What is a sequential circuit?
The main difference between a combinational circuit and a sequential circuit is that in the sequential circuit there is a feedback from one or more of the outputs to one or more of the inputs.
Looking at the following diagram, you can see that the output of the OR gate is feedbacked to the input of the lower AND gate. This means that the future F output (F) depends on the inputs A and B and on the value of the current F output. (Fo)
The truth table would generally be like the one below, where the inputs are: A, B and the current F output F (Fo). The output F is the future output. (look at the following truth table)
It can be seen from the previous truth table that the last column “Future F output” is sometimes different from the “Current F output”, because the “Future F output” not only depends on the current A and B inputs but also that also from the “Current F output”.
This diagram and its respective truth table are a specific example. Other diagrams can have more inputs A, B, C, … etc), more outputs (F1, F2, F3, …, etc) and more feedback outputs (F1, F2, F3, …, etc). In case of having more outputs, it would be necessary to obtain a truth table for each “Future F output” according to the existing inputs and all the “Current F outputs” that are fed back.
- It is important to know that although the current F output column (Fo) shows “0” and “1” for all combinations of A and B, this is not necessarily true.
- The outputs in other sequential circuits can be anywhere within the circuit diagram.