Combinational logic circuits are the circuits that are used to implement Boolean functions. Combination circuits are the circuits where the output is represented as a function of the present input values alone. In this case, when the input values are changed, the previous input values are lost; this shows that combinational logic circuits do have memories. While designing a combinational circuit, the output levels are always dependent on the input levels of that circuit (Nazarian, Fatemi, & Pedram, 2011).
On the other hand, sequential logic circuits are the circuits where the output also depends on the values of past input values. These past input values will also include past output values. This means that the output in a sequential circuit may depend on the values of past output values. This means that, to some extent, this circuit has some form of memory. The output levels of a sequential circuit not only depend on the values of the external inputs but also the current state of the memory device. Unlike a combinational circuit, a sequential circuit has a clock as part of its design. In the design of a sequential circuit, there is also a box that is integrated inside that is referred to as a State. This box has flip flops of a given number. The flip flops are used to store the current state number (Ryu et al., 2011).
The design of combinational circuits makes use of basic gates. They can also be constructed from universal gates. On the other hand, sequential logic circuits can be designed by adding some memory device on the combinational logic circuits that are already in place. The memory device is a device which can remember the values that have, previously, been used in the circuit.
References
Nazarian, S., Fatemi, H., & Pedram, M. (2011). Accurate timing and noise analysis of combinational and sequential logic cells using current source modeling. Very Large Scale Integration (VLSI) Systems, IEEE Transactions on, 19 (1), 92-103.
Ryu, H., Kalblein, D., Zschieschang, U., Schmidt, O. G., & Klauk, H. (2011, June). Combinational and sequential logic with transistors based on individual carbon nanotubes. At Device Research Conference (DRC), 2011 69th Annual (pp. 177-178). IEEE.
