An input signal having a logic low level at a first voltage and a logic high level at a second voltage is received by a Schmitt trigger. A voltage generator outputs a reference voltage generated from a third voltage that is higher than the second voltage. A first transistor coupled between the third voltage and a power supply node of the Schmitt trigger is biased by the reference voltage to apply a fourth voltage to the power supply node of the Schmitt trigger that is dependent on the reference voltage. The reference voltage has a value which causes the fourth voltage to be less than or equal to the second voltage. A second transistor coupled between the input signal and the input of the Schmitt trigger circuit is also biased by the reference voltage to control the logic high level voltage of the input signal at the Schmitt trigger.