Logic systems

• Need for multi-valued logic system.
– Conventional logic systems have only three values i.e. ‘0’, ‘1’, ‘Z’

• Example
– Consider the truth-table for AND gate.




HOW TO RESOLVE THIS CONDITION ?

• For this problem a 9-valued logic system or package was developed that is called “STD_LOGIC_1164” and it is accepted as IEEE STD 1164-1993

• Multi-valued logic
– Unknown: value was known but not anymore.
– Un-initialized: value was never known.
– High impedance: net has no driver.
– Drive strengths: handle different output drivers.
– Don’t care: value is immaterial.

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