ELECTRICAL ENGINEERS OF BESU - Microprocessor 8051

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Lesson 8

Lesson 8

Lesson 10

Lesson 11

                                               PART-1

1.1 Introduction to microcontrollers

A micro-controller can be compared to a small stand alone computer, it is a very powerful device, which is capable of executing a series of pre-programmed tasks and interacting with other hardware devices. Being packed in a tiny integrated circuit (IC) whose size and weight is usually negligible, it is becoming the perfect controller for robots or any machines requiring some kind of intelligent automation. A single microcontroller can be sufficient to control a small mobile robot, an automatic washer machine or a security system. Any microcontroller contains a memory to store the program to be executed, and a number of input/output lines that can be used to interact with other devices, like reading the state of a sensor or controlling a motor.

Nowadays, microcontrollers are so cheap and easily available that it is common to use them instead of simple logic circuits like counters for the sole purpose of gaining some design flexibility and saving some space. Some machines and robots will even rely on a multitude of microcontrollers, each one dedicated to a certain task. Most recent microcontrollers are 'In System Programmable', meaning that you can modify the program being executed, without removing the microcontroller from its place.

Today, microcontrollers are an indispensable tool for the robotics hobbyist as well as for the engineer. Starting in this field can be a little difficult, because you usually can't understand how everything works inside that integrated circuit, so you have to study the system gradually, a small part at a time, until you can figure out the whole image and understand how the system works.

1.2 The 8051 microcontroller architecture

The 8051 is the name of a big family of microcontrollers. The device which we are going to use along this tutorial is the 'AT89S52' which is a typical 8051 microcontroller manufactured by Atmel™. Note that this part doesn't aim to explain the functioning of the different components of a 89S52 microcontroller, but rather to give you a general idea of the organization of the chip and the available features, which shall be explained in detail along this tutorial.

The block diagram provided by Atmel™ in their datasheet showing the architecture the 89S52 device can seem very complicated, and since we are going to use the C high level language to program it, a simpler architecture can be represented as the figure 1.2.A.

This figures shows the main features and components that the designer can interact with. You can notice that the 89S52 has 4 different ports, each one having 8 Input/output lines providing a total of 32 I/O lines. Those ports can be used to output DATA and orders do other devices, or to read the state of a sensor, or a switch. Most of the ports of the 89S52 have 'dual function' meaning that they can be used for two different functions: the fist one is to perform input/output operations and the second one is used to implement special features of the microcontroller like counting external pulses, interrupting the execution of the program according to external events, performing serial data transfer or connecting the chip to a computer to update the software.

Each port has 8 pins, and will be treated from the software point of view as an 8-bit variable called 'register', each bit being connected to a different Input/Output pin.

You can also notice two different memory types: RAM and EEPROM. Shortly, RAM is used to store variable during program execution, while the EEPROM memory is used to store the program itself, that's why it is often referred to as the 'program memory'. The memory organization will be discussed in detail later.

The special features of the 89S52 microcontroller are grouped in the blue box at the bottom of figure 1.2.A. At this stage of the tutorial, it is just important to note that the 89S52 incorporates hardware circuits that can be used to prevent the processor from executing various repetitive tasks and save processing power for more complex calculations. Those simple tasks can be counting the number of external pulses on a pin, or generating precise timing sequences.

Figure 1.2.A

It is clear that the CPU (Central Processing Unit) is the heart of the microcontrollers, It is the CPU that will Read the program from the FLASH memory and execute it by interacting with the different peripherals discussed above.

Figure 1.2.B shows the pin configuration of the 89S52, where the function of each pin is written next to it, and, if it exists, the dual function is written between brackets. The pins are written in the same order as in the block diagram of figure 1.2.A, except for the VCC and GND pins which I usually note at the top and the bottom of any device.

Note that the pin that have dual functions, can still be used normally as an input/output pin. Unless you program uses their dual functions, All the 32 I/O pins of the microcontroller are configured as input/output pins.

Most of the function of the pins of the 89S52 microcontroller will be discussed in detail, except for the pins required to control an external memory, which are the pins number 29, 30 and 31. Since we are not going to use any external memory, pins 29 and 30 will be ignored through all the tutorial, and pin 31 (EA) always connected to VCC (5 Volts) to enable the micro-controller to use the internal on chip memory rather than an external one (connecting the pin 31 to ground would indicate to the microcontroller that an external memory is to be used instead of the internal one).

Figure 1.2.B

1.3. Memory organization

A RAM stands for Random Access Memory, it has basically the same purpose of the RAM in a desktop computer, which is to store some data required during the execution time of different programs. While an EEPROM, also called FLASH memory is a more elaborated ROM (Read Only Memory) which is the memory where the program being executed is stored. Even if that's not exactly true, you can compare an EEPROM to the Hard-Disk of a desktop computer from a general point of view. The EEPROM term stands for Electronically Erasable and Programmable Read Only Memory.

In microcontrollers, like in any digital system, memory is organized in Registers, Which is the basic unit of construction of a memory. Each register is composed of a number of bits (usually where the data can be stored. In the 8051 family of microcontrollers for example, most registers

are 8-bit register, capable of storing values ranging from 0 to 255. In order to use bigger values, various register can be used simultaneously. Figure 1.3.A shows a typical 8-bit registers, where the notation D0 to D7 stands for the 8 DATA bits of the register.

Typical register

D7

D6

D5

D4

D3

D2

D1

D0

Figure 1.3.A

As you shall see, the RAM memory of the 89S52, which contains 256 registers, is divided into to main parts, the GPR part, and the SFR part. GPR stands for 'General Purpose Register' and are the registers that you can use to store any data during the execution of your program. SFRs (Special function Register) are registers used to control the functioning of the microcontroller and to assist the processor through the various operations being executed. For example, SFRs can be used to control Input/Output lines, to retrieve data transmitted through the serial port of a desktop computer, or to configure one of the on-chip counters and timers.

In a memory each register has a specific address which is used by the processor to read and write from specific memory location. Figure 1.3.B shows the memory organization of the 256 registers of the RAM of the 89S52 microcontroller. The address is noted in Hexadecimal format as this notation simplifies digital logic calculations for the designers, 00 corresponds to the first location and FF which is equal to 256 corresponds to the last location.

A programmer that would use the assembly language, have to take this memory organization into consideration while choosing the locations where his variables are stored, as writing general purpose data into special function registers could prevent the microcontroller from working correctly, but since we will use the C language using the KEIL IDE (integrated development environment), this part will be totally handled by the compiler.

Figure 1.3.B

1.4 Clock concept

The clock concept is found in all modern digital electronics, it is a simple circuit that will generate pulses of electricity at a very specific frequency. Those pulses will cadence all the events happening inside a microcontroller, those pulses will also assure the synchronization of the events between various components inside the microcontroller. For example, if the CPU is waiting for some result of mathematical operation from the ALU (Arithmetic and Logic Unit), it will be known - according to very specific protocol - when and where the resulting data will be delivered to the CPU. The synchronization of those two devices is maintained because they share the same clock.

The clock has another very important role which is to enable the microcontroller to count timing. without a precise clock, it would be impossible to build a 'Real Time System', or any other device that relies on time measurements. It can be deduced that the precision of the timing of a microcontroller depends on the frequency of its clock.

In the 89S52 microcontroller, the clock can be fixed to different value by connecting a crystal to the pins 18 and 19. Those crystals are sold with the frequency written on them in Mega Hertz. The maximum operating frequency of the AT89S52 is 33 Mhz, however other manufacturers like philips built similar 8051 microcontrollers that can run at frequencies up to 120 Mhz.

1.5 Life cycle of a microcontroller project

Before passing to the next part of the tutorial, is important to have a general idea of the steps that are followed to realize a project, from the very beginning when you get an idea to the very end when you finalize your project.

As you can see in figure 1.5.A, after you settle on the choice of your project in the 'brain storming' part, its imperative to imagine how it can be implemented from the hardware point of view, before passing to the programming phase, because programming is much more flexible than the hardware design. In other words, you start by designing the hardware, then you work on the programming while taking in consideration the eventual constraints imposed by the hardware design.

Figure 1.5.A

The hardware design includes all the aspects of the electronic connections between other devices, like the compatibility of the voltage levels, or the required number of pins, etc...

After you're done with a first version of your program, you can transfer it to the microcontroller mounted on the board that you realized already, resulting in a first prototype. The transfer of the code is done using a special device called 'burner' or 'programmer' that connect to the computer, reads the HEX file generated by the compiler, and sends it to the 'program memory' of the microcontroller. The prototype will be used to test your project, correct eventual errors and enhance its performance, tacking in account the famous rule that states that any project never works the first time, at least it does not work as you expected!

Your project will always stay in the prototyping cycle, even if you decide that it is functioning correctly, simply because perfect machines or inventions do not exist, so there is always some room for little changes and updates.

Next part: C programming for the 89S52

                                      PART-2

2.1 From the C program to the machine language

The C source code is very high level language, meaning that it is far from being at the base level of the machine language that can be executed by a processor. This machine language is basically just zero's and one's and is written in Hexadecimal format, that why they are called HEX files.

There are several types of HEX files; we are going to produce machine code in the INTEL HEX-80 format, since this is the output of the KEIL IDE that we are going to use. Figure 2.1.A shows that to convert a C program to machine language, it takes several steps depending on the tool you are using, however, the main idea is to produce a HEX file at the end. This HEX file will be then used by the 'burner' to write every byte of data at the appropriate place in the EEPROM of the 89S52.

figure 2.1.A

2.2. Variables and constants

Variables
One of the most basic concepts of programming is to handle variables. knowing the exact type and size of a variable is a very important issue for microcontroller programmers, because the RAM is usually limited is size. There are two main design considerations to be taken in account when choosing the variables types: the occupied space in ram and the processing speed. Logically, a variable that occupies a big number of registers in RAM will be more slowly processed than a small variable that fits on a single register.

For you to chose the right variable type for each one of your applications, you will have to refer to the following table:

Data Type

Bits

Bytes

Value Range

bit

1

--

0 to 1

signed char

8

1

-128 to +127

unsigned char

8

1

0 to 255

signed int

16

2

-32768 to +32767

unsigned int

16

2

0 to 65535

signed long

32

4

-2147483648 to 2147483647

unsigned long

32

4

0 to 4294967295

float

32

4

±1.175494E-38 to ±3.402823E+38

This table shows the number of bits and bytes occupied by each types of variables, noting that each byte will fit into a register. You will notice that most variables can be either 'signed' or unsigned 'unsigned', and the major difference between the tow types is the range, but both will occupy the same exact space in memory.

The names of the variables shown in the table are the same that are going to be used in the program for variables declarations. Note that in C programming language, any variable have to be declared to be used. Declaring a variable, will attribute a specific location in the RAM or FLASH memory to that variable. The size of that location will depend on the type of the variable that have been declared.

To understand the difference between those types, consider the following example source code where we start by declaring three 'unsigned char' variables, and one 'signed char' and then perform some simple operations:

unsigned char a,b,c;
signed char d;
a = 100;
b = 200;
c = a - b;
d = a - b;

In that program the values of 'c' will be equal to '155'! and not '-100' as you though, because the variable 'c' is an unsigned type, and when a the value to be stored in a variable is bigger than the maximum value range of this variable, it overflows and rolls back to the other limit. Back to our example, the program is trying to store '-100' in 'c', but since 'c' is unsigned, its range of values is from '0 to 255' so, trying to store a value below zero, will cause the the variable to overflow, and the compiler will subtract the '-100' from the other limit plus 1, from '255 + 1' giving '156'. We add 1 to the range because the overflow and roll back operation from 0 to 255 counts for the subtraction of one bit. On the other hand, the value of 'd' will be equal to '-100' as expected, because it is a 'signed' variable. Generally, we try to avoid storing value that are out of range, because sometime, even if the compiler doesn't halt on that error, the results can be sometimes totally un-expected.

Note that in the C programming language, any code line is ended with a semicolon ';', except for the lines ending with brackets '{' '}'.

Like in any programming language, the concept of a variables 'array' can also be used for microcontrollers programming. an array is like a table or a group of variables of the same type, each one can be called by a specific number, for example an array can be declared this way:

char display[10];

this will create a group of 10 variables. Each one of them is accessible by its number, example:

display[0] = 100;
display[3] = 60;
display[1] = display[0] - display[3];

where 'display[1]' will be equal to '40'. Note that 'display' contains 10 different variables, numbered from 0 to 9. In that previous example, according to the variable declaration, there is not such variable location as 'display[10]', and using it will cause an error in the compiler.

Constants
Sometimes, you want to store a very large amount of constant values, that wouldn't fit in the RAM or simply would take too much space. you can store this DATA in the FLASH memory reserved for the code, but it wont be editable, once the program is burned on your chip. The advantage of this technique is that it can be used to store a huge amount of variables, noting that the FLASH memory of the 89S52 is 8K bytes, 32 times bigger than the RAM memory. It is, however, your responsibility to distribute this memory between your program and your DATA.

To specify that a variable is to be stored in the FLASH memory, we use exactly the same variable types names but we add the prefix 'code' before it. Example:

code unsigned char message[500];

This line would cause this huge array to be stored in the FLASH memory. This can be interesting for displaying messages on an LCD screen.

To access the pins and the ports through programming, there are a number of pre-defined variables (defined in the header file, as you shall see later) that dramatically simplifies that task. There are 4 ports, Port 0 to Port 3, each one of them can be accessed using the char variablesP0, P1, P2 and P3 respectively. In those char types variables, each one of the 8 bits represents a pin on the port. Additionally, you can access a single pin of a port using the bit type variablesPX_0 to PX_7, where X takes a value between 0 and 3, depending on the port being accessed. For example P1_3 is the pin number 3 of port 1.

You can also define your own names, using the '#define' directive. Note that this is compiler directive, meaning that the compiler will use this directive to read and understand the code, but it is not a statement or command that can be translated to machine language. For example, you could define the following:

#define LED1 P1_0

With the definition above, the compiler will replace every occurrence of LED1 by P1_0. This makes your code much more easier to read, especially when the new names you give make more sense.

You could also define a numeric constant value like this:

#define led_on_time 184

Then, each time you write led_on_time, it will be replaced by 184. Note that this is not a variable and accordingly, you cannot write something like:

led_on_time = 100; //That's wrong, you cannot change a constant's value in code.

The utility of using defined constants, appears when you want to adjust some delays in your code, or some constant variables that are re-used many times within the code: With a predefined constant, you only change it's value once, and it's applied to the whole code. that's for sure apart from the fact that a word like led_on_time is much more comprehensive than simply '184'!

Along this tutorial you will see how port names, and special function registers are used exactly as variables, to control input/output operations and other features of the microcontroller like timers, counters and interrupts.

2.3. Mathematical & logic operations

Now that you know how to declare variables, it is time to know how to handle them in your program using mathematical and logic operations.

Mathematical operations:
The most basic concept about mathematical operations in programming languages, is the '=' operator which is used to store the content of the expression at its right, into the variable at its left. For example the following code will store the value of 'b' into 'a' :

a = b;

And subsequently, the following expression in totally invalid:

5 = b;

Since 5 in a constant, trying to store the content of 'b' in it will cause an error.

You can then perform all kind of mathematical operations, using the operators '+','-','*' and '/'. You can also use brackets '( )' when needed. Example:

a =(5*b)+((a/b)*(a+b));

If you include 'math.h' header file, you will be able to use more advanced functions in your equations like Sin, Cos and Tan trigonometric functions, absolute values and logarithmic calculations like in the following example:

a =(c*cos(b))+sin(b);

To be able to successfully use those functions in your programs, you have to know the type of variables that those functions take as parameter and return as a result. For example a Cosine function takes an angle in radians whose value is a float number between -65535 and 65535 and it will return a float value as a result. You can usually know those data types from the 'math.h' file itself, for example, the cosine function, like all the others is declared in the top of the math header file, and you can read the line:

extern float cos (float val);

from this line you can deduce that the 'cos' function returns a float data type, and takes as a parameter a float too. (the parameter is always between brackets.). Using the same technique, you can easily know how to deal with the rest of the functions of the math header file. the following table shows a short description of those functions:

Function

Description

char cabs (char val);

Return an the absolute value of a char variable.

int abs (int val);

Return an the absolute value of a int variable.

long labs (long val);

Return an the absolute value of a long variable.

float fabs (float val);

Return an the absolute value of a float variable.

float sqrt (float val);

Returns the square root of a float variable.

float exp (float val);

Returns the value of the Euler number 'e' to the power of val

float log (float val);

Returns the natural logarithm of val

float log10 (float val);

Returns the common logarithm of val

float sin (float val);

A set of standard trigonometric functions. They all take angles measured in radians whose value have to be between -65535 and 65535.

float cos (float val);

float tan (float val);

float asin (float val);

float acos (float val);

float atan (float val);

float sinh (float val);

float cosh (float val);

float tanh (float val);

float atan2 (float y, float x);

This function calculates the arc tan of the ratio y / x, using the signs of both x and y to determine the quadrant of the angle and return a number ranging from -pi to pi.

float ceil (float val);

Calculates the smallest integer that is bigger than val. Example: ceil(4.3) = 5.

float floor (float val);

Calculates the largest integer that is smaller than val. Example: ceil(4.8) = 4.

float fmod (float x, float y);

Returns the remainder of x / y. For example: fmod(15.0,4.0) = 3.

float pow (float x, float y);

Returns x to the power y.

Logical operations:
You can also perform logic operations with variables, like AND, OR and NOT operations, using the following operators:

Operator

Description

!

NOT (bit level) Example: P1_0 = !P1_0;

~

NOT (byte level) Example: P1 = ~P1;

&

AND

|

OR

Note that those logic operation are performed on the bit level of the registers. To understand the effect of such operation on registers, it's easier to look at the bits of a variable (which is composed of one or more register). For example, a NOT operation will invert all the bit of a register. Those logic operators can be used in many ways to merge different bits of different registers together.

For example, consider the variable 'P1', which is of type 'char', and hence stored in an 8-bit register. Actually P1 is an SFR, whose 8 bits represents the 8 I/O pins of Port 1. It is required in that example to clear the 4 lower bits of that register without changing the state of the 4 other which may be used by other equipment. This can be done using logical operators according to the following code:

P1 = P1 & 0xF0; (Adding '0x' before a number indicates that it is a hexadecimal one)

Here, the value of P1 is ANDed with the variable 0xF0, which in the binary base is '11110000'. Recalling the two following relations:

1 AND X = X
0 AND X = 0
(where 'X' can be any binary value)

You can deduce that the 4 higher bits of P1 will remain unchanged, while the 4 lower bits will be cleared to 0.

By the way, note that you could also perform the same operation using a decimal variable instead of a hexadecimal one, for example, the following code will have exactly the same effect than the previous one (because 240 = F0 in HEX):

P1 = P1 & 240;

A similar types of operations that can be performed on a port, is to to set some of its bits to 1 without affecting the others. For example, to set the first and last bit of P1, without affecting the other, the following source code can be used:

P1 = P1 | 0x81;

Here, P1 is ORed with the value 0x81, which is '10000001' in binary. Recalling the two following relations:

1 OR X = 1
0 OR X = X
(where 'X' can be any binary value)

You can deduce that the first and last pins of P1 will be turned on, without affecting the state of the other pins of port 1. Those are just a few example of the manipulations that can be done to registers using logical operators. Logic operators can also be used to define very specific conditions, as you shall see in the next section.

The last types of logic operation studied in this tutorial is the shifting. It can be useful the shift the bit of a register the right or to the left in various situations. this can be done using the following two operators:

Operator

Description

>>

Shift to the right

<<

Shift to the left

The syntax is is quite intuitive, for example:

P1 = 0x01;       // After that operation, in binary, P1 = 0000 0001
P1 = (P1 << // After that operation, in binary P1 = 1000 0000

You can clearly notice that the content of P1 have been shifted 8 steps to the left.

2.4. Conditions and loops

In most programs, it is required at a certain time, to differentiate between different situations, to make decision according to specific input, or to direct the flow of the code depending on some criteria. All the above situation describe an indispensable aspect of programming: 'conditions'. In other words, this feature allows to execute a block of code only under certain conditions, and otherwise execute another code block or continue with the flow of the program.

The most famous way to do that is to use the 'if' statement, according to the following syntax.

if (expression) {
...
code to be executed
...
}

It is important to see how the code is organized in this part. The 'expression' is the condition that shall be valid for the 'code block' to be executed. the code block is all
Notice the use of the two equal signs (==) between two variables or constants, In C language, this means that you are asking whether P1 equals 0 or not. Writing this expression with only one equal sign, would cause the the compiler to store 0 in P1. This issue is a source of logical error for many beginners in C language, this error won’t generate any alert from the compiler and is very hard to identify in a big program, so pay attention, it can save you lot of debugging time. Otherwise it is clear that in that previous example, the code block is only executed if both the two expressions are true. Here is a list of all the operators you can use to write an expression describing a certain condition:

Operator

Description

==

Equal to

<, >

Smaller than, bigger than.

<=, >=

Smaller than or equal to, bigger than or equal to.

!=

Not equal to

The 'If' code block can get a little more sophisticated by introducing the 'else' and 'else if' statement. Observe the following example source code:

if (expression_1) {
...
code block 1
...
}else if(expression_2) {
...
code block 2
...
}else if(expression_3) {
...
code block 3
...
}else{
...
code block 4
...
}

Here, There are four different code blocks, only one shall be executed if and only if the corresponding condition is true. The last code block will only be executed if none of the previous expression is valid. Note that you can have as many 'else if' blocks as you need, each one with its corresponding condition, BUT you can only have one 'else' block, which is completely logical. However you can chose not to have and 'else' block at all if you want.

There are some other alternatives to the 'if...else' code block, that can provide faster execution speeds, but also have some limitations and restrictions like the 'Select...case' code block. For now, it is enough to understand the 'if...else' code block, whose performance is quite fair and have a wide range of applications.

Another very important tool in the programming languages is the loop. In C language like in many others, loops are usually restricted to certain number of loops like in the 'for' code block or restricted to a certain condition like the 'while' block.

Let's start with the 'for' code block, which is a highly controllable and configurable loop. consider the following example source code:

for(i=0;i<10;i++){

P0 = i;

}

Here the code between the the two brackets '{' '}' will be be executed a certain number of times, each time with the counting variable 'i' increasing by 1 according to the statement 'i++'. The code will keep looping as long as the condition 'i<10' is true. Usually the counting value 'i' is reused in the body of the loop, which makes the particularity of this loop. The 'for' loop functioning can be recapitulated by the following syntax:

for(start;condition;step){
...
code block
...
}

Where start represents the start value assigned to the count value before the loop begins. Thecondition is the expression that is is to remain true for the loop to continue; as long as this conditions is satisfied, the code will keep looping. Finally, step is the increase or decrease of the counting variable, it can be any statement that changes its value, whether by an addition or subtraction.

The second type of loop that we are going to study is the 'while' loop. the syntax of this one is simpler than the previous one, as you can observe in the following example source code, that is equivalent to the previous method:

while(i < 10){
P0 = i;
i = i +1;
}

Here there is only one parameter to be defined, which is the condition to keep this loop alive, which is 'i < 10' in our example. Then, it is the responsibility of the programmer to design the software carefully to provide an exit for that loop, or to make it an infinite loop. Both techniques are commonly used in microcontroller programs, as you shall see later on along this tutorial.

2.5. Functions

Functions are way of organizing your code, reducing its size, and increasing its overall performance, by grouping relatively small parts of code to be reused many times in the same program. A new function can be created according to the following syntax:

Function_name(parameter_1, Parameter_2, Parameter_3){
...
function body
...
return value (optional)
...
}

This is the general form of a function. The number of parameters of the function can be more than the three parameters of the examples above, as it can be zero, all depends on the type and use of the function. The function's body is usually a sub program that implies the parameters to produce the required result. some functions will also generate an output, like the cos() function, through the 'return' command, which will output the value next to it. Usually the 'return' command is used at the end of the function.

A very common use of functions without return value is to create delays in a software, consider the following function:

delay(unsigned int y){
    unsigned int i;
    for(i=0;i<y;i++){
      ;
    }
}

In this last piece of code a function named 'delay' is created, with an unsigned integer 'y' as a parameter, and implying a locally defined unsigned int 'i'. the function will repeat a loop for a couple hundreds or thousand of times to generate precise delays in a program. A function like this can be called from anywhere in the program according to the following syntax:

delay(30000);

this line of code would cause the program to pause for approximately one second on a 12 MHz clock on a 8051 microcontroller.

A common example of a function with a return value, is a function that will calculate the angle in radian of a given angle in degrees, as all the trigonometric functions that are included by default take angles in radians. This function can be as the following:

deg_to_rad(float deg){
    float rad;
    rad = (deg * 3.14)/180;
    retrun rad;
}

This function named 'deg_to_rad' will take as a parameter an angle in degrees and output an angle in radians. It can be called in your program according to this syntax:

angle = deg_to_rad(102,18);

where angle should be already defined as a float, and where will be stored the value returnedby the function, which is the angle in radians equivalent to 102.18°

Another important note about functions in the 'main' function. Any C program must contain a function named 'main' which is the place where the program's execution will start. more precisely, for microcontrollers, it were the execution will start after a reset operation, or when a microcontroller circuit is turned ON. The 'main' function has no parameters, and is written like this:

main(){
...
code of the main functions
...
}

2.6. Organization of a C program

All C programs have this common organization scheme, sometimes it's followed, sometimes it's not, however, it is imperative for this category of programming that this organization scheme be followed in order to be able to develop your applications successfully. Any application can be divided into the following parts, noting that is should be written in this order:

a. Headers Includes and constants definitions
In this part, header files (.h) are included into your source code. those headers files can be system headers to declare the name of SFRs, to define new constants, or to include mathematical functions like trigonometric functions, root square calculations or numbers approximations. Header files can also contain your own functions that would be shared by various programs.

b. Variables declarations
More precisely, this part is dedicated to 'Global Variables' declarations. Variables declared in this place can be used anywhere in the code. Usually in microcontroller programs, variables are declared as global variables instead of local variables, unless your are running short of RAM memory and want to save some space, so we use local variables, whose values will be lost each time you switch from a function to another. To summarize, global variables as easier to use and implement than local variables, but they consume more memory space.

d. functions' body
Here you group all your functions. Those functions can be simple ones that can be called from another place in your program, as they can be called from an 'interrupt vector'. In other words, the sub-programs to be executed when an interrupt occurs is also written in this place.

e. Initialization
The particularity of this part is that it is executed only one time when the microcontroller was just subjected to a 'RESET' or when power is just switched ON, then the processor continue executing the rest of the program but never executes this part again. This particularity makes it the perfect place in a program to initialize the values of some constants, or to define the mode of operation of the timers, counters, interrupts, and other features of the microcontroller.

f. Infinite loop
An infinite loop in a microcontroller program is what is going to keep it alive, because a processor have to be allays running for the system to function, exactly like a heart have to be always beating for a person to live. Usually this part is the core of any program, and its from here that all the other functions are called and executed.

2.7. Simple C program for 89S52

Here is a very simple but complete example program to blink a LED. Actually it is the source code of the example project that we are going to construct in the next part of the tutorial, but for now it is important to concentrate on the programming to summarize the notions discussed above.

#include <REGX52.h>
#include <math.h>

delay(unsigned int y){
    unsigned int i;
    for(i=0;i<y;i++){;}
}

main(){
    while(1){
        delay(30000);
        P1_0 = 0;
        delay(30000);
        P1_0 = 1;
    }
}

After including basic headers for the SFR definitions of the 8952 microcontroller (REGX52.h) and for mathematical functions (math.h), a function named 'delay' is created, which is simple a function to create a delay controlled via the parameter 'y'. Then comes the main function, with an infinite loop (the condition for that loop to remain will always be satisfied as it is '1'). Inside that loop, the pin number 0 of port 1 is constantly turned ON and OFF with a delay of approximately one second.

As you will see in the next part, A simple circuit can be constructed and a LED can be connected to the pin P1_0 to see how software and hardware adjustments can affect the behavior of you circuits.

2.8. Using the KEIL environment

KEIL uVision is the name of a software dedicated to the development and testing of a family of microcontrollers based on 8051 technology, like the 89S52 which we are going to use along this tutorial. You can can download an evaluation version of KEIL at their website:http://www.keil.com/c51/. Most versions share merely the same interface, this tutorial uses KEIL C51 uVision 3 with the C51 compiler v8.05a.

To create a project, write and test the previous example source code, follow the following steps:

Open Keil and start a new project:

Figure: 2.8.a

You will prompted to chose a name for your new project, Create a separate folder where all the files of your project will be stored, chose a name and click save. The following window will appear, where you will be asked to select a device for Target 'Target 1':

Figure: 2.8.b

From the list at the left, seek for the brand name ATMEL, then under ATMEL, select AT89S52. You will notice that a brief description of the device appears on the right. Leave the two upper check boxes unchecked and click OK. The AT89S52 will be called your 'Target device', which is the final destination of your source code. You will be asked whether to 'copy standard 8051 startup code' click No.

click File, New, and something similar to the following window should appear. The box named 'Text1' is where your code should be written later.

Figure: 2.8.c

Now you have to click 'File, Save as' and chose a file name for your source code ending with the letter '.c'. You can name is 'code.c' for example, and click save. Then you have to add this file to your project work space at the left as shown in the following screen shot:

Figure: 2.8.d

After right-clicking on 'source group 1', click on 'Add files to group...', then you will be prompted to browse the file to add to 'source group 1', chose the file that you just saved, eventually 'code.c' and add it to the source group. You will notice that the file is added to the project tree at the left.

In some versions of this software you have to turn ON manually the option to generate HEX files. make sure it is turned ON, by right-clicking on target 1, Options for target 'target 1', then under the 'output' tab, by checking the box 'generate HEX file'. This step is very important as the HEX file is the compiled output of your project that is going to be transferred to the microcontroller.

You can then start to write the source code in the window titled 'code.c' then before testing your source code, you have to compile your source code, and correct eventual syntax errors. In KEIL IDE, this step is called 'rebuild all targets' and has this icon: .

Figure: 2.8.e

You can use the output window to track eventual syntax errors, but also to check the FLASH memory occupied by the program (code = 49) as well as the registers occupied in the RAM (data = 9). If after rebuilding the targets, the 'output window' shows that there is 0 error, then you are ready to test the performance of your code. In keil, like in most development environment, this step is called Debugging, and has this icon:. After clicking on the debug icon, you will notice that some part of the user interface will change, some new icons will appear, like the run icon circled in the following figure:

Figure: 2.8.f

You can click on the 'Run' icon and the execution of the program will start. In our example, you can see the behavior of the pin 0 or port one, but clicking on 'peripherals, I/O ports, Port 1'. You can always stop the execution of the program by clicking on the stop button () and you can simulate a reset by clicking on the 'reset' button .

You can also control the execution of the program using the following icons: which allows you to follow the execution step by step. Then, when you're finished with the debugging, you can always return to the programming interface by clicking again on the debug button ().

There are many other features to discover in the KEIL IDE. You will easily discover them in first couple hours of practice, and the more important of them will be presented along the rest of this tutorial.

This concludes this second part of the 89S52 tutorial. I now invite you to start building a real hardware project in the next part.

Next part: Basic input/output operations

                                       PART-3

3.1. I/O port detailed structure

It is important to have some basic notions about the structure of an I/O port in the 8051 architecture. You will notice along this tutorial how this will affect our choices when it comes to connect I/O devices to the ports. Actually, the I/O ports configuration and mechanism of the 8051 can be confusing, due to the fact that a pin acts as an output pin as well as an input pin in the same time.

Figure 3.1.A shows the internal diagram of a single I/O pin of port 1. The first thing you have to notice, is that there are tow different direction for the data flow from the microcontroller's processor and the external pin: The Latch value and the Pin value. The latch value is the value that the microcontroller tries to output on the pin, while the pin value, is the actual logic state of the pin, regardless of the latch value that was set by the processor in the first place. The microcontroller reads the state of a pin through the Pin value line, and writes through the latch value line. If you imagine the behavior of the simple circuit in figure 3.1.A, you'll notice that the I/O pin should follow the voltage of the Latch value, providing 5V through the pull-up resistor, or 0V by connecting the pin directly to the GND through the transistor.

When the pin is pulled high by the pull-up resistor, the pin can output 5V but can also be used as an input pin, because there is no any risk of short-circuit due to the presence of a resistor. This can be easily verified by connecting the pin to 0V or to 5V, the tow possible outcomes are both unharmful for the microcontroller, and the PIN value line will easily follow the value imposed by the external connection.

Now imagine the opposite configuration, where the latch value would be low, causing the pin to provide 0V, being directly connected to GND through the transistor. If in this situation, an external device tries to raise the pin's voltage to 5V, a short circuit will occur and some damage may be

figure 3.1.A: Basic I/O pin internal diagram

made to the microcontroller's port or to the external device connected to that pin.

To summarize, in the 8051 architecture, to use a PIN as an input pin, you have to output '1', and the pin value will follow the value imposed by the device connected to it (switch, sensor, etc...). If you plan to use the pin as an output pin, then just output the required value without taking any of this in consideration.

Even if some ports like P3 and P0 can have a slightly different internal composition than P1, due to the dual functions they assure, understanding the structure and functioning of port 1 as described above is fairly enough to use all the ports for basic I/O operations.

3.2 Simple output project: Blinking a led.

A first simple project to experiment with the output operations is to blink a LED. Assuming you have successfully written and compiled the code as explained in the previous part of the tutorial, now we are going to transfer the HEX file corresponding to that code on the 89s52 microcontroller. Let us recall that the HEX file is a machine language file, generated by the compiler, originally from a C code.

The code for blinking a LED is as follow:

#include <REGX52.h>
#include <math.h>

delay(unsigned int y){
    unsigned int i;
    for(i=0;i<y;i++){;}
}

main(){
    while(1){
        delay(30000);
        P1_0 = 0;
        delay(30000);
        P1_0 = 1;
    }
}

Before transferring the HEX file to the target microcontroller, the hardware have to be constructed. First you have to provide a clean (noiseless) 5V power supply, by connecting the Vcc pin (40) to 5V and the GND pin (20) to 0V. Then you have provide a mean of regulating or generating the clock of the processor. The easiest and most efficient way to do this is to add a crystal resonator and 2 decoupling capacitors of approximately 30pF (see the crystal X1 and the capacitors C1 and C2 on figure 3.1). Then, you have connect pin 31 (EA) to 5V. The EA pin is an active low' pin that indicate the presence of an external memory. Activating this pin by providing 0V on it will tell the internal processor to use external memories and ignore the internal built-in memory of the chip. By providing 5V on the EA pin, its functionality is deactivated and the processor uses the internal memories (RAM and FLASH). At last, you have to connect a standard reset circuitry on pin 9 composed of the 10Kohm resistor R2 and the 10 uF capacitor C3, as you can see in the schematic. You can also add a switch to short-circuit pin 9 (RST) and 5V giving you the ability to reset the microcontroller by pressing on the switch (the processor resets in a high level is provided on the RST pin for more than 2 machine cycles).

Those were the minimum connections to be made for the microcontroller to be functional and able to operate correctly. According to the fact that we are going to use an ISP programmer, A connector is added by default to allow easy in system programming.

For our simple output project, a LED is connected to P1.0 through a 220 ohm resistor R1, as you can see in figure 3.2.A below. Note that there are other ways to connect the LED, but now that you understand the internal structure of the port, you can easily deduce that this is the only way to connect the LED so that the current is fully controlled by the external resistor R1. Any other connection scheme would involve the internal resistor of the port, which is 'uncontrollable'.

figure 3.2.A LED blinking project hardware

In order get rid of any confusion, a picture of the implementation of this simple project on a bread board is provided to help you visualize the hardware part of the project:

figure 3.2.B: LED blinking project hardware implementation

Note that the reset switch and R/C filter are not present on this breadboard, the resetfunctionality of the ISP cable was used instead.

At this stage, you can finally connect your ISP programmer, launch the ISPprog software, browse the HEX file for programming the FLASH, and transfer it to the microcontroller, as described in theISP page. You can eventually use any other available programming hardware and/or software.

If all your connections are correct, you should see the LED blinking as soon as the programming (transfer) is finished. You can experiment with different delay in the code to change the blinking frequency. Don't forget that for any change to take place, you have to rebuild your source code, generating a new hex file (replacing the old one) and retransfer the freshly generated HEX file to the microcontroller.

3.3 Simple Input/Output project

The most simple input operation you can implement to the previous project is a push button, to control the LED. The schematic below (figure 3.3) shows how a switch is added on another pin of Port 1. We could have connected the switch on another port, but i preferred to stress on capability of the 8051 architecture to share input and output pins on the same port. Notice how the switch is connected to ground, and without a pull up resistor. recalling the internal structure of a pin, you'll notice that this is the simplest way to connect a switch, and also the most adapted to the 8051 architecture, making use of the internal pull up resistor, and preventing any eventual short circuits if the port is not well configured.

figure 3.3: LED blinking project hardware implementation

Now, that the hardware is finalized, an adequate software have to be designed and written to assure the correct functioning of the system. To control a led, there are many possible solutions. The first one I propose is the simplest one: a software that turns on the LED as long as the push button is pressed and turn it off otherwise and whose source code would be as the following:

#include <REGX52.h>
#include <math.h>

delay(unsigned int y){
    unsigned int i;
    for(i=0;i<y;i++){;}
}

main(){
    P1_3 = 1 //Set up P1_3 as input pin
    while(1){
        if(P1_3 == 0){
            P1_0 = 0 //Turn ON the LED
        }else{
            P1_0 = 1 //Turn OFF the LED
        }
    }
}

The other solution I propose is a software that turns ON the LED for a couple of seconds each time the switch is pressed, then turn it off automatically. The source code would be as the following:

#include <REGX52.h>
#include <math.h>

unsigned long time, ON_time; //Global Variables

void main(){
    P1_3 = 1;     //Set up P1_3 as input pin
    ON_time = 100000;
    while(1){
         if (time < ON_time){
            time++;    // start or continue counting
            P1_0 = 0;     //Turn ON the LED
         }else{
            P1_0 = 1;    // Turn OFF the LED
        }
        if (P1_3 == 0){     // if the switch is pressed,
            time = 0;    // reset 'time' to 0
        }
    }
}

The source code above may need some explanation: First you can notice that there is no 'delay' function, as it is not needed anymore. Tow variables are defined 'time' and 'ON_time', they are both 'usigned long' type, so that they can manage relatively huge numbers, required to generate dozens of seconds delays. The variable 'time' will be used to count the elapsed time (in number of code cycles), while the 'ON_time' is used to store the fixed time period which the LED should stay ON after the push button is released. Then those tow variables as constantly compared, and as soon as the elapsed time reaches the required 'ON_time', the led switches off, and the 'time' counting stops, to prevent eventual overflow. A push on the button would set the 'time' back to 0, and the whole process can start again.

You can try on your own to figure out other ways of optimizing the control of a LED or a number of LEDs.

Exercise:
To conclude this part of the tutorial, i suggest this simple exercise:

"Using the same schematic (figure 3.3), build a software that would allow you to toggle the state of a led on simple button press. A press on the button to turn the LED ON or OFF, depending on its initial state"

You can try your source code by simulating it in KEIL IDE, or by testing it directly on your breadboard. IF you can't find the solution, you can seek for help in the forums.

Next part: Interrupts, timers and counters

                                       PART-4

4.1. Introduction to 89S52 Peripherals

Figure 4.1 below shows a simplified diagram of the main peripherals present in the 89S52 and their interaction with the CPU and with the external I/O pins. You can notice that there are 3 timers/Counters. We use the expression "Timer/Counter" because this unit can be a counterwhen it counts external pulses on it's corresponding pin, and it can be a timer when it counts the pulses provided by the main clock oscillator of the microcontroller. Timer/Counter 2 is a special counter, that does not behave like the tow others, because it have a couple of extra functionality.

The serial port, using a UART (Universal Asynchronous Receive Transmit) protocol can be used in a wide range of communication applications. With the UART provided in the 89S52 you can easily communicate with a serial port equipped computer, as well as communicate with another microcontroller. This last application, called Multi-processor communication, is quite interesting, and can be easily implemented with 2 89S52 microcontrollers to build a very powerful multi-processor controllers.

If all the peripherals described above can generate interrupt signals in the CPU according to some specific events, it can be useful to generate an interrupt signal from an external device, that may be a sensor or a Digital to Analog converter. For that purpose there are 2 External Interrupt sources (INT0 and INT1).

figure 4.1: 89s52 Peripherals

This was a presentation of the available peripheral features in a 89S52 microcontroller. Through this tutorial, we are going to study how to setup and use external interrupts and the 2 standard timers (T0 and T1). For simplicity, and to keep this tutorial a quick and straight forward one, The UART and the Timer/Counter 2 shall be discussed in separate tutorials.

4.2 External Interrupts

Let's start with the simplest peripheral which is the external interrupt, which can be used to cause interruptions on external events (a pin changing its state from 0 to 1 or vice-versa). In case you don't know, interruption is a mean of stopping the flow of a program, as a response to a certain event, to execute a small program called 'interrupt routine'.

As you noticed in figure 4.1, in the 89S52, there are two external interrupt sources, one connected to the pin P3.2 and the other to P3.3. They are configured using a number of SFRs (Special Function Registers). Most of those SFRs are shared by other peripherals as you shall see in the rest of the tutorial.

The IE register

figure 4.2.A: IE Register

The first register you have to configure (by turning On or Off the right bits) is the IE register, shown in figure 4.2.A. IE stands for 'Interrupt Enable', and it is used to allow different peripherals to cause software interruption. To use any of the interrupts, the bit EA (Enable ALL) must be set to 1, then, you have enable each one of the interrupts to be used with its individual enable bit. For the external interrupts, the two bits EX0 and EX1 are used for External Interrupt 0 and External Interrupt 1.

Using the C programming language under KEIL, it is extremely simple to set those bits, simply by using their name as any global variables, Using the following syntax:

EA = 1;
EX0 = 1;
EX1 = 1;

The rest of the bits of IE register are used for other interrupt sources like the 3 timers overflow (ETx) and the serial interface (ES).

The TCON register

figure 4.2.B: TCON Register

Similarly, you have to set the bits IT0 and IT1 in the TCON register, shown in figure 4.2.B. The bits IT0/IT1 are used to configure the type of signal on the corresponding pins (P3.2/P3.3) that generated an interrupt according to the following table:

IT0/IT1 = 1

External interrupt caused by a falling edge signal on P3.2/P3.3

IT0/IT1 = 0

External interrupt caused by a low level signal on P3.2/P3.3

If IT0 or IT1 is set to 0, an interruption will keep reoccurring as long as P3.2 or P3.3 is set to 0. This mode isn't easy to mange, and most programmers tends to use external interrupts triggered by a falling edge (transition from 1 to 0).

Again, this register is 'bit addressable' meaning you can set or clear each bit individually using their names, like in the following example:

IT0 = 1;
IT1 = 1;

Example Program
Here is an example program to demonstrate the External Interrupt peripheral feature of the 89s52. We are going to build a simple digital low pass filter.

External Interrupt 0 is set in 'Falling Edge' mode, and is used to check for noise on a signal and reset a counter in case noise is detected. Since the noise is interpreted by digital devices as a succession of high and low levels, any 'high to low' level transition is easily detected in the 'Falling edge' mode.

If the counter reaches a pre-calibrated value, then the signal is considered to be stable, and can be sampled, otherwise, if the signal bounces between 0 and 1 before the counter reaches the pre-defined value, the external interrupt resets the counter, and the signal is not taken in account.

Since we will be using External Interrupt 0, the signal to be checked for noise and sampled is imperatively connected to pin P3.2, and the clean, filtered output signal is to be generated on P1.0.

// Include standard headers
#include <REGX52.h>
#include <math.h>

unsigned int counter; //Variable used for our counter

setup_interrupts () // Function to setup the External interrupt 0 in the required mode
{
    EA = 1;
    EX0 = 1;
    IT0 = 1;
}

filter () interrupt 0 //The function the be executed when external interrupt occurs
{
    counter = 0; //Reset the counter to 0
}

main()
{
    time_constant = 40000; //Define the response time of our filter
    setup_interrupts (); //setup the External interrupt
    while(1)
    {
        if (counter < time_constant) // Count until the pre-defined time_constant
        {
        counter++;
        }

        if (counter == time_constant) // if the counter was not interrupted by any noise,
        {
        P1_0 = P3_2; // output the valid signal on P1_0
        }
    }
}

Exercise:
To make sure you've correctly assimilated the functioning of the external interrupts, try to build a program that decodes the pulses coming from an incremental encoder to determine an

absolute position.

Incremental encoder are rotational encoder that generate 2 square waves, shifted by 90 degrees (or by a quarter of a period), as you can see in figure 4.2.C. The main idea of operation is that for a same direction of rotation, the falling edges of signal A will occur at the same time with respect to the signal B.

In other words, during clockwise rotation, the falling edge of signal will always occur while signal B is at high level. On the other hand, during counterclockwise rotation, the falling edges of signal A will always occur while signal B is at a low level.

This mechanism can be used to detect the 'quantity' of rotation in number of pulses as well as direction of the rotation

Using this method, build a program to decode the signals coming from an incremental encoder, and update the position of the encoder at each falling edge. You will need

figure 4.2.C: Incremental Encoders wave forms

only one External interruption.

You can try your source code by simulating it in KEIL IDE, or by testing it directly on your breadboard. IF you can't find the solution, you can seek for help in the forums.

4.3 Timer/Counter

For this part, I'll often use the notation 1/0 adjacent to a register name, which means that there are two of that register, one of them for timer/counter 0, and the other for timer/counter 1, and that the description applies to both of them.

The timer is a very interesting peripheral, that is imperatively present in every microcontroller. It can be used in tow distinct modes:

Timer: Counting internal clock pulses, which are fixed with time, hence, we can say that it is very precise timer, whose resolution depends on the frequency of the main CPU clock (note that CPU clock equals the crystal frequency over 12).

Counter: Counting external pulses (on the corresponding I/O pin), which can be provided by a rotational encoder, an IR-barrier sensor, or any device that provide pulses, whose number would be of some interest.

Sure, the CPU of a microcontroller could provide the required timing or counting, but the timer/counter peripheral relieves the CPU from that redundant and repetitive task, allowing it to allocate maximum processing power for more complex calculations.

So, like any other peripheral, a Timer/Counter can ask for an interruption of the program, which - if enabled - occurs when the counting registers of the Timer/Counter are full and overflow. More precisely, the interruption will occur at the same time the counting register will be reinitialized to its initial value.

So to control the behavior of the timers/counters, a set of SFR are used, most of them have already been seen at the top of this tutorial.

The IE register
First, you have to Enable the corresponding interrupts, but writing 1's to the corresponding bits in the IE register. The following table shows the names and definitions of the concerned bits of the IR register (you can always take a look at the complete IE register in figure 4.2.A):

EA

Enable All interrupts

ET2

Enable Timer 2 interrupts (will not be treated in this tutorial)

ET1

Enable Timer 1 interrupts

ET0

Enable Timer 0 interrupts

You can access those special bits by their names, as simply as it seems, example:
ET0 = 1;

The TCON register
The TCON register is also shared between more than one peripherals. It can be used to configure timers or, as you saw before, external interrupts. The following table shows the names and definitions of the concerned bits of the TCON register (available in figure 4.2.B):

TF1

Overflow interrupt flag, used by the processor.

TR1

Timer/counter 1 RUN bit, set it to 1 to enable the timer to count, 0 to stop counting.

TF0

Overflow interrupt flag, used by the processor.

TR0

Timer/counter 0 RUN bit, set it to 1 to enable the timer to count, 0 to stop counting.

As the IE register, TCON is also bit-addressable, so you can set its bit using its names, like we did before. Example

TR0 = 1;

The TMOD register
Before explaining the TMOD register, let us agree and make it clear that the register IS NOT BIT-ADDRESSABLE, meaning you have to write the 8 bits of the register in a single instruction, by coding those bits into a decimal or hexadecimal number, as you shall see later.

So, as you can see in figure 4.2.C, the TMOD register can be divided into two similar set of bits, each group being used to configure the mode of operation of one of the two timers.

figure 4.2.C: TCON Register

For the a given Timer/Counter, the corresponding bits of TMOD can be defined as in the following table:

G

Gate signal. For normal operation clear this bit to 0.

If you want to use the timers to capture external events's length, set it to 1, and the timer 1/0 will stop counting when External Interrupt 1/0 pin is low (set to 0 V). Note that this feature involves both a timer and an external interrupt, It you're responsibility to write the code to manage the operation of those two peripherals.

C/T'

Set to 1 to use the timer/counter 1/0 as a Counter, counting external events on P3_4/P3_5, cleared to 0 to use it as timer, counting the main oscillator frequency divided by 12.

M1

Timer MODE: Those two last bits combine as 2 bit word that defines the mode of operation, defined as the table below.

M0

Timer/counter modes of operation
Each timer/counter has two SFR called TL0 and TH0 (for timer/counter0) and TL1 and TH1 (for timer/counter 1). TL stands for timer LOW, and is used to store the lower bits of the number being counted by the timer/counter. TH stands for TH, and is used to store the higher bits of the number being counted by the timer/counter.

M1

M0

Mode

Description

0

0

0

Only TH0/1 is used, forming an 8bit timer/counter.
Timer/counter will count up from the value initially stored in TH0/1 to 255, and then overflow back to 0.
If an interrupt is enabled, an interrupt will occur upon overflow.
If used as timer, pulses from the processor are divided by 32 (after being divided by 12). The result is the main oscillator frequency divided by 384.
If used as counter, external pulses are only divided by 32.

0

1

1

Both TH0/1 and TL0/1 are used, forming a 16 bit timer/counter.
Timer/counter will count up from the 16 bit value initially stored in TH0/1 and TL0/1 to 65535, and then overflow back to 0.
If an interrupt is enabled, an interrupt will occur upon overflow.
If used as timer, pulses from the processor are only divided by 12.
If used as counter, external pulses are not divided, but the maximum frequency that can be accurately counted equals the oscillator frequency divided by 24.

1

0

2

TL0/1 is used for counting, forming an 8 bit timer/counter. TH0/1 is used to hold the value to be restored in TL upon overflow.
Timer/counter will count up from the 8 bit value initially stored in TL0/1 and to 255, and then overflow, setting the value of TH0/1 in TL0/1. This is called the auto-reload function.
If an interrupt is enabled, an interrupt will occur upon overflow.
If used as timer, pulses from the processor are only divided by 12.
If used as counter, external pulses are not divided, but the maximum frequency that can be accurately counted equals the oscillator frequency divided by 24.

1

1

3

This mode is beyond the scope of this tutorial.

Timer modes 1 and 2 are the most used in 8051 microcontroller projects, since they offer a wide range of possible customizations.

4.3 Exercise project

To conclude this tutorial, a simple project is proposed, to help you assimilate the functioning of the timers, counter and interrupts.

Consider the following problem. A motor is being operated by an outdated motor controller. We want to add some security to the system, by stopping the whole system in case the motor heats up too much, or turns too fast. A temperature sensor is already set up and give a low signal 0 when the temperature is too high, and an optical encoder output a pulse for each revolution of the motor. We need to write the code to stop the motor incase it heats up or in case it reached 10 000 r.p.m.

Considering that the temperature sensor is connected to P3.2 (External interrupt 0), the encoder is connected to P3.5 (Timer/Counter 1), that the system can be stopped by a high level signal on P1_0, and that we are using a 24MHz crystal oscillator, the software for that controller would be like the following:

// Include standard headers
#include <REGX52.h>
#include <math.h>

#define limit 12
#define stop_signal P1_0

unsigned char sub_counter;

setup_peripherals(){
   //setup external interrupt
   EA = 1;
   EX0 = 1;
   IT0 = 1;

   TMOD = 0x52; // timer 0 mode 2, counter 1 mode 1

   //setup timer 0
   TR0 = 1;
   TH0 = 5; //makes the timer to overflow every 125 uS (divide by 250).
   ET0 = 1;

   //setup timer 1
   TR1 = 1 ;
}

timer_0 () interrupt 1{
   sub_counter++ ;
   if (sub_counter == 10){ // divide the overflow frequency further more by 10
      sub_counter = 0;
      // this part is executed every 1.25 mS
      if ((TL1 + (TH1 * 256)) > limit){ // 12/0.00125 = 9600 (~ 10000)s
         //Stop the motor
         stop_signal = 1;
         TL1 = 0;
         TH1 = 0;
      }
   }
}

over_heat_alarm () interrupt 0{
   stop_signal = 1;
}

void main(){
   stop_signal = 0; // the motor runs normally
   setup_peripherals();
   while(1){
      // Do nothing, the whole program is carried out by interrupts!
   }
}

You should be able to understand and calculate all the choices of timings in that source code, especially the values related with the timer 0 that have to be executed at a very precise time. For more information about RPM measurements, see this project: Contact less digital tachometer.

That concludes this 89S52 tutorial, you can always seek for additional help in the forums.

END of the 89S52 beginner tutorial. Go back to the beginning