Following are very basic topics for beginners (taken directly from my book),
1. INTRODUCTION
2. OVERVIEW OF AVR
3. COMPILER /IDE
4. BASIC SERIAL PROGRAMMER
5. I/O PORTS
6. LCD INTERFACING
7 TIMERS AND PWM CHANNELS
8 . ADC
9. UART
10. SERIAL PERIPHERAL INTERFACE
11. INTERRUPTS
1. Introduction to Micro controller
New to micro-controllers ? Wanna learn about it ? I was also in the same condition few years back ! (May 2006) Got some help from seniors and almighty Google then things got started.
So lets start from the very basic !
You must be knowing about Digital Integrated Circuits (ICs) right ? For example:
There are AND, XOR, NAND, NOR, OR logic gate ICs, Counters, Timers, Seven Segment Display Drivers and much more. Just check out 7400 Series and 4000 Series of Integrated Circuits.
Now lets take Quad 2 input AND gate IC. It has 4 AND gates, each having 2 pins for input and 1 pin for output. The truth table or the function table of each gate is fixed. Which is as folllows:
Input 1 | Input 2 | Output |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
Similarly all the Integrated circuits have there function tables and input and output pins fixed. You can not change the function and no input pin act as output and vice versa. So whenever you want to design some circuit you first have to get the output as a function of inputs and then design it using gates or whatever the requirement is.
So once a circuit is built we can not change its function ! Even if u want to make some changes again you have to consider all the gates and components involved. Now if you are designing any circuit which involves change of the function table every now and then you are in trouble ! For example if I want to design an Autonomous Robot which should perform various tasks and I don’t just want to fix trhe task. Suppose I make it to move in a path then I want to change the path ! How to do that ?
Here comes the use of Microcontrollers ! Now if I give you an Integrated Circuit with 20 pins and tell you that you can make any pin as output or input also you can change the function table by programming the IC using your computer ! Then your reactions will be wow ! that’s nice :) That’s what the most basic function of a microcontroller is. It has set of pins called as PORT and you can make any pin either as output or input. After configuring pins you can program it to perform according to any function table you want. You can change the configuration or the function table as many times you want.
There are many Semiconductor Companies which manufactures microcontrollers.
Some of them are:
- Intel
- Atmel
- Microchip
- Freescale
- Motorola
- ZiLOG
We will discuss about Atmel Microcontrollers commonly known as AVR in this section.
Question: How a microcontroller works !
Answer: Well I can not go into lot of details about the working because it is a vast topic in itself. I can just give an overview.
Microcontroller consists of an Microprocessor (CPU that is Central processing Unit) which is interfaced to RAM (Random Access Memory) and Flash Memory (one your pen drive has !). You feed your program in the Flash Memory on the microcontroller. Now when you turn on the microcontroller CPU access the instruction from RAM which access your code from Flash. It sets the configuration of pins and start performing according to your program. For details refer,
- Wikipedia (good link for knowing about anything ! )
- AVR Freaks (nice community for AVR users)
Question: How to make the code ?
Answer: You basically write the program on your computer in any of the high level languages like C, C++, JAVA etc. Then you compile the code to generate the machine file. Now you will ask what this machine file is ? All the machines understands only one language, 0 & 1 that is on and off. Now this 0 & 1 both corresponds to 2 different voltage levels for example 0 volt for 0 logic and +5 volt for 1 logic. Actually the code has to be written in this 0,1 language and then saved in the memory of the microcontroller. But this will be very difficult for us ! So we write the code in the language we understand (C) and then compile and make the machine file (.hex) After we make this machine file we feed this to the memory of the microcontroller.
Question: How to feed the code in the flash of Microcontroller ?
Answer: Assuming you have the machine file (.hex) ready and now you want to feed that to the flash of the microcontroller. Basically you want to make communication between your computer and microcontroller. Now computer has many communication ports such as Serial Port, Parallel Port and USB (Universal Serial Bus).
Lets take Serial Port, it has its own definition that is voltage level to define 0 & 1 (yeah all the data communication is a just collection of 0 & 1 ) Serial Port's protocol is called as UART (Universal Asynchronous Reciever & Transmitter) Its voltage levels are, -12 volt for 0 logic and +12 volt for 1 logic.
Now the voltage levels of our micrcontroller is based on CMOS (Complimentary Metal Oxide Semiconductor) technology which has 0 volt for 0 logic and +5 volt for 1 logic.
Two different machines with 2 different ways to define 0 & 1 and we want to exchange information between them. Consider microcontroller as a French and Computer's Serial Port as an Indian person (obviously no commom language in between !) If they want to exchange information they basically needs a mediator who knows both the language. He will listen one person and then translate to other person. Similarly we need a circuit which converts CMOS (microcontroller) to USART (serial port) and vice versa. This circuit is called as programmer. Using this circuit we can connect computer to the microcontroller and feed the machine file to the flash.
Compiler / IDE (Integrated Development Environment)
Atmel Microcontrollers are very famous as they are very easy to use. There are many development tools available for them. First of all we need an easy IDE for developing code. I suggest beginners to use CVAVR (Code Vision AVR) Evaluation version is available for free download from the website. It has limitaion of code size. It works on computers with Windows platform that is Windows XP & Vista.
Some famous compilers/development tools supporting Windows for Atmel Microcontrollers are:
- WINAVR (AVRGCC for Windows)
- Code Vision AVR (CVAVR)
- AVR Studio (Atmel's free developing tool)
AVRGCC is a very nice open source compiler used by most of the people.
Programmer
Programmer basically consists of two parts:
- Software (to open .hex file on your computer)
- Hardware (to connect microcontroller)
Hardware depends on the communication port you are using on the computer (Serial, Parallel or USB). I suggest beginners to use Serial Programmer as it is very easy to build. Software for that is Pony Prog or At-prog. Some famous Windows (XP, Vista) programmers are:
Assuming you must have got a basic idea about microcontrollers now. So lets start some real stuff ! go to the next link :-)
2....
- Advanced RISC Architecture
- Up to 16 MIPS Throughput at 16 MHz
- 16K Bytes of In-System Self-Programmable Flash
- 512 Bytes EEPROM
- 1K Byte Internal SRAM
- 32 Programmable I/O Lines
- In-System Programming by On-chip Boot Program
- 8-channel, 10-bit ADC
- Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes
- One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture
- Four PWM Channels
- Programmable Serial USART
- Master/Slave SPI Serial Interface
- Byte-oriented Two-wire Serial Interface
- Programmable Watchdog Timer with Separate On-chip Oscillator
- External and Internal Interrupt Sources
Pin Configuration
Pin Description
GND: Ground. (0 V) Note there are 2 ground Pins.
Port A (PA7 - PA0)
Port A serves as the analog inputs to the A/D Converter. Port A also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. When pins PA0 to PA7 are used as inputs and are externally pulled low, they will source current if the internal pull-up resistors are activated. The Port A pins are tri-stated when a reset condition becomes active, even if the clock is not running.
Port B (PB7 - PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). Port B also serves the functions of various special features of the ATmega16 as listed on page 58 of datasheet.
Port C (PC7 - PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). Port C also serves the functions of the JTAG interface and other special features of the ATmega16 as listed on page 61 of datasheet. If the JTAG interface is enabled, the pull-up resistors on pins PC5 (TDI), PC3 (TMS) and PC2 (TCK) will be activated even if a reset occurs.
Port D (PD7 - PD0)
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). Port D also serves the functions of various special features of the ATmega16 as listed on page 63 of datasheet.
RESET: Reset Input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running.
XTAL1: External oscillator pin 1
XTAL2: External oscillator pin 2
AVCC: AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.
AREF: AREF is the analog reference pin for the A/D Converter.
Block Diagram
3 Compiler / IDE (Integrated Development Enviornment)
An IDE has following functions:
- Preprocessing
- Compilation
- Assembly
- Linking
- Object Translation
- Text Editor
If we just use compiler and linker independently we still need to get a text editor. So combining everything will actually mess things up. So the best way is to get a Software which has it all. Thats called an Integrated Development Enviroment, in short IDE.
I consider Code-Vision-AVR to be the best IDE for getting started with AVR programming on Windows XP, Vista. It has a very good Code Wizard which generate codes automatically ! You need not mess with the assembly words. So in all my tutorials I will be using CVAVR. You can download evaluation version for free which has code size limitation but good enough for our purpose.
For all my examples I will be using Atmega-16 as default microcontroller because it very easily available and is powerful enough with sufficent number of pins and peripherals we use. You can have a look on the datasheet of Atmega-16 in the datasheet section.
Lets take a look on the software. The main window looks like following (click to enlarge the pic)
Now click on File ---> New --->Project
A pop up window will come asking whether you want to use Code Wizard AVR, obviously select yes because that is the reason we are using CVAVR !
Now have a look on this Wizard. It has many tabs where we can configure PORTS, TIMERS, LCD, ADC etc. I am explaining some of them
CHIP:
Select the chip for which you are going to write the program. Then select the frequnecy at which Chip is running. By default all chips are set on Internal Oscialltor of 1 Mhz so select 1 MHz if that is the case. If you want to change the running clock frequency of the chip then you have to change its fuse bits (I will talk more about this in fuse bits section).
PORT:
PORT is usually a collection of 8 pins.
From this tab you can select which pin you want to configure as output and which as input. It basically writes the DDR and PORT register through this setting. Registers are basically RAM locationswhich configures various peripherals of microcontroller and by changing value of these registers we can change the function it isperforming. I will talk more about registers later. All the details are provided inthe datasheet.
So you can configure any pin as output or input by clicking the box.
For Atmega-16 which has 4 Ports we can see 4 tabs each corresponding to one Port. You can also set initial value of the Pins you want to assign. or if you are using a pin as input then whether you want to make it as pullup or tristated, again I will talk in details about these functions later.
Similarly using this code wizard you can very easily configure all the peripherals on the Atmega.
Now for generating code just go to File ---->Generate, Save and Exit (of the code wizard obviously !)
Now it will ask you name and location for saving three files. Two being project files and one being the .C file which is your program. try to keep same names of all three files to avoid confusion. By default these files are generated in C:\CVAVR\bin
The generated program will open in the text editor. Have a look it has some declarations like PORT, DDR, TCCR0 and many more. These are all registers which configures various functions of Atmega and by changing these value we make different functions. All the details about the registers are commented just below them. Now go down down and find following infinite while loop there. We can start writing our part of program just before the while loop. And as for most of the applications we want microcontroller to perform the same task forever we put our part of code in the infinite while loop provided by the code wizard !
while (1)
{
// Place your code here};
}
See how friendly this code wizard is, all the work (configuring registers) automatically done and we dont even need to understand and go to the details about registers too !
Now we want to generate the hex file, so first compile the program. Either press F9 or go to Project--->Compile.
It will show compilation errors if any. If program is error free we can proceed to making of hex file. So either press Shift+F9 or go to Project---->Make. A pop up window will come with information about code size and flash usage etc.
So the machine file is ready now ! It is in the same folder where we saved those 3 files. Now we want to make a programmer and feed our code to the microcontroller (Atmega-16)
4. Serial Programmer's Circuit (Hardware)
This is the easiest programmer circuit to make. You just have to get Serial Port connector and three 1K resistors and you are done ! Circuit Diagram is attached.
Now open the datasheet of Atmega you are using (I am using Atmega-16). Go to the pin configuration and find the following pins and connect the programmer. Programming is done through SPI (Serial Peripheral Interface) which involves MISO, MOSI and SCK pins. RESET is used to reset the chip. 0 volt on this pin will reset the chip and for normal running it should be pulled up to +5V.
- MOSI (Master Out Slave In)
- MISO (Master In Slave Out)
- SCK (Serial Clock)
- RESET
- GND (Ground)
Now connect the power supplies that is Vcc and GND to the micro controller.
Vcc = +5V and GND = 0V
Do not forget to connect Reset to Vcc with a 1K/10K resistor for pulling up.
Thats it ! We are ready with the hardware :)
Software
As I told you there are two parts of a programmer, hardware and software. We built this hardware as it is very easy with just 3-4 components. Now we need software which support this hardware and can comminucate with micro controller using this circuit. There are 2 good softwares for Windows. They are,
Both of them support Serial as well as Parallel port, but I have always preferred serial port because it has only 9 pins, hence a smaller connector is required.
Now let us discuss about them.
At-Prog
I consider it as simplest programmer ever ! It was my first programmer. The circuit we discussed is actually based on this programmer. Download the folder from the website, unzip it. It has an executable file named at-prog.exe double click to execute it.
Go to PORT and select the address of serial port (by default COM1). Once you click that Serecon2 detected should appear in the Activility tab. That is the name of the programmer circuit.
Turn on the power supply and connect the circuit to the micro controller.
Click on Check, it should show OK in the Activity tab and device name (ATMEGA16) will automatically come in the tab below check option. This is the autodetect option of this programmer which autodetects the device name connected by its signature.
Now just open the device file from File--->Open and click on Write. It will write and verify the program. Done ! So simple isnt it ? :)
Pony Prog
One of the most commonly used programmers on Windows. Download it from the website and install it. Now lets set it up for our hardware.
First select AVR micro and AVR-Auto (you can also specify device name, Atmega16) in the chip options (last two drop down tabs)
Go to the Setup---->Interface-Setup. Then do the settings as shown in the picture below. Then connect the circuit to the microcontroller, turn on the power supply. Now click on Probe. You should get Test OK message. If not, check your connections again.
Now lets read the Microcontroller. Go to Command---->Read All. It should start reading the signature and the flash memory. You should get Read Successful message after that.
So we are done with the settings and testings. Everything is working fine :) Now just open the hex file from File---->Open-Device-File
Go to Command---->Write-Program(Flash). It will start writing and then veryfying the code. Congratulations :) you did it !
5 I/O PORTS PROGRAMMING
So lets start with understanding the functioning of AVR. We will first discuss about I/O Ports. Again I remind you that I will be using and writing about Atmega-16. Lets first have a look at the Pin configuration of Atmega-16. Image is attached, click to enlarge.
You can see it has 32 I/O (Input/Output) pins grouped as A,B,C & D with 8 pins in each group. This group is called as PORT.
- PA0 - PA7 (PORTA)
- PB0 - PB7 (PORTB)
- PC0 - PC7 (PORTC)
- PD0 - PD7 (PORTD)
Notice that all these pins have some function written in bracket. These are additonal function that pin can perform other than I/O. Some of them are.
- ADC (ADC0 - ADC7 on PORTA)
- UART (Rx,Tx on PORTD)
- TIMERS (OC0 - OC2)
- SPI (MISO, MOSI, SCK on PORTB)
- External Interrupts (INT0 - INT2)
REGISTERS:
All the configurations in microcontroller is set through 8 bit (1 byte) locations in RAM (RAM is a bank of memory bytes) of the microcontroller called as Registers. All the functions are mapped to its locations in RAM and the value we set at that location that is at that Register, configures the functioning of microcontroller. There are total 32 x 8bit registers in Atmega-16. As Register size of this microcontroller is 8 bit, it called as 8 bit microcontroller.
All the information is given in the datasheet. Just open the datasheet of Atmega 16 and go to last lines in page 4 and start reading the Pin Descriptions. You will understand about functioning of each pin.
I/O PORTS:
Input Output functions are set by Three Registers for each PORT.
- DDRX ----> Sets whether a pin is Input or Output of PORTX.
- PORTX ---> Sets the Output Value of PORTX.
- PINX -----> Reads the Value of PORTX.
Go to the page 50 in the datasheet or you can also see the I/O Ports tab in the Bookmarks.
DDRX (Data Direction Register)
First of all we need to set whether we want a pin to act as output or input. DDRX register sets this. Every bit corresponds to one pin of PORTX. Lets have a look on DDRA register.
Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
PIN | PA7 | PA6 | PA5 | PA4 | PA3 | PA2 | PA1 | PA0 |
Now to make a pin act as I/O we set its corresponding bit in its DDR register.
- To make Input set bit 0
- To make Output set bit 1
If I write DDRA = 0xFF (0x for Hexadecimal number system) that is setting all the bits of DDRA to be 1, will make all the pins of PORTA as Output.
Similarly by writing DDRD = 0x00 that is setting all the bits of DDRD to be 0, will make all the pins of PORTD as Input.
Now lets take another example. Consider I want to set the pins of PORTB as shown in table,
PORT-B | PB7 | PB6 | PB5 | PB4 | PB3 | PB2 | PB1 | PB0 |
Function | Output | Output | Input | Output | Input | Input | Input | Output |
DDRB | 1 | 1 | 0 | 1 | 0 | 0 | 0 | 1 |
For this configuration we have to set DDRB as 11010001 which in hexadecimal is D1. So we will write DDRB=0xD1
Summary
- DDRX -----> to set PORTX as input/output with a byte.
- DDRX.y ---> to set yth pin of PORTX as input/output with a bit (works only with CVAVR).
PORTX (PORTX Data Register)
This register sets the value to the corresponding PORT. Now a pin can be Ouput or Input. So lets discuss both the cases.
If a pin is set to be output, then by setting bit 1 we make output High that is +5V and by setting bit 0 we make output Low that is 0V.
Lets take an example. Consider I have set DDRA=0xFF, that is all the pins to be Output. Now I want to set Outputs as shown in table,
PORT-A | PA7 | PA6 | PA5 | PA4 | PA3 | PA2 | PA1 | PA0 |
Value | High(+5V) | High(+5V) | Low(0V) | Low(0V) | Low(0V) | High(+5V) | High(+5V) | Low(0V) |
PORTA | 1 | 1 | 0 | 0 | 0 | 1 | 1 | 0 |
For this configuration we have to set PORTA as 11000110 which in hexadecimal is C6. So we will write PORTA=0xC6;
If a pin is set to be input, then by setting its corresponding bit in PORTX register will make it as follows,
- Set bit 0 ---> Tri-Stated
- Set bit 1 ---> Pull Up
Tristated means the input will hang (no specific value) if no input voltage is specified on that pin. Pull Up means input will go to +5V if no input voltage is specified on that pin.
- PORTX ----> to set value of PORTX with a byte.
- PORTX.y --> to set value of yth pin of PORTX with a bit (works only with CVAVR).
PINX (Data Read Register)
This register is used to read the value of a PORT. If a pin is set as input then corresponding bit on PIN register is,
- 0 for Low Input that is V < 2.5V
- 1 for High Input that is V > 2.5V (Ideally, but actually 0.8 V - 2.8 V is error zone !)
For an example consider I have connected a sensor on PC4 and configured it as an input pin through DDR register. Now I want to read the value of PC4 whether it is Low or High. So I will just check 4th bit of PINC register.
We can only read bits of the PINX register, can never write on that as it is meant for reading the value of PORT.
Summary
- PINX ----> Read complete value of PORTX as a byte.
- PINX.y --> Read yth pin of PORTX as a bit (works only with CVAVR).
I hope you must have got basic idea about the functioning of I/O Ports. For detailed reading you can always refer to datasheet of Atmega.
6 LCD Interfacing with AVR
Now we need to interface an LCD to our microcontroller so that we can display messages, outputs, etc. Sometimes using an LCD becomes almost inevitable for debugging and calibrating the sensors (discussed later). We will use the 16x2 LCD, which means it has two rows of 16 characters each. Hence in total we can display 32 characters.
Cell coordinates : (x,y)
| 0,0 | 1,0 | 2,0 | 3,0 | 4,0 | 5,0 | 6,0 | 7,0 | 8,0 | 9,0 | 10,0 | 11,0 | 12,0 | 13,0 | 14,0 | 15,0 |
| 0,1 | 1,1 | 2,1 | 3,1 | 4,1 | 5,1 | 6,1 | 7,1 | 8,1 | 9,1 | 10,1 | 11,1 | 12,1 | 13,1 | 14,1 | 15,1 |
Circuit Connection
There are 16 pins in an LCD; See reverse side of the LCD for the PIN configuration.
The connections have to be made as shown in image.
Settings in CVAVR
When we connect an LCD to Atmega16, one full PORT is dedicated to it, denoted by PORT-X in the figure. To enable LCD interfacing in the microcontroller, just click on the LCD tab in the Code Wizard and select the PORT at which you want to connect the LCD. We will select PORTC. Also select the number of characters per line in your LCD. This is 16 in our case. Code Wizard now shows you the complete list of connections which you will have to make in order to interface the LCD. These are nothing but the same as in the above figure for general PORT-X.
As you can see, there are some special connections other than those to uC, Vcc and gnd. These are general LCD settings. Pin 3 (VO) is for the LCD contrast, ground it through a <1kΩ resistance/ potentiometer for optimum contrast. Pin 15 & 16 (LEDA and LEDK) are for LCD backlight, give them permanent +5V and GND respectively as we need to glow it continuously.
Printing Functions
Now once the connections have been made, we are ready to display something on our screen. Displaying our name would be great to start with. Some of the general LCD functions which you must know are:
lcd_clear()
Clears the lcd. Remember! Call this function before the while(1) loop, otherwise you won’t be able to see anything!
lcd_gotoxy(x,y)
Place the cursor at coordinates (x,y) and start writing from there. The first coordinate is (0,0). Hence, x ranges from 0 to 15 and y from 0 to 1 in our LCD. Suppose you want to display something starting from the 5th character in second line, then the function would be
lcd_gotoxy(5,1);
lcd_putchar(char c)
To display a single character.
Ex,
lcd_putchar(‘H’);
lcd_putchar(‘H’);
lcd_putsf(constant string)
To display a constant string.
Ex,
lcd_putsf(“IIT Kanpur”);
lcd_puts(char_array)
To display a variable string, which is nothing but an array of characters (data type char) in C language. e.g., You have an array char c[10] which keeps on changing. Then to display it, the function would be called as
lcd_puts(c);
Now we have seen that only characters or strings (constant or variable) can be displayed on the LCD. But quite often we have to display values of numeric variables, which is not possible directly. Hence we need to first convert that numeric value to a string and then display it. For e.g., if we have a variable of type integer, say int k, and we need to display the value of k (which changes every now and then, 200 now and 250 after a second... and so on). For this, we use the C functions itoa() and ftoa(), but remember to include the header file stdlib.h to use these C functions.
itoa(int val, char arr[])
It stores the value of integer val in the character array arr. E.g., we have already defined
int i and char c[20], then
itoa(i,c);
lcd_puts(c);
lcd_puts(c);
Similarly we have
ftoa(float val, char decimal_places, char arr[])
It stores the value of floating variable f in the character array arr with the number of decimal places as specified by second parameter. E.g., we have already defined float f and char c[20], then
ftoa(f,4,c); // till 4 decimal places
lcd_puts(c);
lcd_puts(c);
Now we are ready to display anything we want on our LCD. Just try out something which you would like to see glowing on it ! I know you are going to print your name first :-)
7 Timers Basic Overview
- Timer 0, 8 bit
- Timer 1, 16 bit consisting of two 8 bit parts, A and B
- Timer 2, 8 bit
Now there are two clocks,
- System Clock (fs): This is the clock frequency at which Atmega is running. By default it is 1 MHz which can be changed by setting fuse bits.
- Timer Clock (ft): This is the clock frequency at which timer module is running. Each timer module has different clocks.
Now ft can be in ratios of fs. That is, ft = fs, fs/8, fs/64 ...
For example, if we keep fs = 8 MHz then available options for ft are: 8 MHz, 1 MHz, 125 KHz ... (look in the image)
There are total 4 settings for Timers,
- Clock Source: Source for timer clock, keep it as system clock. You can also provide external clock. Read datasheet for more information about external clock source.
- Clock value: This is value of ft. Drop down for available options of ft. Chose whichever is required
- Mode: There are many modes of timers. We will be discussing following 2 modes,
- Fast PWM top = FFh
- CTC top=OCRx (x=0, 1A, 2)
- Output: Depending upon the mode we have chosen there are options for output pulse. We will look in detail later.
Basically each Timer has a counter unit with size 8 bit for Timer 0, 2 and 16 bit for Timer 1. I will be talking about Timer 0 and same will follow for other Timers.
Each counter has a register which increments by one on every rising edge of timer clock. After counting to its full capacity, 255 for 8 bit, it again starts from 0. By using this register we can have different modes.
- Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers.
- TOP: The counter reaches the TOP when it becomes equal to the highest value in the count sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value stored in the OCR0 Register. The assignment is dependent on the mode of operation.
Fast PWM Mode
PWM = Pulse Width Modulation.This mode is used to generate pulse with
- Fixed Frequency (F)
- Variable Duty Cycle (D)
F = Ft / 256
D = OCR0 / 255 (non inverted output)
D = (255-OCR0) / 255 (inverted output)
By changing OCR0 value we can change the duty cycle of the output pulse.
As OCR0 is an 8bit register it can vary from 0 to 255.
0 ≤ OCR0 ≤ 255
Pins for Output pulse,
Timer 0 : OC0, pin 4
Timer 1A : OC1A, pin 19
Timer 1B : OC1B, pin 18
Timer 2 : OC2, pin 21
CTC Mode
This mode is to generate pulse with,
- Fixed Duty Cycle (D = 0.5)
- Variable Frequency (F)
(corrected on 8th Feb, thanks to Ankur for pointing mistake)
D = 0.5
By changing value of OCR0 we can change the value of output pulse frequency. As OCR0 is an 8bit register it can vary from 0 to 255.
0 ≤ OCR0 ≤ 255
8 ADC PROGRAMMING IN AVR
Theory of Operation
What we have seen till now that the input given to uC was digital, i.e., either +5 V (logic 1) or 0V (logic 0). But what if we have an analog input, i.e., value varies over a range, say 0V to +5V? Then we require a tool that converts this analog voltage to discrete values. Analog to Digital Converter (ADC) is such a tool.
ADC is available at PORTA of Atmega16. Thus we have 8 pins available where we can apply analog voltage and get corresponding digital values. The ADC register is a 10 bit register, i.e., the digital value ranges from 0 to 1023. But we can also use only 8 bit out of it (0 to 255) as too much precision is not required.
Reference voltage is the voltage to which the ADC assigns the maximum value (255 in case of 8 bit and 1023 for 10 bit). Hence, the ADC of Atmega16 divides the input analog voltage range (0V to Reference Voltage) into 1024 or 256 equal parts, depending upon whether 10 bit or 8 bit ADC is used. For example, if the reference voltage is 5V and we use 10bit ADC, 0V has digital equivalent 0, +5V is digitally 1023 and 2.5V is approximately equal to 512.
ADC =Vin x 255/Vref (8 bit)
ADC =Vin x 1023/Vref (10 bit)
Setting up Microcontroller
To enable ADC in Atmega16, click on the ADC tab in Code Wizard and enable the checkbox. You can also check “use 8 bits” as that is sufficient for our purpose and 10 bit accuracy is not required. If the input voltage ranges from 0 to less than +5V, then apply that voltage at AREF (pin 32) and select the Volt. Ref. as AREF pin. But if it ranges from 0 to +5 V, you can select the Volt. Ref. as AVCC pin itself. Keep the clock at its default value of 125 kHz and select the Auto Trigger Source as Free Running. You can also enable an interrupt function if you require.
Function for getting ADC value
unsigned char read_adc(unsigned char adc_input).
This function returns the digital value of analog input at that pin of PORTA whose number is passed as parameter, e.g., if you want to know the digital value of voltage applied at PA3, and then just call the function as,
read_adc(3);
If the ADC is 8 bit, it will return a value from 0 to 255. Most probably you will need to print it on LCD. So, the code would be somewhat like
int a; char c[10]; // declare in the section of global variables
a=read_adc(3);
itoa(a,c);
lcd_puts(c);
9 UART (Universal Asynchronous Receiver Transmitter)
It is a way of communication between the microcontroller and the computer system or another microcontroller. There are always two parts to any mode of communication-a Receiver and a Transmitter. Hence, our Atmega can receive data as well as send data to other microcontroller, computer or any other device.
UART: Theory of Operation
Figure 16 illustrates a basic UART data packet. While no data is being transmitted, logic 1 must be placed in the Tx line. A data packet is composed of 1 start bit, which is always a logic 0, followed by a programmable number of data bits (typically between 6 to 8), an optional parity bit, and a programmable number of stop bits (typically 1). The stop bit must always be logic 1.
Most UART uses 8bits for data, no parity and 1 stop bit. Thus, it takes 10 bits to transmit a byte of data.
Figure 17: Basic UART packet format: 1 start bit, 8 data bits, 1 parity bit, 1 stop bit.
BAUD Rate: This parameter specifies the desired baud rate (bits per second) of the UART. Most typical standard baud rates are: 300, 1200, 2400, 9600, 19200, etc. However, any baud rate can be used. This parameter affects both the receiver and the transmitter. The default is 2400 (bauds).
In the UART protocol, the transmitter and the receiver do not share a clock signal. That is, a clock signal does not emanate from one UART transmitter to the other UART receiver. Due to this reason the protocol is said to be asynchronous.
Since no common clock is shared, a known data transfer rate (baud rate) must be agreed upon prior to data transmission. That is, the receiving UART needs to know the transmitting UART’s baud rate (and conversely the transmitter needs to know the receiver’s baud rate, if any). In almost all cases the receiving and transmitting baud rates are the same. The transmitter shifts out the data starting with the LSB first.
Once the baud rate has been established (prior to initial communication), both the transmitter and the receiver’s internal clock is set to the same frequency (though not the same phase). The receiver "synchronizes" its internal clock to that of the transmitter’s at the beginning of every data packet received. This allows the receiver to sample the data bit at the bit-cell center.
The receiver detects the start bit by detecting the transition from logic 1 to logic 0 (note that while the data line is idle, the logic level is high). In the case of 16450 UART, once the start-bit is detected, the next data bit’s "center" can be assured to be 24 ticks minus 2 (worse case synchronizer uncertainty) later. From then on, every next data bit center is 16 clock ticks later. Figure 2 illustrates this point.
Once the start bit is detected,the subsequent data bits are assembled in a de-serializer. Error condition maybe generated if the parity/stop bits are incorrect or missing.
Serial Port of Computer
We will be using Serial Port for communication between the uC and the computer. A serial port has 9 pins as shown. If you have a laptop, then most probably there won’t be a serial port. Then you can use a USB to serial Converter.
If you have to transmit one byte of data, the serial port will transmit 8 bits as one bit at a time. The advantage is that a serial port needs only one wire to transmit the 8 bits.
Pin 3 is the Transmit (TX) pin, pin 2 is the Receive (RX) pin and pin 5 is Ground pin. Other pins are used for controlling data communication in case of a modem. For the purpose of data transmission, only the pins 3 and 5 are required.
The standard used for serial communication is RS-232 (Recommended Standard 232). The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels. Valid signals are plus or minus 3 to 15 volts. The range near zero volts is not a valid RS-232 level; logic one is defined as a negative voltage, the signal condition is called marking, and has the functional significance of OFF. Logic zero is positive; the signal condition is spacing, and has the function ON.
Now we know that this is not the voltage level at which our microcontroller works. Hence, we need a device which can convert this voltage level to that of CMOS, i.e., logic 1 = +5V and logic 0 = 0V. This task is carried out by an IC MAX 232, which is always used with four 10uF capacitors.
The circuit is shown:
The Rx and Tx shown in above figure (pins 11 and 12 of MAX232) are the Rx and Tx of Atmega 16 (PD0 and PD1 respectively).
Setting up UART in microcontroller
Once our electronic circuit is complete, we can start with coding. To enable UART mode in Atmega16, click on the USART tab in Code Wizard. Now depending upon your requirement, you can either check receiver, transmitter or both. You get a list of options to select from for the baud rate. Baud Rate is the unit of data transfer, defined as bits per second. We will select 9600 as it is fair enough for our purpose, and default setting in most of the applications (like Matlab, Docklight, etc.). Keep the communication parameters as default, i.e., 8 data, 1 stop and no parity and mode asynchronous. You also have option of enabling Rx and Tx Interrupt functions, but we won’t do at this point.
Once you generate and save the code, all the register values are set by CVAVR and you only need to know some of the C functions to transfer data. Some of them are:
putchar()
To send one character to the buffer, which will be received by the device (uC or computer)?
E.g.,
putchar(‘A’); //sends character ‘A’ to the buffer
putchar(c); // sends a variable character c
putchar(c); // sends a variable character c
getchar()
To receive one character from the buffer, which might have been sent by the other uC or the computer. E.g., if we have already defined a variable char c, then
c = getchar(); // receives the character from buffer and save it in variable c
putsf()
To send a constant string. Eg,
putsf(“Robocon Team IIT Kanpur”);
Docklight
To communicate with the computer, you need a terminal where you can send data through keyboard and the received data can be displayed on the screen. There are many softwares which provide such terminal, but we will be using Docklight. Its evaluation version is free for download on internet, which is sufficient for our purpose.
To start with, check the Terminal Settings in Docklight. Go to Tools -> Project Settings. Select the Send/Receive communication channel, i.e., the name by which the serial port is known in your computer (like COM1…). In the COM port settings, select the same values as you had set while coding Atmega 16. So, we will select Baud Rate 9600, Data Bits 8, Stop Bits 1, Parity Bits none. You can select ‘none’ in Parity Error Character.
Click OK.
We are now ready to send/receive data, so, select Run -> Start Communication, or, press F5. If your uC is acting as a transmitter, then the characters it sends will appear in the Communication window of Docklight. E.g.,
putchar(‘K’);
delay_ms(500); // Sends character K after every 500ms
Hence what you get on the screen is a KKKKKKKKKKKKKKKKKKKKKKKKK…….. one K increasing every 500ms. To stop receiving characters, select Run -> Stop Communication, or press F6.
If the receiver option is also enabled in Atmega16, then whatever you type from keyboard will be received by it. You can either display these received characters on an LCD, control motors depending on what characters you send, etc. e.g.,
c = getchar() ; // receive character
lcd_putchar(c); // display it on LCD
if (c==’A’)
PORTA=255; // if that character is A, make all pins of PORTA high.
When you are done with sending data, select Run -> Stop Communication, or press F6.
10 SERIAL PERIPHERAL INTERFACE
The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link used to communicate between two or more microcontroller and devices supporting SPI mode data transfer. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select (chip select) lines.
This communication protocol consists of folliwng lines or pins,
- MOSI : Master Out Slave In (Tx for Master and Rx for Slave)
- MISO : Master In Slave Out (Rx for Master Tx for Slave)
- SCK : Serial Clock (Clock line)
- SS : Slave Select (To select Slave chip) (if given 0 device acts as slave)
Master: This device provides the serial clock to the other device for data transfer. As a clock is used for the data transfer, this protocol is Synchronous in nature. SS for Master will be disconnected.
Slave: This device accepts the clock from master device. SS for this has to be made 0 externally.
Setting up SPI in Microcontroller
Master Microcontroller
Open Code Wizard and go to SPI tab. Enable SPI and choose Master in SPI Type.
Select clock rate depending upon your data transfer speed requirement.
Keep other settings as default.
Slave Microcontroller
Open Code Wizard and go to SPI tab. Enable SPI and choose Slave in SPI Type.
Select clock rate depending upon your data transfer speed requirement.
Keep other settings as kept in the master microcontroller.
Data Transfer Functions
You can transmit or receive 1 byte of data at a time.
Transmit Data
spi(1 byte data);
Example: spi(‘A’);
Receive Data
c = spi(0); // c is 1 byte variable
Example: char c = spi(0);
11 INTERRUPT
An interrupt is a signal that stops the current program forcing it to execute another program immediately. The interrupt does this without waiting for the current program to finish. It is unconditional and immediate which is why it is called an interrupt.
The whole point of an interrupt is that the main program can perform a task without worrying about an external event.
For example if you want to read a push button connected to one pin of an input port you could read it in one of two ways either by polling it or by using interrupts.
Polling
Polling is simply reading the button input regularly. Usually you need to do some other tasks as well e.g. read an RS232 input so there will be a delay between reads of the button. As long as the delays are small compared to the speed of the input change then no button presses will be missed.
If however you had to do some long calculation then a keyboard press could be missed while the processor is busy. With polling you have to be more aware of the processor activity so that you allow enough time for each essential activity.
Hardware Interrupt
By using a hardware interrupt driven button reader the calculation could proceed with all button presses captured. With the interrupt running in the background you would not have to alter the calculation function to give up processing time for button reading.
The interrupt routine obviously takes processor time – but you do not have to worry about it while constructing the calculation function.
You do have to keep the interrupt routine small compared to the processing time of the other functions - it’s no good putting tons of operations into the ISR. Interrupt routines should be kept short and sweet so that the main part of the program executes correctly e.g. for lots of interrupts you need the routine to finish quickly ready for the next one.
Hardware Interrupt or polling?
The benefit of the hardware interrupt is that processor time is used efficiently and not wasted polling input ports. The benefit of polling is that it is easy to do.
Another benefit of using interrupts is that in some processors you can use a wake-from-sleep interrupt. This lets the processor go into a low power mode, where only the interrupt hardware is active, which is useful if the system is running on batteries.
Hardware interrupt Common terms
Terms you might hear associated with hardware interrupts are ISR, interrupt mask, non maskable interrupt, an asynchronous event, and interrupt vector and context switching.
Setting up Hardware Interrupt in Microcontroller
There are 3 external interrupts in Atmega 16. They are
- INT0 : PD2, Pin 16
- INT1 : PD3, Pin 17
- INT2 : PB2, Pin 3
There are 3 modes of external Interrupts,
- Low Level: In this mode interrupt occurs whenever it detects a ‘0’ logic at INT pin. To use this, you should put an external pull up resistance to avoid interrupt every time.
- Falling Edge: In this mode interrupt occurs whenever it detects a falling edge that is ‘1’ to ‘0’ logic change at INT pin.
- Rising Edge: In this mode interrupt occurs whenever it detects a falling edge that is ‘0’ to ‘1’ logic change at INT pin.
Functions of Interrupt Service Routine
After generating the code, you can see the following function,
// External Interrupt 0 service routine
interrupt [EXT_INT0] void ext_int0_isr(void)
{
// Place your code here
}
You can place your code in the function, and the code will be executed whenever interrupt occurs.
