In the design of processors, the basic elements are logic gates, which in combination are used to create combinational and sequential systems that allow complex memories and processors that computers have used since the dawn of computing. In this article we are going to give you a master class on what logic gates are together with combinational and sequential systems.
We already commented to you previously, through the article ” How is hardware designed? This is how design engineers work ” the tools that are used today for hardware design. In this article, on the other hand, we will introduce you to what logic gates are and the products of their combination, the combination and sequential systems.
What are logic gates?
Logic gates are combinations of transistors that are the minimum units to build any semiconductor, we are already talking about the construction of a memory, whether volatile or non-volatile, as well as any type of processor. So any semiconductor found in a computer is of the type that is made up of logic gates, even the microcontroller that controls the fridge in your kitchen is made up of logic gates.
All logic gates have a certain number of inputs and a single output and are responsible for performing a specific operation on the input data on the pins. In each pin a pulse is emitted that is binary, since it can only have two values that we call high or logical 1 or low or logical 0. Contrary to popular belief, a low signal does not mean a signal without electricity but one with a lower voltage.
In order to better visualize the logical zeros and ones mentally, think of them as a button, which has only two positions.
Types of logic gates
The logic gates used in the design of memories and processors are the following:
- OR gate: the output will be a logical zero when at least one of the input pins of the OR gate has a 1. In the case that all the input values are 0 then the output value will be 0.
- AND gate: the output can only be a logical one if all the input values are logical ones. In any other case the output is always logical zero.
- XOR gate: the output is a logic one when only one of the input pins receives a high signal and the rest are low, in other cases the output pulse will be low or logic zero.
- Inverter: It is not a logic gate, it is simply responsible for changing the signal from high to low. It is used to create the negated logic gates.
- NOR gate: it is the inverse of the OR gate, the output will only be a logical one when all the input signals are logical zero. Contrary to the OR gate, the output will be logical zero if at least one of the input signals is high or a logical one.
- NAND gate: it is the inverse of the AND gate so the output will always be a logical one, except when all the input values are logical one, then the output will be logical zero.
- XNOR gate: This is the inverted XOR gate. The output is logical zero when at least one of the inputs is a logical one, in other conditions the output will always be logical one.
Combinational Systems
Combinational systems are nothing more than complex combinations of logic gates in which the inputs and outputs are combined to obtain more complex circuits. An example of combinational systems are decoders, which convert the input signal into the selection of an output and which are used to access the data in a RAM memory or in the control unit of a processor. Another type of combinational circuits are the ALUs of the execution units of CPUs and GPUs that are in charge of performing arithmetic operations with the input data.
We can divide combinational circuits into two different types:
- Elements used to perform arithmetic and logic operations.
- Elements that serve to transmit data such as multiplexers, demultiplexers, encoders, decoders, etc.
Contrary to sequential systems, combinational systems do not operate on clock cycles and the output signal will be obtained “immediately”.
Sequential Systems
A sequential system is a combination of logic gates, but organized in such a way that it recalls the past. With them you can build elements such as flip-flops and latches that can store bits of information. As a note, the registers of a processor are made up of flip-flops.
Sequential systems need a clock signal to work, which each given amount of time receives a high pulse, the clock speed being the inverse of the time of each cycle. (1 / time of each cycle). For example a 1 MHz processor switches from low to high 1 million times per second. A complex circuit can have multiple clock speeds and phases.
In order to visualize the operation of a sequential system, imagine a group of people who are in a room and who can only go to the next room when a whistle sounds, which is a clock cycle. Therefore, unlike combinational systems where the output is obtained in a single clock cycle, sequential systems work in phases and the signal passes from one part of the circuit to another per clock cycle.