Introduction to Microcontrollers

Introduction to MicrocontrollersElectronics Club, IIT Delhi16th Feb, 2014

Circuits with Discrete Components

Whats the big idea?Lots of functionality packed into a single chip.

Multipurpose Programmable ICs: Microcontrollers.

Micro-ControllersSome popular ones: Atmel AVR, PIC, 8051, MSP430

Well use AVR ATmega8 for now.

Another way to look at a uCTill now we saw the uC as a configurable IC.But actually, it is a miniature computer, found in many devices around us!

Generally speaking, if a device has buttons and a digital display, chances are it also has a programmable microcontroller brain.uCs are used in almost all control applications.

What do they contain?

ATmega8 Pin Diagram

Barebones Circuit for ATmega8Power Pins: Vcc, AVcc, GND

Reset Pin

16 MHz crystal oscillator (X1)

22uF caps C1 & C2

Hardware Programmer: USBaspBurning the code also needs a hardware programmer, which forms the interface between the laptop and the uC. Well use USBasp programmer.

Programming an MCUWell use Win-AVR (avr-gcc + avrdude) to program the ATmega8 MCU (Others like AVR studio & CV-AVR can also be used).Write a C program .c fileCompile the program with AVR-gcc Executable and Linkable Format (.elf file)Copy specific sections of .elf file (Linking) Intel HEX format (.hex file)Burn the HEX file to ATmega8 using AVRdude.

Code Blinking an LED

Compiling Codeavr-gcc -mmcu=atmega8 -Wall -Os -o blink.elf blink.c

-mmcu, tells the compiler which AVR microcontroller the code is being compiled for.-Wall turns on all reasonable compiler warnings which will help make sure you're writing good code.-Os is the optimization flag which tells the compiler to optimize the code for efficient space utilization.-o argument specifies blink.elf as the output filename.Finally blink.c is the input filename.

Generating .ELF fileavr-objcopy -j .text -j .data -O ihex blink.elf blink.hex

-j specifies Memory Sections to copy from the ELF file. It is used twice, once to copy the .text section and once for the .data sections. The .text section contains the machine instructions which make up the program. The .data section contains various static data used in the program.-O ihex option specifies Intel HEX as the output format.blink.elf is passed as the input file and blink.hex is specified as the output file.

Burn the code to MCUavrdude -p m328p -c usbasp -e -U flash:w:blink.hex

-p specifies the AVR microcontroller part number being programmed.-c specifies the AVR programmer you are using.-e erases the chip before writing the new contents.-U flash:w:blink.hex performs a memory operation: flash is the type of memory, w is for a "write" operation, and blink.hex is the name of the file to write. In other words, "write blink.hex to flash memory".

Writing Code General Format

GPIO General Purpose Input OutputWe need to set registers each having 8 (or 16) bits.

DDRx: Data Direction Register for Port x.Used to specify whether the port is input or output.

PORTx: PORT register for Port x.Used to write digital values to pins of port x.

PINx: PIN register for Port x.Used to read digital values at pins of port x.

DDRx RegisterDDRx Data Direction Register for PORT x

Writing 1 makes the pin Output, 0 makes the pin Input.Example: Set PB0 as output.DDRB |= 0x01;DDRB |= 0b00000001;

PORTx RegisterPORTx Data Register for PORT x

Configures the value at a pin, if it is an output.Example:Produce 5 V at pin PB5: PORTB |= 0b00100000;Produce 0 V at pin PB3: PORTB &= ob11110111;

PINx RegisterPINx Input Pins Address for PORT x

Reads the value at a pin, if it is an input pin.Example:Check if pin PB5 is 1: if(PINB &= 0x20) { // Do something }

Bit Operations in CBitwise AND:Eg. 0b00001101 & 0b00000111 = 0b00000101Bitwise OR:Eg. 0b00001001 | 0b00000111 = 0b00001111

Bitwise AND and Bitwise OR are used as mask operators to change only specific bits in an 8-bit value, without disturbing all other bits:PORTB |= 0b00001000 (Will set ONLY the 4th bit as 1)PORTB &= ~0b00001000 (Will set ONLY the 4th bit as 0)

Bit Operations in CEach bit of a register has a special name._BV(bitname) will generate the required binary number.Eg: _BV(PB3) means 0b00001000i.e. 4th bit of PORTB registerSo to set 4th bit of PORTB as:1 => PORTB |= _BV(PB3)0 => PORTB &= ~_BV(PB3)

Writing the blink LED program

Connecting a SwitchIn this configuration, the 10kOhm resistor is called a Pull-down resistor. If it is not used, the input voltage when the switch is open is floating (indeterminate).

Switch closed: 5 V Switch open: 0 V

Program to use a switch

Timers

TimersA Timer is usually an 8-bit or 16-bit register.Values starts from 0 and goes up to 255 (or 65535).Timer value increases by 1, after each clock period.When the timer reaches its maximum value, in the next cycle, its value becomes 0 again and the process repeats itself.

Advantages of TimersThe timer frequency can be factors of the base frequency of the MCU.Timers are independent of the CPU. Dont need CPU processing time.CPU interacts with Timers through Interrupts.

Timer ModesTimers work in three modes: NormalClear Timer on Compare (CTC) and Fast PWM

Lots to explore, but we will learn about Fast PWM mode.

PWM (Pulse Width Modulation) Simple method of obtaining analog output of any value between 0 and 5V. Desired output is x% of 5V. If Ton = x% then average value is x% of 5V.

PWM mode

PWM code

Lots more to Explore!!!Try it on your own in the upcoming workshop.(Sunday, 23rd Feb, 2014: II-401)

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