Introduction (Combinational Logic Circuit)

CMOS logic

• Digital logics are divided into combinational and sequential circuits.
• Combinational circuits are circuits where outputs depend only on the present inputs.
• For sequential or regenerative circuit, the output is not only a function of the current input data, but also of previous values of the input signals.
• A sequential circuit includes a combinational logic portion and a memory module that holds the state. Example are registers, counters and memory.
• The building blocks for combinational circuits are logic gates, while the building blocks for sequential circuits are registers and latches.
• The delay of a logic gate depends on its output current I, load capacitance C and output voltage swing ΔV.

Combinational Logic Circuit

Alternative (ratioed circuits, dynamic circuits and pass transistor circuits) CMOS logic configurations are called circuit families.

nMOS transistors provide more current than pMOS for the same size and capacitance, so nMOS networks are preferred.