Advanced Features of Atmega8 Micro Controller

8/6/2019 Advanced Features of Atmega8 Micro Controller

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Advanced Features of Atmega8

Microcontroller


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Topics to be covered

Timers/counters

ADC


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TIMERS/COUNTERS


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What are timers?

Timer counts the time sequence or the clock

pulses of the microcontroller.

They are set of registers integrated in most of

the microcontroller.


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Number ofTimers in Atmega8There are three timers/Counters in Atmega8

Timer 0 Timer 1 Timer 2


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Number ofTimers in Atmega8There are three timers/Counters in Atmega8

Timer 0 Timer 1 Timer 2

8-bit 16-bit 8-bit


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Registers ofTimers/Counters

There are four registers to control the

operation & data storage. TCNT

x= Timer Counter Register

TCCRx

= Timer/Counter Control Register

TIMSK= Timer Interrupt Mask Register

TIFR= Timer Interrupt Flag Register


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TIMER/COUNTER 0 REGISTER


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TCNTx

This magical 8 bit register increases its ownvalue with a fix rate. This register dont requireCPU to increase its value. TCNTx keeps thecount value from 0-255, it must be initialized

in program code. When the count valuereaches maximum, it raises a interrupt bit inanother register and reset to 0.


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Timer Frequency and Prescaling

Timer frequency is the rate by which TCNTx

register increases its own value.

A prescalar measures an output clock/

time related to an input clock/time by afractional scale factor.


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CS0, CS1. CS2: Prescalar select inputs.


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TIMER/COUNTER CONTROL REGISTER

The timer control register controls Timer

Frequency, this is achieved by setting the

prescalar value. CS00-CS02 are used to select PRESCALER value

and hence determines the frequency of the timer.


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PRESCALAR

TIMER FREQUENCY =CPU CLOCK FREQUENCY / PRESCALER


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TIMER INTERRUPT MASK REGISTER

Bit 0 or TOIE0 bit of TIMSK is used to enable/disableoverflow interrupt of TIMER0.

Storing 1 at this place will Enable the overflow

interrupt ofTI

MER0. Storing 0 at this place will Disable the overflow

interrupt ofTIMER0.

Other bits are used for other timers.


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OVERFLOWMODE

Once the timer reaches it highest counts and

if it not resets it is said to be in OverflowMode.

For this mode we have to set 3 registers

TCNT0 (To initialize counter)

TCCR0 (To set Prescalar)

TIMSK (To enable overflow interrupt)


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Program for creating 1 sec delay using

OVERFLOW mode#include#includevolatile uint8_t count;main(){int second=0;char buffer[10];TCCR0=0b00000011; //to set the prescalar to64

TIMSK |=(1


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Contdcount=0;sei(); //Enable Global Interruptwhile(1);} //main close

ISR(TIMER0_OVF_vect) // interrupt service routine{If(count==61) // if count reaches 1 sec{second++;sprintf(seconds:%d, second);lcd_puts(buffer); // for printing seconds on LCD

count=0; // reset the count variable}elsecount++;} //ISR function close


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Description of program

If(count==61) // if count reaches 1 sec

61 is decided by clock frequency and the prescalar.Formula for calculating number of interrupts for 1 secondtime duration is :

Timer frequency= CPU clock frequency/Prescalar

No of overflow interrupts= Timer frequency/256

In our case:

Timer frequency=1000000/64= 15625

No of interrupts=15625/256 = 61.035 61


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PULSEWIDTH MODULATION(PWM)


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WHATIS PWM?

In this modulation technique, the width of themodulated signal is varied in accordance withinput.


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Why it is used in Microcontroller? Pulse Width Modulation (PWM) is a method

of getting intermediate voltage between 0V

and 5V for e.g. 3.6V.

Intermediate voltage is achieved by increasing

and decreasing the width of digital pulse.

PWM is defined by Duty Cycle of the pulse.


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DUT

Y CYCLE It is the ratio of active time(ON time) to the

total time period of the clock cycle.

total time period= ON time(TON) + OFF time(TOFF)

For e.g.- If an CPU Clock pulse is ON or HIGH for half

time period of the pulse so duty cycle is 50%


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FORMULA OF DUTY CYCLE% DUTY CYCLE= 100

T

otalT

ime Period=

T

ON +T

OFF


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PWM GENERATION in AVR AVR contains separate hardware.

PWM can be generated by Timer1 in ATmega8microcontroller.

TCNTx, OCR1 A/B, ICR1 registers generatesPWM.


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FAST

PW

M GENERATI

ON USI

NGTIMER1

In this mode TCNTx

register count its value fromBOTTOM to TOP.

Reset to BOTTOM as it reaches TOP to countagain. While counting up when TCNT1 matched with

compare register (OCRxx

) an output pinassociated with output compare register pulled

HIGH. When TCNT1 reset to Bottom that pin pulled

LOW.


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FAST PWM MODE For this mode, we have to set

TCNT1 (Timer Counter Register)

OCR1A (Output Compare Register)

ICR1 (Input Compare Register)

TCCR1B & TCCR1A (Timer/Counter Control

Register) TIMSK (Timer Interrupt Mask Register)


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Program to make PWM of 20ms time

period using FAST PWM mode ofTIMER1#include#include#includeint count =1;Void main(){DDRB=0xFF; //DIRECTION OF PORT B AS OUTPUTTCCR1B=0b00011001; //Bit 0-2 for CLOCK SELECT(NO

PRESCALING) bit 3-4 for fastPWMWAVEFORM GENERATION MODE

TCNT1=0; //for initializing timer1 count

TCCR1A=0b10100010; //bit 0-1 for fast PWMWAVEFORMGENERATION MODE bit 4-5 forCOM1B1:0, bit 6-7 for COM1A1:0

(10 for both)ICR1=20000; // setting upper limit for count

register (Input compare register)


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THANK YOU

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Advanced Features of Atmega8 Micro Controller