Beginning Logic Design – Part 3

Hello and welcome to Part 3 of my Beginning Logic Design tutorial series!

In the last post we explored a common combinational circuit, the adder. In this post I’ll explore some of the basic sequential logic elements: latches and flip-flops.

Sequential Circuits

So far, the circuits I’ve shared are pretty linear. The schematic diagrams have had inputs on the left and all signals move toward the outputs on the right. For sequential circuits, there is an element of feedback where some outputs are routed back into an earlier gate.

Let’s look at the concept of feedback using the NOR gate as our base. I’ll provide one switch as input, and I’ll use it’s own output as the second input.

NOR gate with feedback

In this configuration, the NOR cannot yet determine what it’s output should be, let’s look at the truth table for NOR to understand why.

NOR truth table

Looking at the truth table, if we only know one of the inputs and we know that it’s 0, we can’t determine the output. On the other hand if we know one of the inputs is 1 we can safely say the output is 0.

Now the second input is also known, but something interesting happens if we turn the switch off now.