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Currents and Voltages

Consider the output of one gate connected to the input of another gate. It is important to understand the allowable ranges of voltages at gate outputs and inputs, and the currents that can flow between gates.

• V_IH(min) The minimum voltage level at the input to be recognised as H.
• V_IL(max) The maximum voltage level at the input to be recognised as L.
• V_OH(min) The minimum voltage at the output when in state H.
• V_OL(max) The maximum voltage at the output when in state L.

Thus any voltage greater than V_IH(min) applied to an input will be interpreted as H, and any voltage less than V_IL(max) will be interpreted as L. For unambiguous communication of logic level from one gate to another we need

$$\begin{array}{rl} V_{OH(min)} \geq V_{IH(min)}, \\ \\ V_{OL(max)} \leq V_{IL(max)} \end{array}$$ (1)

The currents are

• I_IH The current flowing into the input when a specified H voltage is applied.
• I_IL The current flowing into the input when a specified L voltage is applied.
• I_OH The current flowing out of the output when in state H.
• I_OL The current flowing out of the output when in state L.

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