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How do you stop an inferred latch?
If you intended to infer a latch for the specified signal or variable, no action is required. Otherwise, to avoid a latch, explicitly assign a new value to the signal or variable in every possible path through the process statement.
What is an inferred latch VHDL?
Latches are inferred in VHDL by using the IF statement without its matching ELSE. This causes the synthesis to make the logical decision to “hold” the value of a signal when not told to do anything else with it. The inferred latch is a transparent latch.
Why should we avoid latch?
The reason that latches should never be used is twofold: Often the user who created the latch did so unintentionally. It is highly likely that the HDL code written is not actually what the designer intended. They can be very difficult for the FPGA tools to create properly.
How a latch gets inferred in RTL design?
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.
How to avoid latches in combinational always blocks?
To avoid this, make sure that all of your if statements that are used in combinational always blocks always have one last else statement to catch all missing conditions! Another place where a latch could be generated in Verilog is in a case statement that does not include the default assignment.
How are flip flops different from latches in VHDL?
In contrast, the positive-edge D flip-flop (DFF), shown in Figure 1 (b), is transparent at the rising edge of clk; otherwise, the output retains its previous value. Use of flip-flops is common in sequential circuit design, whereas latches are utilized only occasionally.