The situation we find ourselves today in the field of microcontrollers has its beginnings in the development of technology of integrated circuits. It enabled us to store hundreds of thousands of transistors into one chip, which was a precondition for the manufacture of microprocessors. The first computers were made by adding external peripherals, such as memory, input/output lines, timers and other circuits, to it. Further increasing of package density resulted in designing an integrated circuit which contained both processor and peripherals. This is how the first chip containing a microcomputer later known as the microcontroller was developed.
Novices in electronics usually think that the microcontroller is the same as the microprocessor. That’s not true. They differ from each other in many ways. The first and most important difference in favour of the microcontroller is its functionality. In order that the microprocessor may be used, other components, memory comes first, must be added to it. Even though it is considered a powerful computing machine, it is not adjusted to communicating to peripheral environment. In order to enable the microprocessor to communicate with peripheral environment, special circuits must be used. This is how it was in the beginning and remains the same today.
On the other hand, the microcontroller is designed to be all of that in one. No other specialized external components are needed for its application because all necessary circuits which otherwise belong to peripherals are already built in it. It saves time and space needed to design a device.
ALL THE MICROCONTROLLER CAN DO
In order to make it easier for you to understand the reasons for such a great success of microcontrollers, we will call your attention for a few minutes to the following example.
About ten years ago, designing of an electronic device controlling the elevator in a multistory building was enormously difficult, even for a team of experts. Have you ever thought about what requirements an ordinary elevator must meet? How to deal with the situation when two or more people call the elevator at the same time? Which call has priority? How to handle security question? Loss of electricity? Failure? Misuse?…What comes after solving these basic questions is a painstaking process of designing appropriate electronics using a large number of specialized chips. Depending on device complexity, this process can take weeks or months. When finished, its time to design a printed circuit board and assemble device. A huge device! It is another long-lasting and trying work. Finally, when everything is finished and tested for many times, the crucial moment comes when you concentrate, take a deep breath and switch the power supply on.
This is usually the point at which the party turns into a real work since electronic devices almost never starts to operate immediately. Get ready for many sleepless nights, corrections, improvements… and don’t forget, we are still talking about running an ordinary elevator.
When your device finally starts to operate perfectly and everybody is satisfied and you finally get paid for the work you have done, many constructing companies will become interested in your work. Of course, if you are lucky, another day will bring you a locking offer from a new investor. However, a new building has four stories more. You know what it is about? You think you can control destiny? You are going to make a universal device which can be used in buildings of 4 to 40 stories, a masterpiece of electronics? All right, even if you manage to make such an electronic jewel, your investor will wait in front of your door asking for a camera in elevator. Or for relaxing music in the event of the failure of elevator. Or for two-door elevator. Anyway, Murphy’s law is inexorable and you will certainly not be able to make an advantage of all the effort you have made. Unfortunately, everything that has been said now is true. This is what ‘handling electronics’ really means. No, wait, let us correct ourself, that is how it was until the first microcontrollers were designed – small, powerful and cheap microcontrollers. Since the moment their programming stopped being a science, everything took another direction…
Electronics capable of controlling a small submarine, a crane or the above mentioned elevator is now built in one single chip. Microcontrollers offer a wide range of applications and only some of them are normally used. It’s up to you to decide what you want the microcontroller to do and dump a program containing appropriate instructions into it. Prior to turning on the device, its operation should be tested by a simulator. If everything works fine, build the microcontroller into your device. If you ever need to change, improve or upgrade the program, just do it. Until when? Until you feel satisfied. That’s all.
Do you know that all people can be classified into one out of 10 groups- those who are familiar with binary number system and those who are not familiar with it. You don’t understand? It means that you still belong to the latter group. If you want to change your status read the following text describing briefly some of the basic concepts used further in this book (just to be sure we are on the same page).
1.2 NUMBERS, NUMBERS, NUMBERS…
Mathematics is such a good science! Everything is so logical… The whole universe can be described with ten digits only. But, does it really have to be like that? Do we need exactly ten digits? Of course not, it is only a matter of habit. Remember the lessons from the school. For example, what does the number 764 mean: four units, six tens and seven hundreds. It’s as simple as that! Could it be described in a more complicated way? Of course it could: 4 + 60 + 700. Even more complicated? Yes: 4*1 + 6*10 + 7*100. Could this number look more scientific? The answer is yes again: 4*100 + 6*101 + 7*102. What does it actually mean? Why do we use exactly these numbers: 100, 101 and 102 ? Why is it always about the number 10? Because we use ten different digits (0, 1, 2, … 8, 9). In other words, we use base-10 number system, i.e. decimal number system.
BINARY NUMBER SYSTEM
What would happen if only two digits are used- 0 and 1? Or if we don’t not know how to determine whether something is 3 or 5 times greater than something else? Or if we are restricted when comparing two sizes, i.e. if we can only state that something exists (1) or does not exist (0)? The answer is ‘nothing special’, we would keep on using numbers in the same way as we do now, but they would look a bit different. For example: 11011010. How many pages of a book does the number 11011010 include? In order to learn that, you just have to follow the same logic as in the previous example, but in reverse order. Bear in mind that all this is about mathematics with only two digits- 0 and 1, i.e. base-2 number system (binary number system).
It is obviously the same number represented in two different number systems. The only difference between these two representations is the number of digits necessary for writing a number. One digit (2) is used to write the number 2 in decimal system, whereas two digits (1 and 0) are used to write it in binary system. Do you now agree that there are 10 groups of people? Welcome to the world of binary arithmetic! Do you have any idea where it is used?
Except for strictly controlled laboratory conditions, the most complicated electronic circuits cannot accurately determine the difference between two sizes (two voltage values, for example) if they are too small (lower than several volts). The reasons are electrical noises and something called the ‘real working environment’ (unpredictable changes of power supply voltage, temperature changes, tolerance to values of built-in components etc.). Imagine a computer which operates upon decimal numbers by treating them in the following way: 0=0V, 1=5V, 2=10V, 3=15V, 4=20V…9=45V.
Did anybody say batteries?
A far simpler solution is a binary logic where 0 indicates that there is no voltage and 1 indicates that there is a voltage. It is easier to write 0 or 1 instead of full sentences ‘there is no voltage’ or ‘there is voltage’, respectively. It is about logic zero (0) and logic one (1) which electronics perfectly cope with and easily performs all those endlessly complex mathematical operations. Obviously, the electronics we are talking about applies mathematics in which all the numbers are represented by two digits only and where it is only important to know whether there is a voltage or not. Of course, we are talking about digital electronics.
HEXADECIMAL NUMBER SYSTEM
At the very beginning of computer development it was realized that people had many difficulties in handling binary numbers. For this reason, a new number system, using 16 different symbols was established. It is called hexadecimal number system and consists of the ten digits we are used to (0, 1, 2, 3,… 9) and six letters of alphabet A, B, C, D, E and F. You probably wonder about the purpose of this seemingly bizarre combination? Just look how perfectly it fits the story about binary numbers and you will understand.
The largest number that can be represented by 4 binary digits is the number 1111. It corresponds to the number 15 in decimal system, whereas in hexadecimal system it is represented by only one digit F. It is the largest 1-digit number in hexadecimal system. Do you see how skillfully it is used? The largest number written with eight binary digits is at the same time the largest 2-digit hexadecimal number. Don’t forget that computers use 8-digit binary numbers. By chance?
BCD code is a binary code for decimal numbers only (Binary-Coded Decimal). It is used to enable electronic circuits to communicate either with peripherals using decimal number system or within ‘their own world’ using binary system. It consists of 4-digit binary numbers which represent the first ten digits (0, 1, 2, 3 … 8, 9). Even though four digits can give in total of 16 possible combinations, the BCD code normally uses only the first ten.
NUMBER SYSTEM CONVERSION
Binary number system is most commonly used, decimal system is most understandable, while hexadecimal system is somewhere between them. Therefore, it is very important to learn how to convert numbers from one number system to another, i.e. how to turn a sequence of zeros and ones into understandable values.
BINARY TO DECIMAL NUMBER CONVERSION
Digits in a binary number have different values depending on the position they have in that number. Additionally, each position can contain either 1 or 0 and its value may be easily determined by counting its position from the right. To make the conversion of a binary number to decimal it is necessary to multiply values with the corresponding digits (0 or1) and add all the results. The magic of binary to decimal number conversion works…You doubt? Look at the example below:
It should be noted that in order to represent decimal numbers from 0 to 3, you need to use only two binary digits. For larger numbers, extra binary digits must be used. Thus, in order to represent decimal numbers from 0 to 7 you need three binary digits, for the numbers from 0 to 15 you need four digits etc. Simply put, the largest binary number consisting of n digits is obtained when the base 2 is raised by n. The result should then be subtracted by 1. For example, if n=4:
24 – 1 = 16 – 1 = 15
Accordingly, by using 4 binary digits it is possible to represent decimal numbers from 0 to 15, which amounts to 16 different values in total.
HEXADECIMAL TO DECIMAL NUMBER CONVERSION
In order to make the conversion of a hexadecimal number to decimal, each hexadecimal digit should be multiplied with the number 16 raised by its position value. For example:
HEXADECIMAL TO BINARY NUMBER CONVERSION
It is not necessary to perform any calculations in order to convert hexadecimal numbers to binary. Hexadecimal digits are simply replaced by appropriate binary digits. Since the maximum hexadecimal digit is equivalent to the decimal number 15, we need to use four binary digits to represent one hexadecimal digit. For example:
A comparative table below contains the values of numbers 0-255 in three different number systems. This is probably the easiest way to understand the common logic applied to all the systems.
Hexadecimal number system is along with binary and decimal systems considered to be the most important number system for us. It is easy to make conversion of any hexadecimal number to binary and it is also easy to remember it. However, these conversions may cause confusion. For example, what does the sentence ‘It is necessary to count up 110 products on the assembly line’ actually mean? Depending on whether it is about binary, decimal or hexadecimal system, the result could be 6, 110 or 272 products, respectively! Accordingly, in order to avoid misunderstanding, different prefixes and suffixes are directly added to the numbers. The prefix $ or 0x as well as the suffix h marks the numbers in hexadecimal system. For example, the hexadecimal number 10AF may look as $10AF, 0x10AF or 10AFh. Similarly, binary numbers usually get the prefix % or 0b. If a number has neither suffix nor prefix it is considered decimal. Unfortunately, this way of marking numbers is not standardized, thus depends on concrete application.
Theory says a bit is the basic unit of information…Let’s forget this for a moment and take a look at what it is in practice. The answer is- nothing special- a bit is just a binary digit. Similar to decimal number system in which digits of a number do not have the same value (for example digits in the decimal number 444 are the same, but have different values), the ‘significance’ of bit depends on its position in the binary number. Since there is no point talking about units, tens etc. in binary numbers, their digits are referred to as the zero bit (rightmost bit), first bit (second from the right) etc. In addition, since the binary system uses two digits only (0 and 1), the value of one bit can be either 0 or 1.
Don’t be confused if you come across a bit having value 4, 16 or 64. It just means that its value is represented in decimal system. Simply put, we have got so much accustomed to the usage of decimal numbers that such expressions became common. It would be correct to say for example, ‘the value of the sixth bit of any binary number is equivalent to the decimal number 64’. But we are human and old habits die hard…Besides, how would it sound ‘number one-one-zeroone- zero…’?
A byte consists of eight bits grouped together. If a bit is a digit, it is logical that bytes represent numbers. All mathematical operations can be performed upon them, like upon common decimal numbers. Similar to digits of any number, byte digits do not have the same significance either. The greatest 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 eight 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 a zero).
A nibble is referred to as half a byte. Depending on which half of the register we are talking about (left or right), there are ‘high’ and ‘low’ nibbles, respectively.
Have you ever wondered what electronics within digital integrated circuits, microcontrollers or processors look like? What do circuits performing complicated mathematical operations and making decisions look like? Do you know that their seemingly complicated schematic comprise only a few different elements called logic circuits or logic gates?
1.3 MUST KNOW DETAILS
The operation of these elements is based on principles established by a British mathematician George Boole in the middle of the 19th century- even before the first bulb was invented. Originally, the main idea was to express logical forms through algebraic functions. Such thinking was soon transformed into a practical product which far later evaluated in what today is known as AND, OR and NOT logic circuits. The principle of their operation is known as Boolean algebra.
Some of the program instructions give the same results as logic gates. The principle of their operation will be discussed in the text below.
The logic gate ‘AND’ has two or more inputs and one output. Let us presume that the gate used in this example has only two inputs. A logic one (1) will appear on its output only if both inputs (A AND B) are driven high (1). Table on the right shows mutual dependence between inputs and the output.
When used in a program, a logic AND operation is performed by the program instruction, which will be discussed later. For the time being, it is enough to remember that logic AND in a program refers to the corresponding bits of two registers.
Similarly, OR gates also have two or more inputs and one output. If the gate has only two inputs the following applies. Alogic one (1) will appear on its output if either input (A OR B) is driven high (1). If the OR gate has more than two inputs then the following applies. Alogic one (1) appears on its output if at least one input is driven high (1). If all inputs are at logic zero (0), the output will be at logic zero (0) as well.
In the program, logic OR operation is performed in the same manner as logic AND operation.
The logic gate NOT has only one input and only one output. It operates in an extremely simple way. When logic zero (0) appears on its input, a logic one (1) appears on its output and vice versa. It means that this gate inverts the signal and is often called inverter, therefore.
In the program, logic NOT operation is performed upon one byte. The result is a byte with inverted bits. If byte bits are considered to be a number, the inverted value is actually a complement thereof. The complement of a number is a value which added to the number makes it reach the largest 8-digit binary number. In other words, the sum of an 8-digit number and its complement is always 255.
EXCLUSIVE OR GATE
In short, a register or a memory cell is an electronic circuit which can memorize the state of one byte.
In addition to registers which do not have any special and predetermined function, every microcontroller has a number of registers (SFR) whose function is predetermined by the manufacturer. Their bits are connected (literally) to internal circuits of the microcontroller such as timers, A/D converter, oscillators and others, which means that they are directly in command of the operation of these circuits, i.e. the microcontroller. Imagine eight switches which control the operation of a small circuit within the microcontroller- Special Function Registers do exactly that.
INPUT / OUTPUT PORTS
In order to make the microcontroller useful, it has to be connected to additional electronics, i.e. peripherals. Each microcontroller has one or more registers (called ports) connected to the microcontroller pins. Why input/output? Because you can change a pin function as you wish. For example, suppose you want your device to turn on/off three signal LEDs and simultaneously monitor the logic state of five sensors or push buttons. Some of the ports need to be configured so that there are three outputs (connected to LEDs) and five inputs (connected to sensors). It is simply performed by software, which means that a pin function can be changed during operation.
One of important specifications of input/output (I/O) pins is the maximum current they can handle. For most microcontrollers, current obtained from one pin is sufficient to activate an LED or some other low-current device (10-20 mA). The more I/O pins, the lower maximum current of one pin. In other words, the maximum current stated in the data specifications sheet for the microprocessor is shared across all I/O ports.
Another important pin function is that it can have pull-up resistors. These resistors connect pins to the positive power supply voltage and come into effect when the pin is configured as an input connected to a mechanical switch or a push button. Newer versions of microcontrollers have pull-up resistors configurable by software.
Each I/O port is usually under control of the specialized SFR, which means that each bit of that register determines the state of the corresponding microcontroller pin. For example, by writing logic one (1) to a bit of the control register (SFR), the appropriate port pin is automatically configured as an input and voltage brought to it can be read as logic 0 or 1. Otherwise, by writing zero to the SFR, the appropriate port pin is configured as an output. Its voltage (0V or 5V) corresponds to the state of appropriate port register bit.
Memory is part of the microcontroller used for data storage. The easiest way to explain it is to compare it with a filing cabinet with many drawers. Suppose, the drawers are clearly marked so that their contents can be easily found out by reading the label on the front of the drawer.
READ ONLY MEMORY (ROM)
Read Only Memory (ROM) is used to permanently save the program being executed. The size of program that can be written depends on the size of this memory. Today’s microcontrollers commonly use 16-bit addressing, which means that they are able to address up to 64 Kb of memory, i.e. 65535 locations. As a novice, your program will rarely exceed the limit of several hundred instructions. There are several types of ROM.
Masked ROM (MROM)
Masked ROM is a kind of ROM the content of which is programmed by the manufacturer. The term ‘masked’ comes from the manufacturing process, where regions of the chip are masked off before the process of photolithography. In case of a large-scale production, the price is very low. Forget it…
One Time Programmable ROM (OTP ROM)
One time programmable ROM enables you to download a program into it, but, as its name states, one time only. If an error is detected after downloading, the only thing you can do is to download the correct program to another chip.
UV Erasable Programmable ROM (UV EPROM)
Installation of this window is complicated, which normally affects the price. From our point of view, unfortunately-negative…
This type of memory was invented in the 80s in the laboratories of INTEL and was represented as the successor to the UV EPROM. Since the content of this memory can be written and cleared practically an unlimited number of times, microcontrollers with Flash ROM are ideal for learning, experimentation and small-scale production. Because of its great popularity, most microcontrollers are manufactured in flash technology today. So, if you are going to buy a microcontroller, the type to look for is definitely Flash!
RANDOM ACCESS MEMORY (RAM)
Once the power supply is off the contents of RAM is cleared. It is used for temporary storing data and intermediate results created and used during the operation of the microcontroller. For example, if the program performs an addition (of whatever), it is necessary to have a register representing what in everyday life is called the ‘sum’. For this reason, one of the registers of RAM is called the ‘sum’ and used for storing results of addition.
ELECTRICALLY ERASABLE PROGRAMMABLE ROM (EEPROM)
The contents of EEPROM may be changed during operation (similar to RAM), but remains permanently saved even after the loss of power (similar to ROM). Accordingly, EEPROM is often used to store values, created during operation, which must be permanently saved. For example, if you design an electronic lock or an alarm, it would be great to enable the user to create and enter the password, but it’s useless if lost every time the power supply goes off. The ideal solution is a microcontroller with an embedded EEPROM.
Most programs use interrupts in their regular execution. The purpose of the microcontroller is mainly to respond to changes in its surrounding. In other words, when an event takes place, the microcontroller does something… For example, when you push a button on a remote controller, the microcontroller will register it and respond by changing a channel, turn the volume up or down etc. If the microcontroller spent most of its time endlessly checking a few buttons for hours or days, it would not be practical at all.
This is why the microcontroller has learnt a trick during its evolution. Instead of checking each pin or bit constantly, the microcontroller delegates the ‘wait issue’ to a ‘specialist’ which will respond only when something attention worthy happens.
The signal which informs the central processor unit about such an event is called an INTERRUPT.
CENTRAL PROCESSOR UNIT (CPU)
As its name suggests, this is a unit which monitors and controls all processes within the microcontroller. It consists of several subunits, of which the most important are:
- Instruction Decoder is a part of electronics which decodes program instructions and runs other circuits on the basis of that. The ‘instruction set’ which is different for each microcontroller family expresses the abilities of this circuit;
- Arithmetical Logical Unit (ALU) performs all mathematical and logical operations upon data; and
- Accumulator is an SFR closely related to the operation of the ALU. It is a kind of working desk used for storing all data upon which some operation should be performed (addition, shift/move etc.). It also stores results ready for use in further processing. One of the SFRs, called a Status Register (PSW), is closely related to the accumulator. It shows at any given time the ‘status’ of a number stored in the accumulator (number is larger or less than zero etc.). Accumulator is also called working register and is marked as W register or just W, therefore.
A bus consists of 8, 16 or more wires. There are two types of buses: the address bus and the data bus. The address bus consists of as many lines as necessary for memory addressing. It is used to transmit address from the CPU to the memory. The data bus is as wide as the data, in our case it is 8 bits or wires wide. It is used to connect all the circuits within the microcontroller.
Parallel connection between the microcontroller and peripherals via input/output ports is the ideal solution on 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 not possible to use parallel connection. Instead, serial communication is used.
Today, most microcontrollers have built in several different systems for serial communication as a standard equipment. Which of these systems 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 microcontroller automatically takes care of this, so that the work of the programmer/user is reduced to simple write (data to be sent) and read (received data).
The term baud rate is used to denote the number of bits transferred per second [bps]. Note that it refers to bits, not bytes. It is usually required by the protocol that each byte is transferred along with several control bits. It means that one byte in serial data stream may consist of 11 bits. For example, if the baud rate is 300 bps then maximum 37 and minimum 27 bytes may be transferred per second.
The most commonly used serial communication systems are:
I2C (INTER INTEGRATED CIRCUIT)
Inter-integrated circuit is a system for serial data exchange between the microcontrollers and specialized integrated circuits of a new generation. It is used when the distance between them is short (receiver and transmitter are usually on the same printed board). Connection is established via two conductors. One is used for data transfer, the other is used for synchronization (clock signal). As seen in figure below, one device is always a master. It performs addressing of one slave chip before communication starts. In this way one microcontroller can communicate with 112 different devices. Baud rate is usually 100 Kb/sec (standard mode) or 10 Kb/sec (slow baud rate mode). Systems with the baud rate of 3.4 Mb/sec have recently appeared. The distance between devices which communicate over an I2C bus is limited to several meters.
SPI (SERIAL PERIPHERAL INTERFACE BUS)
A serial peripheral interface (SPI) bus is a system for serial communication which uses up to four conductors, commonly three. One conductor is used for data receiving, one for data sending, one for synchronization and one alternatively for selecting a device to communicate with. It is a full duplex connection, which means that data is sent and received simultaneously.
The maximum baud rate is higher than that in the I2C communication system.
UART (UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER)
This sort of communication is asynchronous, which means that a special line for transferring clock signal is not used. In some applications, such as radio connection or infrared waves remote control, this feature is crucial. Since only one communication line is used, both receiver and transmitter operate at the same predefined rate in order to maintain necessary synchronization. This is a very simple way of transferring data since it basically represents the conversion of 8-bit data from parallel to serial format. Baud rate is not high, up to 1 Mbit/sec.
POWER SUPPLY CIRCUIT
There are two things worth attention concerning the microcontroller power supply circuit:
- Brown out is a potentially dangerous condition which occurs at the moment the microcontroller is being turned off or when the power supply voltage drops to a minimum due to electric noise. As the microcontroller consists of several circuits with different operating voltage levels, this state can cause its out-of-control performance. In order to prevent it, the microcontroller usually has a built-in circuit for brown out reset which resets the whole electronics as soon as the microcontroller incurs a state of emergency.
- Reset pin is usually marked as MCLR (Master Clear Reset). It is used for external reset of the microcontroller by applying a logic zero (0) or one (1) to it, which depends on the type of the microcontroller. In case the brown out circuit is not built in, a simple external circuit for brown out reset can be connected to the MCLR pin.
The microcontroller oscillator uses quartz crystal for its operation. Even though it is not the simplest solution, there are many reasons to use it. The frequency of such oscillator is precisely defined and very stable, so that pulses it generates are always of the same width, which makes them ideal for time measurement. Such oscillators are also used in quartz watches. If it is necessary to measure time between two events, it is sufficient to count up pulses generated by this oscillator. This is exactly what the timer does.
Most programs use these miniature electronic ‘stopwatches’. These are commonly 8- or 16-bit SFRs the contents of which is automatically incremented by each coming pulse. Once a register is completely loaded, an interrupt may be generated!
If the timer uses an internal quartz oscillator for its operation then it can be used to measure time between two events (if the register value is T1 at the moment measurement starts, and T2 at the moment it terminates, then the elapsed time is equal to the result of subtraction T2-T1). If 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 is however more complicated in practice.
HOW DOES THE TIMER OPERATE?
In practice, pulses generated by the quartz oscillator are once per each machine cycle, directly or via a prescaler, brought to the circuit which increments the number stored in the timer register. If one instruction (one machine cycle) lasts for four quartz oscillator periods then this number will be incremented a million times per second (each microsecond) by embedding quartz with the frequency of 4MHz.
It is easy to measure short time intervals, up to 256 microseconds, in the way described above because it is the largest number that one register can store. This restriction may be easily overcome in several ways such as by using a slower oscillator, registers with more bits, prescaler or interrupts. The first two solutions have some weaknesses so it is more recommended to use prescalers or interrupts.
USING A PRESCALER IN TIMER OPERATION
A prescaler is an electronic device used to reduce frequency by a predetermined factor. In order to generate one pulse on its output, it is necessary to bring 1, 2 , 4 or more pulses on its input. Most microcontrollers have one or more prescalers built in and their division rate may be changed from within the program. The prescaler is used when it is necessary to measure longer periods of time. If one prescaler is shared by timer and watchdog timer, it cannot be used by both of them simultaneously.
USING INTERRUPT IN TIMER OPERATION
If the timer register consists of 8 bits, the largest number it can store is 255. As for 16-bit registers it is the number 65.535. If this number is exceeded, the timer will be automatically reset and counting will start at zero again. This condition is called an overflow. If enabled from within the program, the overflow can cause an interrupt, which gives completely new possibilities. For example, the state of registers used for counting seconds, minutes or days can be changed in an interrupt routine. The whole process (except for interrupt routine) is automatically performed behind the scenes, which enables the main circuits of the microcontroller to operate normally.
This figure illustrates the use of an interrupt in timer operation. Delays of arbitrary duration, having almost no influence on the main program execution, can be easily obtained by assigning the prescaler to the timer.
If the timer receives pulses frm the microcontroller input pin, then it turns into a counter. Obviously, it is the same electronic circuit able to operate in two different modes. The only difference is that in this case pulses to be counted come over the microcontroller input pin and their duration (width) is mostly undefined. This is why they cannot be used for time measurement, but for other purposes such as counting products on an assembly line, number of axis rotation, passengers etc. (depending on sensor in use).
A 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 maximum value, the microcontroller reset occurs and the program execution starts from the first instruction. The point is to prevent this from happening by using a specific command.
Anyway, the whole idea is based on the fact that every program is executed in several longer or shorter loops. If instructions which reset 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 timer will not be cleared, so the register’s value being constantly incremented will reach the maximum et voila! Reset occurs!
The result of measurement is a number (digital value) used and processed later in the program.
All upgraded microcontrollers use one of two basic design models called Harvard and von-Neumann architecture.
They represent two different ways of exchanging data between CPU and memory.
- Read the part of the program instruction specifying WHAT should be done (in this case it is the ‘incf’ instruction for increment).
- Read the other part of the same instruction specifying upon WHICH data it should be performed (in this case it is the ‘SUM’ register).
- After being incremented, the contents of this register should be written to the register from which it was read (‘SUM’ register address).
The same data bus is used for all these intermediate operations.
The advantages of such design are the following:
- All data in the program is one byte (8 bits) wide. As the data bus used for program reading has 12, 14 or 16 lines, both instruction and data can be read simultaneously using these spare bits. For this reason, all instructions are single-cycle instructions, except for the jump instruction which is two-cycle instruction.
- Owing to the fact that the program (ROM) and temporary data (RAM) are separate, the CPU can execute two instructions at a time. Simply put, while RAM read or write is in progress (the end of one instruction), the next program instruction is read through the other bus.
- When using microcontrollers with von-Neumann architecture, one never knows how much memory is to be occupied by the program. Basically, most program instructions occupy two memory locations (one contains information on WHAT should be done, whereas the other contains information upon WHICH data it should be done). However, it is not a hard and fast rule, but the most common case. In microcontrollers with Harvard architecture, the program word bus is wider than one byte, which allows each program word to consist of instruction and data, i.e. one memory location – one program instruction.
RISC (REDUCED INSTRUCTION SET COMPUTER)
In this case, the microcontroller recognizes and executes only basic operations (addition, subtraction, copying etc.). Other, more complicated operations are performed by combining them. For example, multiplication is performed by performing successive addition. It’s the same as if you try to explain to someone, using only a few different words, how to reach the airport in a new city. However, it’s not as black as it’s painted. First of all, this language is easy to learn. The microcontroller is very fast so that it is not possible to see all the arithmetic ‘acrobatics’ it performs. The user can only see the final results. At last, it is not so difficult to explain where the airport is if you use the right words such as left, right, kilometers etc.
CISC (COMPLEX INSTRUCTION SET COMPUTER)
CISC is the opposite to RISC! Microcontrollers designed to recognize more than 200 different instructions can do a lot of things at high speed. However, one needs to understand how to take all that such a rich language offers, which is not at all easy…
HOW TO MAKE THE RIGHT CHOICE?
Ok, you are the beginner and you have made a decision to go on an adventure of working with the microcontrollers. Congratulations on your choice! However, it is not as easy to choose the right microcontroller as it may seem. The problem is not a limited range of devices, but the opposite!
Before you start to design a device based on the microcontroller, think of the following: how many input/output lines will I need for operation? Should it perform some other operations than to simply turn relays on/off? Does it need some specialized module such as serial communication, A/D converter etc.? When you create a clear picture of what you need, the selection range is considerably reduced and it’s time to think of price. Are you planning to have several same devices? Several hundred? A million? Anyway, you get the point.
If you think of all these things for the very first time then everything seems a bit confusing. For this reason, go step by step. First of all, select the manufacturer, i.e. the microcontroller family you can easily get. Study one particular model. Learn as much as you need, don’t go into details. Solve a specific problem and something incredible will happen- you will be able to handle any model belonging to that microcontroller family.
Remember learning to ride a bicycle. After several bruises at the beginning, you were able to keep balance, then to easily ride any other bicycle. And of course, you will never forget programming just as you will never forget riding bicycles!
1.4 PIC MICROCONTROLLERS
PIC microcontrollers designed by Microchip Technology are likely the best choice for beginners. Here is why…
The original name of this microcontroller is PICmicro (Peripheral Interface Controller), but it is better known as PIC. Its ancestor, called the PIC1650, was designed in 1975 by General Instruments. It was meant for totally different purposes. Around ten years later, this circuit was transformed into a real PIC microcontroller by adding EEPROM memory. Today, Microchip Technology announces the manufacture of the 5 billionth sample.
If you are interested in learning more about it, just keep on reading.
The main idea with this book is to provide the user with necessary information so that he is able to use microcontrollers in practice after reading it. In order to avoid tedious explanations and endless story about the useful features of different microcontrollers, this book describes the operation of one particular model belonging to the ‘high middle class’. It is the PIC16F887- powerful enough to be worth attention and simple enough to be easily presented to everybody. So, the following chapters describe this microcontroller in detail, but refer to the whole PIC family as well.
|Family||ROM [Kbytes]||RAM [bytes]||Pins||Clock Freq. [MHz]||A/D Inputs||Resolution of A/D Converter||Compar- ators||8/16 – bit Timers||Serial Comm.||PWM Outputs||Others|
|Base-Line 8 – bit architecture, 12-bit Instruction Word Length|
|PIC10FXXX||0.375 – 0.75||16 – 24||6 – 8||4 – 8||0 – 2||8||0 – 1||1 x 8||–||–||–|
|PIC12FXXX||0.75 – 1.5||25 – 38||8||4 – 8||0 – 3||8||0 – 1||1 x 8||–||–||EEPROM|
|PIC16FXXX||0.75 – 3||25 – 134||14 – 44||20||0 – 3||8||0 – 2||1 x 8||–||–||EEPROM|
|PIC16HVXXX||1.5||25||18 – 20||20||–||–||–||1 x 8||–||–||Vdd = 15V|
|Mid-Range 8 – bit architecture, 14-bit Instruction World Length|
|PIC12FXXX||1.75 – 3.5||64 – 128||8||20||0 – 4||10||1||1 – 2 x 8 1 x 16||–||0 – 1||EEPROM|
|PIC12HVXXX||1.75||64||8||20||0 – 4||10||1||1 – 2 x 8 1 x 16||–||0 – 1||–|
|PIC16FXXX||1.75 – 14||64 – 368||14 – 64||20||0 – 13||8 or 10||0 – 2||1 – 2 x 8 1 x 16||USART I2C SPI||0 – 3||–|
|PIC16HVXXX||1.75 – 3.5||64 – 128||14 – 20||20||0 – 12||10||2||2 x 8 1 x 16||USART I2C SPI||–||–|
|High-End 8 – bit architecture, 16-bit Instruction Word Length|
|PIC18FXXX||4 – 128||256 – 3936||18 – 80||32 – 48||4 – 16||10 or 12||0 – 3||0 – 2 x 8 2 – 3 x 16||USB2.0 CAN2.0 USART I2C SPI||0 – 5||–|
|PIC18FXXJXX||8 – 128||1024 – 3936||28 – 100||40 – 48||10 – 16||10||2||0 – 2 x 8 2 – 3 x 16||USB2.0 USART Ethernet I2C SPI||2 – 5||–|
|PIC18FXXKXX||8 – 64||768 – 3936||28 – 44||64||10 – 13||10||2||1 x 8 3 x 16||USART I2C SPI||2||–|
All PIC microcontrollers use Harvard architecture, which means that their program memory is connected to the CPU over more than 8 lines. Depending on the bus width, there are 12-, 14- and 16-bit microcontrollers. Table above shows the main features of these three categories.
As seen in the table on the previous page, excepting ‘16-bit monsters’- PIC 24FXXX and PIC 24HXXX- all PIC microcontrollers have 8-bit Harvard architecture and belong to one out of three large groups. Thus, depending on the size of the program word there are first, second and third microcontroller category, i.e. 12-, 14- or 16-bit microcontrollers. Having similar 8-bit core, all of them use the same instruction set and the basic hardware ‘skeleton’ connected to more or less peripheral units.
The instruction set for the 16F8XX includes 35 instructions in total. The reason for such a small number of instructions lies in the RISC architecture. It means that instructions are well optimized from the aspects of operating speed, simplicity in architecture and code compactness. The bad thing about RISC architecture is that the programmer is expected to cope with these instructions. Of course, this is relevant only if you use assembly language for programming. This book refers to programming in the higher programming language C, which means that most work has been done by somebody else. You just have to use relatively simple instructions.
INSTRUCTION EXECUTION TIME
All instructions are single-cycle instructions. The only exception may be conditional branch instructions (if condition is met) or instructions performed upon the program counter. In both cases, two cycles are required for instruction execution, while the second cycle is executed as an NOP (No Operation). Single-cycle instructions consist of four clock cycles. If 4MHz oscillator is used, the nominal time for instruction execution is 1μS. As for jump instructions, the instruction execution time is 2μS.
Instruction set of 14-bit PIC microcontrollers:
|Data Transfer Instructions|
|MOVLW k||Move constant to W||k -> w||1|
|MOVWF f||Move W to f||W -> f||1|
|MOVF f,d||Move f to d||f -> d||Z||1||1, 2|
|CLRW||Clear W||0 -> W||Z||1|
|CLRF f||Clear f||0 -> f||Z||1||2|
|SWAPF f,d||Swap nibbles in f||f(7:4),(3:0) -> f(3:0),(7:4)||1||1, 2|
|ADDLW k||Add W and constant||W+k -> W||C, DC, Z||1|
|ADDWF f,d||Add W and f||W+f -> d||C, DC ,Z||1||1, 2|
|SUBLW k||Subtract W from constant||k-W -> W||C, DC, Z||1|
|SUBWF f,d||Subtract W from f||f-W -> d||C, DC, Z||1||1, 2|
|ANDLW k||Logical AND with W with constant||W AND k -> W||Z||1|
|ANDWF f,d||Logical AND with W with f||W AND f -> d||Z||1||1, 2|
|ANDWF f,d||Logical AND with W with f||W AND f -> d||Z||1||1, 2|
|IORLW k||Logical OR with W with constant||W OR k -> W||Z||1|
|IORWF f,d||Logical OR with W with f||W OR f -> d||Z||1||1, 2|
|XORWF f,d||Logical exclusive OR with W with constant||W XOR k -> W||Z||1||1, 2|
|XORLW k||Logical exclusive OR with W with f||W XOR f -> d||Z||1|
|INCF f,d||Increment f by 1||f+1 -> f||Z||1||1, 2|
|DECF f,d||Decrement f by 1||f-1 -> f||Z||1||1, 2|
|RLF f,d||Rotate left f through CARRY bit||C||1||1, 2|
|RRF f,d||Rotate right f through CARRY bit||C||1||1, 2|
|COMF f,d||Complement f||f -> d||Z||1||1, 2|
|BCF f,b||Clear bit b in f||0 -> f(b)||1||1, 2|
|BSF f,b||Clear bit b in f||1 -> f(b)||1||1, 2|
|Program Control Instructions|
|BTFSC f,b||Test bit b of f. Skip the following instruction if clear.||Skip if f(b) = 0||1 (2)||3|
|BTFSS f,b||Test bit b of f. Skip the following instruction if set.||Skip if f(b) = 1||1 (2)||3|
|DECFSZ f,d||Decrement f. Skip the following instruction if clear.||f-1 -> d skip if Z = 1||1 (2)||1, 2, 3|
|INCFSZ f,d||Increment f. Skip the following instruction if set.||f+1 -> d skip if Z = 0||1 (2)||1, 2, 3|
|GOTO k||Go to address||k -> PC||2|
|CALL k||Call subroutine||PC -> TOS, k -> PC||2|
|RETURN||Return from subroutine||TOS -> PC||2|
|RETLW k||Return with constant in W||k -> W, TOS -> PC||2|
|RETFIE||Return from interrupt||TOS -> PC, 1 -> GIE||2|
|NOP||No operation||TOS -> PC, 1 -> GIE||1|
|CLRWDT||Clear watchdog timer||0 -> WDT, 1 -> TO, 1 -> PD||TO, PD||1|
|SLEEP||Go into sleep mode||0 -> WDT, 1 -> TO, 0 -> PD||TO, PD||1|
*1 When an I/O register is modified as a function of itself, the value used will be that value present on the pins themselves. *2 If the instruction is executed on the TMR register and if d=1, the prescaler will be cleared. *3 If the PC is modified or test result is logic one (1), the instruction requires two cycles.
The architecture of 8-bit PIC microcontrollers. Which of these modules are to belong to a microcontroller depends on its type.