Timers

Timer Module
Eman Aboelatta

Copyright © 2012 Embedded Systems
Committee

Atmega16Timer Modules:
ONE

TWO
8-bit Timer/Counters with
Separate Prescalers and
Compare Modes

16-bit Timer/Counter with
Separate Prescaler,
Compare Mode, and Capture
Mode

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Atmega32Timer Modules:
The timer of the AVR can be monitor three Events:
Timer Overflow.
Compare Match.
Input Capture(in timer1)
Also, Timer use in PWM Generation.

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Timer Overflow
 timer overflow means that the counter has counted up to its maximum
A
value and is reset to zero in the next timer clock cycle.
The resolution of the timer determines the maximum value of that timer.

The timer overflow event causes the Timer Overflow Flag (TOVx) to be set
in the Timer Interrupt Flag Register (TIFR).

There are two timers with 8-bit resolution and one timer with 16-bit
resolution on the ATmega16
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Compare Match




In cases where it is not sufficient to monitor a timer overflow.
the compare match interrupt can be used.



The Output Compare Register (OCRx) can be loaded with a value [0..
MaxVal] which the timer will be checked against every timer cycle.



When the timer reaches the compare value, the corresponding Output
Compare Flag (OCFx) in the TIFR register is set.



The Timer can be configured to clear the count register to “0” on a
compare match.

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Input Capture

The AVR has an input pin to trigger the input capture event.

A signal change at this pin causes the timer value to be read and saved in
the Input Capture Register (ICRx).

 the same time the Input Capture Flag (ICFx) in the TIFR will be set.
At

This is useful to measure the width of external pulses.

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Timer / Counter0


Timer0 is an 8 bit timer/counter which can count from 0 to 0xFF.



In the timer mode this peripheral uses an internal clock signal.



in the counter mode an external signal on PORTB.0.



the timer can be operated either in the polling mode or in the interrupt
mode.

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Timer / Counter0 „cont
Used registers:

• Timer registers:


TCCR0 (Timer/Counter 0 Control Register)



TCNT0 (Timer/Counter 0 Value)

• Interrupt registers:


TIFR (Timer Interrupt Flag Register).



TIMSK (Timer Interrupt Mask Register)



SREG (I bit - General Interrupt Enable)

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 Define module ON/OFF
 Define module T/C
 Prescaler

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TCNT0 (Timer / Counter 0 Register):
The initial value of the counter is set by writing it

.

Output Compare Register –OCR0

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TIMSK (Timer / Counter Interrupt Mask)

TOIE0: Timer Overflow Interrupt Enable
OCIE0: Output Compare Match Interrupt Enable

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TIFR (Timer / Counter Interrupt Flag Register)

TOV0: Timer/Counter0 Overflow Flag
OCF0: Output Compare Flag 0

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Initializations example :
// Timer/Counter 0 initialization with clock = oscillator / 1024
// Timer initial value is 0xf0;
TCCR0=0x05;
TCNT0=0xf0;
// Timer/Counter 0 initialization
// Clock source: T0 pin Rising Edge
TCCR0=0x07;

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Wave Generation Mode

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Clear Timer on Compare Match (CTC) Mode (WGM01:0 = 2):

the OCR0 Register is used to manipulate the counter resolution.
In CTC mode the counter is cleared to zero when the counter value (TCNT0)
matches the OCR0.

Note

To be able to generate an output from the PWM, the Output Compare Pin
of Timer (OCn) is set up as output.
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Note
that the Data Direction Register (DDR) bit corresponding to the OC0 pin
must be set in order to enable the output driver.

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PWM


The PWM is used to generate a pulse train with varying duty-cycle.



An analog filter can be used to shape the digital PWM output to obtain
an analog signal such as a sine wave.



In the AVR, the timer/counters are used to generate PWM signals.



To change the PWM base frequency, the timer clock frequency and top
counter value is changed. Faster clock and/or lower top value will
increase the PWM base frequency, or timer overflow frequency.

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Fast PWM Mode


The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3)
provides a high frequency.



PWM waveform generation option.



The fast PWM differs from the other PWM option by its single-slope
operation.



The counter counts from BOTTOM to MAX then restarts from
BOTTOM.

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TIMER0 for PWM generation


8bit counter counting from 0-255 and the goes to 0 and so on.



We can store any value between 0-255 in OCR0,



say we store 64 in OCR0 then it would appear in the graph as follows
(the RED line).

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

When the TIMER0 is configured for fast PWM mode,



while up counting whenever the value of TIMER0 counter (TCNT0 register) matches
OCR0 register an output PIN is pulled low (0)



when counting sequence begin again from 0 it is SET again (pulled high=VCC).



This PIN is named OC0 and you can find it in the PIN configuration

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.


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

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0 pin.



Setting the COM01:0 bits to 2 will produce a non-inverted PWM .



Inverted PWM output can be generated by setting the COM01:0 to 3.

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Phase Correct PWM mode


The phase correct PWM mode (WGM01:0 = 1) provides a high
resolution phase correct PWM



waveform generation option.



The phase correct PWM mode is based on a dual slope operation.



The counter counts repeatedly from BOTTOM to MAX and then from
MAX to BOTTOM.

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

The dual-slope operation has lower maximum operation frequency than
single slope operation.



However, due to the symmetric feature of the dual-slope PWM modes,
these modes are preferred for motor control applications

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.



The PWM resolution for the phase correct PWM mode is fixed to eight
bits.



In phase correct PWM mode the counter is incremented until the
counter value matches MAX. When the counter reaches MAX, it
changes the count direction.



The TCNT0 value will be equal to MAX for one timer clock cycle

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Note

To be able to generate an output from the PWM, the Output Compare Pin
of Timer (OCn) is set up as output.
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Initializations example :
// Timer/Counter 0 initialization
// Clock source: System Clock / 1024
// Mode: Non-Inverted Fast PWM
TCCR0=0x6D;
TCNT0=0x00;
OCR0=0x00;
TIMSK=0x02;
OCR0 = x; // x is a variable which change the duty cycle.

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References
Atmega16 datasheet
http://www.avrfreaks.net/

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info@escommittee.net

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Timers

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