session6_timerrevision&pwm

7/30/2019 Session6_TimerRevision&PWM

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AVR Course

Ashraf Darwish

Bassam Tarek


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6-Aug-112

Session 6

Timers revision& PWM


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6-Aug-113

Timers revision

We have 3 timers/counters in Atmega 16

2 8-Bit timer/counter Timer/counter0 &Timer/counter2

can count from 0x00 to 0xFF

1 16-Bit timer/counter Timer/counter1


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Modes

Normal mode CTC mode

From 0x00 (or initialvalue set by user) to

0xFF

Overflow flag(TOV0,TOV1,TOV2) is

set when timer/counterreach to maximumvalue 0xFF (timer0 &timer2) or 0xFFFF(timer1).

From 0x00 (or initial valueset by user) to certain value

also set by user.

Output compare matchflag (OCF0, OCF1,OCF2) isset when timer reach the

compare match value


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Modes

Normal mode CTC mode

Timer overflow interruptcan be enabled when

TOIE0 bit=1 (TIMSKregister)

Timer compare matchinterrupt can be enabled

when OCIE0 bit =1(TIMSK register)

We can choose theoutput of the(OC0,OC1,OC2) PINs to be(logic1, logic0, toggle ) atcompare match


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Registers

1- TCNT0: Timer/Counter0

This register holds the value of timer/counter.We can use this register to:

read the value of the timer at any time.

Put an initial value for the timer to start from iteither in normal mode or CTC mode.


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Registers

2- OCR0: output compare register 0

Used only in CTC mode.

Holds the value of compare match ( the endvalue of the timer/counter in CTC mode)


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Registers

3- TCCR0: Timer/counter control register 0

To adjust:The mode (Normal,CTC,PWM,Phase correction PWM)

The prescaler value (1, 8, 64, 256,1024)

Timer output (OC0,OC1,OC2) in case of CTC mode


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Registers

4- TIFR: Timer Interrupt Flag Register 0

Holds:Timer overflow interrupt flag ( TOV0, TOV1, TOV2 ) in case ofnormal mode flag is set when timer reach0xff

Timer Output compare match Flag ( OCF0, OCF1,OCF2)in case ofCTC mode flag is set when timer reach thecompare match value


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Registers

5- TIMSK: Timer Interrupt mask register

To: enable or disable timer interrupts.

if we want to enable timer overflow interrupt,

TOIE0=1

If we want to enable compare match interrupt,OCIE0=1


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Code wizard

Over flow interrupt enable

Compare match interrupt

enable

Timer initial value

Compare match value


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Code wizard

Clock value of the timer after

prescaling


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Code wizard

Mode of operation


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Pulse Width Modulation(PWM)


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PWMWhat is PWM ?

PWM represents Pulse Width Modulation we can

also call it DAC (Digital To Analog Converter).

PWM is a very efficient way to provideintermediate amounts of electrical powerbetween fully and fully off.


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6-Aug-1117

When we have a motor (DC motor) and we want to make

control on it.

we can use a variable resistance connected in series

with the motor to control the current . But it is inefficient

way because of losses.

Another way is to use a variable power supply. But it is

expensive way.

PWM is one of the most efficient way used to switch

between fully and fully off to make a good control on DCmotor.


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Square wave and duty cycle

Any square wave has its duty cycle which can beconstant along the time or variable.

Duty cycle is the percentage between the period

that the wave is high and the periodic time.


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To generate PWM signal we need:1- Timer

2- Comparator


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As mentioned before, the AVR Timers has 4modes:

1- Normal mode.2- CTC mode.3- Fast PWM mode.4- Phase correction mode.

We will talk about third and fourth modes ( PWMmodes )


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6-Aug-1121

Fast PWM mode:

Timer counts from

bottom to themaximum then restartsto bottom(Single slope operation)

Phase correction mode:

The timer counts frombottom to maximum andthen counts from maximum tobottom (Dual slopeoperation).

PWM


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The basic idea of PWM is if we change the value of

OCR (output compare register).

In CTC mode the value of OCR is constant


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Applications:

1-power controlwe can use PWM to control the delivered power to aload.

2-Voltage regulationSwitching voltage to load with appropriate dutycycle.

3-Audio effectsPWM sometimes used in audio effects.


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Code Wizard

Choose clock value

Fast PWM mode

Select the output shape


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Thank you

session6_timerrevision&pwm

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