned to a known value. Assign a net to itself will still infer a latch. Latches can also be inferred by missing signals form a sensitivity list and feedback loops.
How to avoid unintentional latch inference in VHDL?
“If” statements without an “else” branch can lead to undesired latch inference. A flip-flop is an edge-sensitive memory element, and a latch is level-sensitive. We can use a default assignment before the “if” statement to avoid unintentional latch inference, even when no “else” branch is included.
How to keep the state in VHDL latches?
If you want to keep the state, then consider making a clocked process instead; a template is provided in this answer. If you want a combinatorial output, then get ridge of the internal ONOFF process variable, and make sure that an explicit value is assigned in all branches of the if statement.
When to assign D to Q in VHDL?
The lower latch is a negative-level D latch that samples the data input just before the rising clk edge. If we apply the clock to both latches and to a multiplexer that selects between the outputs of the two latches, we can assign D to Q every time the clock transitions.
Why does VHDL process statement warning at < location >?
Quartus II Integrated Synthesis warns about foo because the structure of the if statements implies the possibility of a path were foo holds its previous value, that is, when sel != ‘1’ and when sel != ‘0’. Though this condition may occur during simulation, it cannot occur in the synthesized netlist because sel always has a binary value.