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Shown below are the defined logic levels for TTL and CMOS integrated circuits. Pay close attention to the undefined regions for each type of integrated circuit. Those voltage regions do not have the acceptable logic level to be either a logical low or high by definition. The logic low for a TTL IC is defined as 0.75 volts or less and its logic high is defined as 2.0 volts or greater. While the CMOS IC logic low is defined as 1.5 volts or less and its logic high is defined as 3.5 volts or greater.  |