![]() ![]() ![]() Lastly, VHDL has the dedicated "don't care" value of '-', which is purely used as a hint to the synthesizer to help optimize the logic. ![]() (And in the actual circuit, the voltage will be recognized as being either HIGH or LOW, but we cannot know which one!) Another four of these values, 'X', 'U', 'W', and 'Z', all represent unknown voltages so the logic value cannot be assumed to be HIGH or LOW it could be either one.Four of these values, '1', '0', 'H', and 'L' represent known logic levels with various drive strengths.In VHDL std_logic there are 9 distinct logic values which we can roughly group as follows: In general it is very helpful to think about the intended meaning of all these values. To add a bit to the other answers, which are all correct. ![]()
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