How do I stop my design from latching?
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.
What is a latch in 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.
How are latches created in a VHDL process?
Latches are created when you create a combinational process or conditional assignment (in VHDL) or a combinational always block (in Verilog) with an output that is not assigned under all possible input conditions. This creates what is known as incomplete assignment by the synthesis tools.
Why are latches bad and how to avoid them?
But the latch samples the value on the falling edge of the Enable input. And it lets the input propagate during the high period of the Enable (usually the clock), while the flip-flop doesn’t. But most latches are accidentally inferred because there’s something wrong with your VHDL process.
How to fix code that does not create latch?
To fix the code above, the Digital Designer can either specify what happens when i_select = “11” or can use the else clause with no when attached to it. I recommend the latter because it is more flexible. If the width of i_select changes from two bits to three bits your code will still not create a latch.
How to avoid latches in your FPGA code?
Let’s look at one more example in Verilog: This code above will generate a latch, because the output o_latch is not defined when the input i_select is equal to 2’b11. 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!