Introduction to 8051 Microcontroller

If you are a beginner to the world of microcontrollers and embedded systems, you have several cheap and easy-to-use development boards in the form of Arduino UNO and ESP32. These are fine if your aim is to quickly design a system for a specific task. However, if you want a profession out of these platforms, then you might need to understand the absolute basics. I am talking about the architecture, peripherals, writing drivers for those peripherals, etc. Technically, you can do all these with Arduino UNO and ESP32. But if you start with something much simpler, you can understand the concepts better. And what better way to start your microcontroller and embedded systems journey other than the famous 8051 Microcontroller.

Isn’t 8051 a bit outdated in 2025? Well, yes and no. If you compare the 8051 Microcontroller with the likes of ESP32 (which has a very powerful CPU, integrated Wi-Fi and Bluetooth, and a whole lot of peripherals), then yes, the 8051 is outdated.

However, if you want to learn the basics of a microcontroller and its programming (especially if you are a high school student or freshman in college), then the 8051 Microcontroller is as basic as it can get. You can quickly finish learning about the 8051 and move on to the latest microcontroller (ARM, RISC-V).

In this Introduction to 8051 Microcontroller, I would like to take you through some of the essential concepts of microcontrollers, the basics of 8051 Microcontroller, and more.

Preface

I was recently interviewing some candidates for an internship program in my company. They all are final year engineering students. During the interview process, I asked them about the hardware they worked on, specifically microcontrollers. The answers ranged from Arduino UNO, Raspberry Pi (both SBC and Pico), ESP32, and STM32.

Surprisingly, one microcontroller was a common answer among the majority of the candidates. It is, of course, the 8051 Microcontroller. Yep, the good old 8051 Microcontroller is still a part of the electronics engineering curriculum (at least where I am from).

I have a bit of sentimental attachment with the 8051 Microcontroller. It was my first microcontroller and I learned embedded C programming on the 8051 Microcontroller.

When they all said they learned about the 8051 Microcontroller (in addition to some advanced ARM-based hardware), it felt nostalgic.

Sadly, when I asked them to write simple C programs for demonstrating 8051 UART and Timer peripherals, most of them couldn’t. This is the reality where students cannot properly explain something that is a part of their course. And I cannot blame them as I was the same when I was in my university (I’m sure most of you agree with me).

This is when it occurred to me, how about a beginner’s guide to one of the simplest and primitive microcontrollers, the 8051. The result is a series of guides which I will be developing on the 8051 Microcontroller, starting with this Introduction to 8051 Microcontroller.

What is a Microprocessor?

Before understanding what a microcontroller is, you should first get an idea about the microprocessor. So, what is a microprocessor? A Microprocessor (also known as the Central Processing Unit or CPU) is an integrated circuit (IC) that ‘fetches’ instructions from a memory/storage device and ‘executes’ those instructions.

When I say ‘execute,’ I mean perform a bunch of arithmetic and logical operations on the data (a group of 1s and 0s).

Even though a microprocessor is an integrated circuit, it contains only the hardware (ALU – Arithmetic Logic Unit, registers, etc.) that ‘fetches’ and ‘executes’ the instructions. Examples of Microprocessors are: 8085, 8086, 80386, etc.

In order to build a ‘computer system’ using a microprocessor, you need several additional integrated circuits (peripherals) such as the RAM (Read-Write Memory), ROM (Read-only Memory), Timer, I/O Port, Serial Interface, etc.

This makes the system large and more expensive. While this makes sense for general purpose computers, having a bulky and costly system just to perform some control operations (embedded systems) is not feasible. This led to the development of Microcontroller.

What is a Microcontroller?

Imagine you are designing the front panel of a washing machine. At the very least, the front panel should allow the user to set the type of clothes, mode of washing (wash, wash+rinse, wash+rinse+spin), the temperature of the water, and the speed of the spin cycle.

We have a rather simple and straightforward set of controls here. If you design the control panel with a general purpose microprocessor, then you have to build a system around it (with all the necessary components such as RAM, ROM, I/O Port, etc.). The system will not only be bulky, but it will also be extremely expensive.

What if we could integrate all the necessary/essential components into a single integrated circuit so that the size of the control panel will be apt for the washing machine and the overall cost to build such a system is significantly less? This is what gave birth to the Microcontroller.

A Microcontroller is an integrated circuit that not only contains the main processing unit (the CPU) but also some essential components (such as RAM, ROM, I/O Ports, Timers, Communication Interfaces, etc.) that are necessary to build a computer system.

The CPU does the processing (fetch and execute), ROM contains the application/program, RAM contains the temporary data, I/O Ports connect with buttons, LEDs, displays, etc., Timers allow you to set precise timing-related tasks, and with communication interfaces (such as UART, SPI, etc.) you can connect/control additional peripherals/components.

And the best part is, all these components are right inside the same integrated circuit as the main CPU. Hence, a Microcontroller is a true Computer on a Chip.

Key Points About a Microcontroller (In General)

The purpose of a microcontroller is not general purpose computing, but rather to perform a limited-set of control operations. This doesn’t need the computing prowess of a general purpose microprocessor. Hence, the ‘CPU’ in a microcontroller is less powerful, can perform limited computations even though the essence is still the same (fetch and execute).

Power consumption of a microcontroller-based system is considerably less. This is to aid the possibility of battery-operated devices (toys, remote controls, etc.).

The pins of a microcontroller are ‘Programmable’ in the sense that you can configure them to perform different functions depending on the application. For instance, the same pin can read from a button or drive a motor (well, not directly) in different applications.

Before 8051 Microcontroller

Up until the mid 1970s, the combination of Intel Microprocessors (8008, 8080, 4004) and Memories (1103, 1702) was the heart of many consumer products. As I said before, they were too expensive and difficult to program.

Intel saw this problem and started working on a solution where all the essential components (the main processor, memory modules, input/output) are inside of a single package. The result is the 8048, Intel’s first microcontroller launched in 1976. Technically, the architecture is known as the MCS-48.

The 8048 (or rather the MCS-48) has the following features:

• 8-bit CPU
• 1KB ROM (Program Memory)
• 64B RAM (Data Memory)
• 27 I/O Lines
• 8-Bit Timer/Event Counter

While the 8048 is a Microcontroller, it certainly was not the first. That credit goes to Texas Instruments. A couple of years prior to the launch of 8048, Texas Instruments made the world’s first microcontroller, the TMS 1000.

The 8048 soon became a popular IC and found its way in video game consoles, health monitoring devices, etc.

Introduction to 8051 Microcontroller

With the success of the MCS-48 (the 8048), Intel started working on a completely new architecture, known as the MCS-51. In 1980, Intel launched the 8051 Microcontroller.

Like the 8048, 8051 also has an 8-bit CPU. But the rest of the things are much better. For instance, 8051 comes with 4KB ROM, 128B RAM, and more peripherals. With its capabilities, the 8051 Microcontroller soon became an integral part of several embedded systems ranging from toys to car/airplane parts.

The 8051 is a CISC-based microcontroller with a Harvard Architecture (data and program memory are separate). At the core is an 8-bit ALU that performs all the arithmetic and logical operations on 8-bit data.

Originally, the 8051 Microcontroller was fabricated using NMOS Technology (similar to 8048). But the rise of battery-operated devices led to the implementation of CMOS fabrication. So, most of the later versions of the 8051 Microcontroller used CMOS technology.

If you are unfamiliar, NMOS transistors are notoriously famous for their high static power dissipation (the circuit consumes power even when the transistor is not switching). CMOS solved this problem by using both n-Channel and p-Channel MOSFETS to build logic circuits.

Features of 8051 Microcontroller

I will explain the architecture of the 8051 Microcontroller in a separate guide. In the meantime, here’s a simple overview of some of the key features of the original 8051 microcontroller:

• An 8-bit ALU (Arithmetic-Logic Unit) with an internal bus width of 8-bit.
• 4KB on-chip Program Memory (ROM)
• 128B on-chip Data Memory (RAM)
• 32 Bidirectional and Individually Addressable I/O Lines (divided into four 8-bit I/O Ports)
• Two 16-bit Timers/Counters
• Serial Interface (Full Duplex UART)
• Can access up to 64KB of program and 64KB of data external memories.
• Five programmable Interrupts (three internal and two external) with two priority levels
• Other Features: Boolean Processor (capable of 1-bit operations), internal oscillator, Power Down Mode

Second Sourcing 8051

Since the launch of 8051 in 1980, Intel has given licenses to other semiconductor companies to manufacture their own variants of the 8051. The ‘core’ is still compatible with the MCS-51 Instruction Set but these companies could tweak some functionalities.

This made the ‘8051’ even more popular as you could essentially get the same microcontroller from multiple manufacturers with features catered for different applications.

Back in 2007, Intel ended the production of the 8051 Microcontroller. However, we are still getting ‘MCS-51’ compatible microcontrollers from several semiconductor vendors even in 2025.

The Atmel (now Microchip) ‘AT89’ series of microcontrollers are pretty popular 8051 Microcontrollers. These microcontrollers are still available from electronics parts distributors but not sure if Microchip is still manufacturing though.

Silicon Labs (the semiconductor company famous for its CP210x USB to UART ICs) produces some ‘8051’ core microcontrollers through their ‘C8051’ and ‘EFM8’ line-up.

Nuvoton and STC are two prominent Asian semiconductor companies who also produce 8051-based microcontrollers.

Variants of 8051

While the original 8051 Microcontroller is quite popular, there are several variants of this that use the same MCS-51 core. The following table shows some popular 8051 variants along with their features.

NOTE: Except 89C1051, all the other variants have one Serial Interface.

Conclusion

This was a brief introduction to 8051 Microcontroller. In the upcoming guides, I will talk more about the different components and features of the 8051 Microcontroller.