Learning FPGAs

Chapter 3. Combinational Logic

Digital circuits can be broken into two main categories: combinational logic and sequential logic (sometimes referred to as synchronous logic).

Combinational logic is a digital circuit whose output depends only on the current inputs. The circuit has no memory or feedback loops. It is important to have a strong understanding of combinational logic before proceeding to sequential logic. Even in sequential circuits, you can isolate large sections of combinational logic that perform all the interesting computation.

An example of a combinational logic circuit is the previous button example. The output, if the LED is on, depends only on the current input, if the button is pressed. Another example is a circuit that takes two numbers and adds them together. The output depends only on the two numbers being added. A circuit that would not be considered combinational logic is one that keeps a running total of a series of numbers, as this requires the circuit to remember the current total.

In this chapter, we will run through several combinational logic examples including basic logic gates, various operators, math functions, and more-complex but common circuits. These form the building blocks for all future designs.

IO Shield

The next section requires the IO Shield, shown in Figure 3-1. Alternatively, you could set up a comparable circuit on a breadboard. Go to Embedded Micro for the schematic of the IO Shield. We will be using the DIP switches and LEDs to ...