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Introduction to 8051 Microcontrollers and Keil µVision

December 22, 2019 by Suketu Shah

Get to know the 8051 MCU architecture and learn Keil IDE and be sure to try it out for your next project!

This article introduces the 8051, including its architecture and how to set up the Keil IDE to write an 8051 program. Specifically, I will cover how to use the AT89C51ED2 microcontroller, which uses the 8051 architecture (and explain why 8051 is now considered an architecture rather than an MCU itself).

The 8051 is for those looking to try something other than rapid prototyping, who want to take a step further and make projects on a larger scale, or who want to learn embedded system design.

Microcontroller Vs. Microprocessor

Before we continue, let’s quickly cover the differences between a microcontroller and a microprocessor. They may sound similar, but they are actually very different.

A microprocessor is an integrated circuit that acts as the central processing unit (CPU) of a system. When programmed, it can perform mathematical calculations, make logical decisions, communicate with external devices (memory, display, printer, mouse). Microprocessors are generally used only in computers and laptops where multitasking is a key factor. It runs on a speed of GHz and requires a large amount of memory (500GB or more).

A microcontroller is considered to be a whole computer on a chip because it not only contains a CPU, but also memory, I/O ports, and many other peripherals embedded on it. Microcontrollers are extensively used in embedded system design where tasks are predefined. Microcontrollers work in MHz and do not require much memory. 

difference between microprocessor and microcontroller

The difference between a microprocessor (left) and a microcontroller (right).

The History of 8051

The 8051 is an old school microcontroller invented back in the 1980s by Intel. It is considered to be one of the most basic microcontrollers in engineering, but since its invention, it has seen a lot of upgrades. Newer 8051 microcontrollers are more powerful and consume less power and are, more importantly, way cheaper.

The original 8051 had only 4KB ROM and 128 bytes of RAM while new 8051 variants have built-in external RAM, on-chip EEPROM memory for data storage, increased FLASH memory to save a program, and more UARTs and timers. PWM generators, ADC, and DAC are also added to some of its variants.

There is a misconception about 8051 that I would like to clear up. 8051 is an architecture, not a microcontroller. 8051’s architecture is free to use and popular vendors like Atmel, Cypress Semiconductor, Nuvoton, and NXP develop microcontrollers with 8051 as the core.

8051 Architecture

A CPU forms the main block of the system and all other peripherals are attached to it with the help of buses. An oscillator outputs clock signals to the CPU which helps to execute instructions, generate time delays and PWM pulses, and implement synchronous protocols to communicate with external devices. 

8051 architecture

Read-only memory (ROM) is a non-volatile memory, meaning data doesn’t erase when the MCU powers OFF. The memory stores the program, so this memory can only be erased through programming.

Contrary to ROM, there is also random access memory (RAM) which stores on-the-fly calculations and data. This memory is erased when power shuts OFF.

Serial and I/O ports are used to communicate with external devices or hardware such as computers, sensors, displays, and more.

Armed with all of that knowledge, let’s dive into 8051 programming!

How to Set up Keil IDE for Programming

Keil µVision IDE is a popular way to program MCUs containing the 8051 architecture. It supports over 50 microcontrollers and has good debugging tools including logic analyzers and watch windows. 

In this article, we will use the AT89C51ED2 microcontroller, which has:

  • On chip EEPROM
  • 256 Bytes RAM and 1792 bytes X-RAM
  • In-System programming for uploading program
  • 3 Timer/counters
keil uvision logo

The Keil µVision icon.

To start writing a new program, you need to create a new project. Navigate to project —> New µVision project. Then save the new project in a folder.

new uvision project

After saving the file, a new window will pop up asking you to select your microcontroller.

As discussed we are using AT89C51ED2, so select this controller under the Microchip section (as Atmel is now a part of Microchip). 

find the mcu you're using in keil uvision

Select ‘No’ in the next pop up, as we do not need this file in our project.


Our project workspace is now ready!

From here, we need to create a file where we can write our C code. Navigate to File —> New. Once the file is created, save it with .c extension in the same project folder.


Next, we have to add that .c file to our project workspace. Select Add Existing Files and then select the created .c file to get it added. 


Hurray! The workspace and project file is ready. This tutorial shows an example of the LED blinking code using timers, which we will look into more in future articles. 


A Comparison Chart of Popular Microcontrollers

Navigating the differences between microcontrollers can be a challenge as you move from using development boards to MCUs. The table below provides a comparison of popular microcontrollers. 

Program and Data Memory Architecture
Modified Harvard architecture
Harvard architecture
Harvard architecture
Modified Harvard architecture
>100 MHz
12-24 MHz
Up to 252 MHz
Up to 1GHz
On-chip Oscillator
Not Present
In-system programming using serial/JTAG
In-system programming and parallel programming
In-system programming using RS-232
In-system programming and in-application programming
On-chip Debugging
Using JTAG or DebugWire
Not present
Using JTAG or SWD protocol
FLASH/ program memory size
Up to 256 KB
Up to 64 KB
Up to 2 MB
Up to 2 MB
Avr-gcc winAvr, AtmelStudio
Keil IDE
Keil, Eclipse IDE

Note: Data shown in this chart is based on the newest variants of each microcontroller.

If you have any questions regarding setting up 8051 with Keil µVision IDE or you have another parameter of comparison between popular MCUs you find helpful, please put them into the comment section below!

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