8051 > Architecture and programming of 8051 MCU's - Chapter1: Introduction to Microcontrollers

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Book Overview

Architecture and Programming of 8051 MicrocontrollersLearn in a quick and easy way how to program 8051 microcontrollers using many practical examples we have provided for you. In spite of the fact that they are relatively old, 8051 microcontrollers are still very popular and commonly used nowadays. In addition to Intel, many other renowned companies, such as Philips, Siemens, etc, are manufacturing this model. This book describes its architecture in detail, gives many practical examples, useful program routines, instructions on handling the programmer for 8051 microcontrollers from Atmel as well as the guide on using development systems for Atmel's microcontrollers. In appendices you will find a description of assembly instructions with examples, glossary and much more.

Title: Architecture and Programming of 8051 Microcontrollers

Author: Milan Verle

Table of Contents


Chapter 1: Introduction to Microcontrollers

Chapter 2 : 8051 Microcontroller Architecture

Chapter 3 : The 8051 Instruction Set

Chapter 4 : AT89S8253 Microcontroller

Chapter 5: Assembly Language

Chapter 6 : Examples

Chapter 7 : Development systems


The content published in mikroElektronika’s online books is subject to copyright and must not be reproduced in any form without an explicit written permission released from the editorial of mikroElektronika. The book was prepared with due care and attention, however the publisher doesn’t accept any responsibility neither for the exactness of the information published therein, nor for any consequences of its application.

Architecture and programming of 8051 MCU's

Chapter1: Introduction to Microcontrollers


It was electricity in the beginning....The people were happy because they did not know that it was all around them and could be utilized. That was good. Then Faraday came and a stone has started to roll slowly...

The first machines using a new sort of energy appeared soon. A long time has passed since then and just when the people finally got used to them and stopped paying attention to what a new generation of specialists were doing, someone came to an idea that electrons could be a very convenient toy being closed in a glass pipe. It was just a good idea at first, but there was no return. Electonics was born and the stone kept on rolling down the hill faster and faster...

A new science - new specialists. Blue coats were replaced with white ones and people who knew something about electronics appeared on the stage. While the rest of humanity were passively watching in disbelief what was going on, the plotters split in two groups - “software-oriented” and “hardware-oriented”. Somewhat younger than their teachers, very enthusiastic and full of ideas, both of them kept on working but separate ways. While the first group was developing constantly and gradually, the hardware-oriented people, driven by success, threw caution to the wind and invented transistors.

Up till that moment, the things could be more or less kept under control, but a broad publicity was not aware of what was going on, which soon led to a fatal mistake! Being naive in belief that cheap tricks could slow down technology development and development of the world and retrieve the good all days, mass market opened its doors for the products of Electronics Industry, thus closing a magic circle. A rapid drop in prices made these components available for a great variety of people. The stone was falling freely...

The first integrated circuits and processors appeared soon, which caused computers and other products of electronics to drop down in price even more. They could be bought everywhere. Another circle was closed! Ordinary people got hold of computers and computer era has begun...

While this drama was going on, hobbyists and professionals, also split in two groups and protected by anonymity, were working hard on their projects. Then, someone suddenly put a question: Why should not we make a universal component? A cheap, universal integrated circuit that could be programmed and used in any field of electronics, device or wherever needed? Technology has been developed enough as well as the market. Why not? So it happened, body and spirit were united and the first integrated circuit was designed and called the MICROCONTROLLER.

1.1 What are microcontrollers and what are they used for?

Like all good things, this powerful component is basically very simple. It is made by mixing tested and high- quality "ingredients" (components) as per following receipt:

  1. The simplest computer processor is used as the "brain" of the future system.
  2. Depending on the taste of the manufacturer, a bit of memory, a few A/D converters, timers, input/output lines etc. are added
  3. All that is placed in some of the standard packages.
  4. A simple software able to control it all and which everyone can easily learn about has been developed.

On the basis of these rules, numerous types of microcontrollers were designed and they quickly became man's invisible companion. Their incredible simplicity and flexibility conquered us a long time ago and if you try to invent something about them, you should know that you are probably late, someone before you has either done it or at least has tried to do it.

The following things have had a crucial influence on development and success of the microcontrollers:

  • Powerful and carefully chosen electronics embedded in the microcontrollers can independetly or via input/output devices (switches, push buttons, sensors, LCD displays, relays etc.), control various processes and devices such as industrial automation, electric current, temperature, engine performance etc.
  • Very low prices enable them to be embedded in such devices in which, until recent time it was not worthwhile to embed anything. Thanks to that, the world is overwhelmed today with cheap automatic devices and various “smart” appliences.
  • Prior knowledge is hardly needed for programming. It is sufficient to have a PC (software in use is not demanding at all and is easy to learn) and a simple device (called the programmer) used for “loading” raedy-to-use programs into the microcontroller.

So, if you are infected with a virus called electronics, there is nothing left for you to do but to learn how to use and control its power.

How does the microcontroller operate?

Even though there is a large number of different types of microcontrollers and even more programs created for their use only, all of them have many things in common. Thus, if you learn to handle one of them you will be able to handle them all. A typical scenario on the basis of which it all functions is as follows:

  1. Power supply is turned off and everything is still…the program is loaded into the microcontroller, nothing indicates what is about to come…
  2. Power supply is turned on and everything starts to happen at high speed! The control logic unit keeps everything under control. It disables all other circuits except quartz crystal to operate. While the preparations are in progress, the first milliseconds go by.
  3. Power supply voltage reaches its maximum and oscillator frequency becomes stable. SFRs are being filled with bits reflecting the state of all circuits within the microcontroller. All pins are configured as inputs. The overall electronis starts operation in rhythm with pulse sequence. From now on the time is measured in micro and nanoseconds.
  4. Program Counter is set to zero. Instruction from that address is sent to instruction decoder which recognizes it, after which it is executed with immediate effect.
  5. The value of the Program Counter is incremented by 1 and the whole process is repeated...several million times per second.
Microcontroller Overview

1.2 What is what in the microcontroller?

As you can see, all the operations within the microcontroller are performed at high speed and quite simply, but the microcontroller itself would not be so useful if there are not special circuits which make it complete. In continuation, we are going to call your attention to them.

Read Only Memory (ROM)

Read Only Memory (ROM) is a type of memory used to permanently save the program being executed. The size of the program that can be written depends on the size of this memory. ROM can be built in the microcontroller or added as an external chip, which depends on the type of the microcontroller. Both options have some disadvantages. If ROM is added as an external chip, the microcontroller is cheaper and the program can be considerably longer. At the same time, a number of available pins is reduced as the microcontroller uses its own input/output ports for connection to the chip. The internal ROM is usually smaller and more expensive, but leaves more pins available for connecting to peripheral environment. The size of ROM ranges from 512B to 64KB.

Random Access Memory (RAM)

Random Access Memory (RAM) is a type of memory used for temporary storing data and intermediate results created and used during the operation of the microcontrollers. The content of this memory is cleared once the power supply is off. For example, if the program performes an addition, it is necessary to have a register standing for what in everyday life is called the “sum” . For that purpose, one of the registers in RAM is called the "sum" and used for storing results of addition. The size of RAM goes up to a few KBs.

Electrically Erasable Programmable ROM (EEPROM)

The EEPROM is a special type of memory not contained in all microcontrollers. Its contents may be changed during program execution (similar to RAM ), but remains permanently saved even after the loss of power (similar to ROM). It is often used to store values, created and used during operation (such as calibration values, codes, values to count up to etc.), which must be saved after turning the power supply off. A disadvantage of this memory is that the process of programming is relatively slow. It is measured in miliseconds.

Microcontroller Overview 2

Special Function Registers (SFR)

Special function registers are part of RAM memory. Their purpose is predefined by the manufacturer and cannot be changed therefore. Since their bits are physically connected to particular circuits within the microcontroller, such as A/D converter, serial communication module etc., any change of their state directly affects the operation of the microcontroller or some of the circuits. For example, writing zero or one to the SFR controlling an input/output port causes the appropriate port pin to be configured as input or output. In other words, each bit of this register controls the function of one single pin.

Program Counter

Program Counter is an engine running the program and points to the memory address containing the next instruction to execute. After each instruction execution, the value of the counter is incremented by 1. For this reason, the program executes only one instruction at a time just as it is written. However…the value of the program counter can be changed at any moment, which causes a “jump” to a new memory location. This is how subroutines and branch instructions are executed. After jumping, the counter resumes even and monotonous automatic counting +1, +1, +1…

Central Processor Unit (CPU)

As its name suggests, this is a unit which monitors and controls all processes within the microcontroller and the user cannot affect its work. It consists of several smaller subunits, of which the most important are:

  • Instruction decoder is a part of the electronics which recognizes program instructions and runs other circuits on the basis of that. The abilities of this circuit are expressed in the "instruction set" which is different for each microcontroller family.
  • Arithmetical Logical Unit (ALU) performs all mathematical and logical operations upon data.
  • Accumulator is an SFR closely related to the operation of ALU. It is a kind of working desk used for storing all data upon which some operations should be executed (addition, shift etc.). It also stores the results ready for use in further processing. One of the SFRs, called the Status Register, is closely related to the accumulator, showing at any given time the "status" of a number stored in the accumulator (the number is greater or less than zero etc.).

If you Have Forgotten
A bit is just a word invented to confuse novices at electronics. Joking aside, this word in practice indicates whether the voltage is present on a conductor or not. If it is present, the approprite pin is set to logic one (1), i.e. the bit’s value is 1. Otherwise, if the voltage is 0 V, the appropriate pin is cleared (0), i.e. the bit’s value is 0. It is more complicated in theory where a bit is referred to as a binary digit, but even in this case, its value can be either 0 or 1.

Input/output ports (I/O Ports)

In order to make the microcontroller useful, it is necessary to connect it to peripheral devices. Each microcontroller has one or more registers (called a port) connected to the microcontroller pins.


Why do we call them input/output ports? Because it is possible to change a pin function according to the user's needs. These registers are the only registers in the microcontroller the state of which can be checked by voltmeter!


Oscillator Overview

Even pulses generated by the oscillator enable harmonic and synchronous operation of all circuits within the microcontroller. It is usually configured as to use quartz-crystal or ceramics resonator for frequency stabilization. It can also operate without elements for frequency stabilization (like RC oscillator). It is important to say that program instructions are not executed at the rate imposed by the oscillator itself, but several times slower. It happens because each instruction is executed in several steps. For some microcontrollers, the same number of cycles is needed to execute any instruction, while it's different for other microcontrollers. Accordingly, if the system uses quartz crystal with a frequency of 20MHz, the execution time of an instruction is not expected 50nS, but 200, 400 or even 800 nS, depending on the type of the microcontroller!


Most programs use these miniature electronic "stopwatches" in their operation. These are commonly 8- or 16-bit SFRs the contents of which is automatically incremented by each coming pulse. Once the register is completely loaded, an interrupt is generated!

If these registers use an internal quartz oscillator as a clock source, then it is possible to measure the time between two events (if the register value is T1 at the moment measurement has started, and T2 at the moment it has finished, then the elapsed time is equal to the result of subtraction T2-T1 ). If the registers use pulses coming from external source, then such a timer is turned into a counter.

This is only a simple explanation of the operation itself. It’s somehow more complicated in practice.


If you Have Forgotten
A register or a memory cell is an electronic circuit which can memorize the state of one byte. Besides 8 bits available to the user, each register has also a number of addressing bits. It is important to remember that:

  • All registers of ROM as well as those of RAM referred to as general-purpose registers are mutually equal and nameless. During programming, each of them can be assigned a name, which makes the whole operation much easier.
  • All SFRs are assigned names which are different for different types of the microcontrollers and each of them has a special function as their name suggests.

Watchdog timer

The Watchdog Timer is a timer connected to a completely separate RC oscillator within the microcontroller.

If the watchdog timer is enabled, every time it counts up to the program end, the microcontroller reset occurs and program execution starts from the first instruction. The point is to prevent this from happening by using a special command. The whole idea is based on the fact that every program is executed in several longer or shorter loops.

If instructions resetting the watchdog timer are set at the appropriate program locations, besides commands being regularly executed, then the operation of the watchdog timer will not affect the program execution.

If for any reason (usually electrical noise in industry), the program counter "gets stuck" at some memory location from which there is no return, the watchdog will not be cleared, so the register’s value being constantly incremented will reach the maximum et voila! Reset occurs!

Power Supply Circuit

There are two things worth attention concerning the microcontroller power supply circuit:

Brown out Overview

Brown out is a potentially dangerous state which occurs at the moment the microcontroller is being turned off or when power supply voltage drops to the lowest level due to electric noise. As the microcontroller consists of several circuits which have different operating voltage levels, this can cause its out of control performance. In order to prevent it, the microcontroller usually has a circuit for brown out reset built-in. This circuit immediately resets the whole electronics when the voltage level drops below the lower limit.

Reset pin is usually referred to as Master Clear Reset (MCLR) and serves for external reset of the microcontroller by applying logic zero (0) or one (1) depending on the type of the microcontroller. In case the brown out is not built in the microcontroller, a simple external circuit for brown out reset can be connected to this pin.

Serial communication

Serial communication

Parallel connections between the microcontroller and peripherals established over I/O ports are the ideal solution for shorter distances up to several meters. However, in other cases, when it is necessary to establish communication between two devices on longer distances it is obviously not possible to use parallel connections. Then, serial communication is the best solution.

Today, most microcontrollers have several different systems for serial communication built in as a standard equipment. Which of them will be used depends on many factors of which the most important are:

  • How many devices the microcontroller has to exchange data with?
  • How fast the data exchange has to be?
  • What is the distance between devices?
  • Is it necessary to send and receive data simultaneously?

One of the most important things concerning serial communication is the Protocol which should be strictly observed. It is a set of rules which must be applied in order that devices can correctly interpret data they mutually exchange. Fortunately, the microcontrollers automatically take care of this, so the work of the programmer/user is reduced to a simple write (data to be sent) and read (received data).

If you Have Forgotten
A byte consists of 8 bits grouped together. If a bit is a digit then it is logical that bytes are numbers. All mathematical operations can be performed upon them, just like upon common decimal numbers, which is carried out in the ALU. It is important to remember that byte digits are not of equal significance. The largest value has the leftmost bit called the most significant bit (MSB). The rightmost bit has the least value and is therefore called the least significant bit (LSB). Since 8 digits (zeros and ones) of one byte can be combined in 256 different ways, the largest decimal number which can be represented by one byte is 255 (one combination represents zero).


Unlike other integrated circuits which only need to be connected to other components and turn the power supply on, the microcontrollers need to be programmed first. This is a so called "bitter pill" and the main reason why hardware-oriented electronics engineers stay away from microcontrollers. It is a trap causing huge losses because the process of programming the microcontroller is basically very simple.

In order to write a program for the microcontroller, several "low-level" programming languages can be used such as Assembly, C and Basic (and their versions as well). Writing program procedure consists of simple writing instructions in the order in which they should be executed. There are also many programs running in Windows environment used to facilitate the work providing additional visual tools.

This book describes the use of Assembly because it is the simplest language with the fastest execution allowing entire control on what is going on in the circuit.

If you Have Forgotten
Interrupt - electronics is usually more faster than physical processes it should keep under control. This is why the microcontroller spends most of its time waiting for something to happen or execute. In other words, when some event takes place, the microcontroller does something. In order to prevent the microcontroller from spending most of its time endlessly checking for logic state on input pins and registers, an interrupt is generated. It is the signal which informs the central processor that something attention worthy has happened. As its name suggests, it interrupts regular program execution. It can be generated by different sources so when it occurs, the microcontroller immediately stops operation and checks for the cause. If it is needed to perform some operations, a current state of the program counter is pushed onto the Stack and the appropriate program is executed. It's the so called interrupt routine.

Stack is a part of RAM used for storing the current state of the program counter (address) when an interrupt occurs. In this way, after a subroutine or an interrupt execution, the microcontroller knows from where to continue regular program execution. This address is cleared after returning to the program because there is no need to save it any longer, and one location of the stack is automatically availale for further use. In addition, the stack can consist of several levels. This enables subroutines’ nest

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