Learning FPGAs

Chapter 4. Sequential Logic

In this chapter, we will cover what sequential logic is and, more specifically, what flip-flops are and how to use them. Sequential logic is important for controlling the flow of data through your design as well as improving efficiency by allowing different sections of combinational logic to operate independently. This chapter starts a new example project; you will create a circuit that can keep track of a running total. We will also introduce how to properly read external inputs and solve the challenges that can be associated with them.

It should come as no surprise that because combinational logic is a type of circuit that depends only on the current inputs, the other type, sequential logic, is a type of circuit whose output depends on not only the current inputs but also the sequence of previous inputs. This means that the circuit has some sort of memory to be able to keep track of what has happened before.

A basic example is a circuit that keeps a running total of numbers. Each cycle (we will define what a cycle is later), it adds the current input to the previous sum, but the previous sum isn’t an input. The circuit “remembers” it.

So how do we create circuits that can remember things? It’s quite simple: we need to have feedback loops. In the example of the running total, we would need to feed the previous output back as another internal input. In practice, it gets a little more complicated than that, though.

Feedback Loops and Flip-Flops

Take ...