The truth table for an AND gate states that it will output a true value if and only if all the input values evaluate to true. The simplest possible circuit for checking that all the bits in a vector are logical '1' values is to use an AND gate. The synthesis tool may choose to implement this differently, depending on the available resources on the target device, but the basic circuit consists of this single elementary logic gate. The output will be set to '1' only if all the input bits are interpreted as '0', that’s how a NOR gate works. The circuit for checking if all bits are zero is a NOR gate with all the bits of the vector as inputs. Signal my_slv : std_logic_vector(7 downto 0) This is the vector that we’re going to use as a test case for all the code examples that are presented: Let’s have a look at the best methods for checking that all the bits in a vector are set or unset. Of course, you know a few ways to do it already, but you want to find the most elegant code that will work with vectors of any length. I know that I have googled this at least a hundred times throughout my career as an FPGA engineer how to check if all bits in a std_logic_vector signal are '0' or '1'.
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December 2022
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