Timers

Timer is an 8 bit register that keeps on increasing its value, so one of the basic conditions is the situation when timer register OVERFLOWS i.e. it has counted up to its maximum value (255 for 8 BIT timers) and rolled back to 0. In this situation timer can issue an interrupt and you must write an Interrupt Service Routine (ISR) to handle the event. There are three different timers available in Atmega16 and all the timers work in almost same way. They are TIMER0, TIMER1 and TIMER2.

 

Prescalar

The Prescalar is a mechanism for generating clock for timer by CPU clock.

Atmega has clocks of several frequencies such as 1 MHz, 8 MHz, 12 MHz, 16 MHz (max). The Prescalar is used to divide this clock frequency and produce a clock for TIMER. The Prescalarcan be set to produce the following types of clocks:

No Clock(Timer stop)

No prescaling (clock frequency = CPU frequency) FCPU/8

FCPU/64

FCPU/256

FCPU/1024

External clock

Timer Mode

Timers are usually used in one of the following modes:

Normal

CTC

Fast PWM

Phase correct PWM

Normal Mode

A timer running in normal mode will count up to its maximum value. When it reaches this maximum value, it issues an Overflow interrupt and resets the value of the timer to its original value.

In the above case, you can see that the time period is 256 times the time period of the clock. 255 clock cycles are required to attain the maximum value and one clock cycle to clear the timer value.

So, ftimer = fclock / 256

this mode has its limitations. We are confined to a very small set of values of frequency for the timer. This limitation is overcome by the compare mode.

CTC Mode

(clear timer on compare match)

Compare mode makes use of a register known as the Output Compare Register which stores a value of our choice. The timer continuously compares its current value with the value on the register and when the two values match, the timer resets itself to 0.

In that case, the output pin will remain high for one time period of the timer and will remain low for another time period. 

So,

tout = 2 * ttimer

From the normal case, we can draw an analogy to find out ttimer.

ttimer = tclock * (OCR + 1)

So, finally, we have the frequency as,

fout = fclock / (2 * (OCR + 1) )

Pulse Width Modulation (PWM) Mode

if you want to control the brightness of an LED (or any lamp), or the speed of DC motor, then digital on/off signals will not suffice. This situation is very smartly handled by a technique called as PWM or Pulse Width Modulation.

PWM is the technique used to generate analog signals from a digital device like a MCU.

In AVR microcontrollers, PWM signals are generated by timers. There are two methods by which you can generate PWM from timers:

1. Fast PWM

2. Phase Correct PWM

1.Fast PWM

Now for PWM generation from this count sequence OCR0 (Output Compare Register Zero) is used (Zero because it is for TIMER0 and there are more of these for TIMER1 & TIMER2). 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).

When the TIMER0 is configured for fast PWM mode, then, while the timer is counting up, whenever the value of TIMER0 counter matches the value in the OCR0 register, an output PIN is pulled low (0) and 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 of ATmega

From the figure, you can see that a wave of duty cycle of 64/256 = 25% is produced by setting OCR0 to 64. You can set OCR0 to any value and get a PWM of duty cycle of (OCR0 / 256). When you set it to 0 you get a 0% duty cycle while setting it to 255 will give you a 100% duty cycle output. Thus by varying duty cycle you can get an analog voltage output from the OC0 PIN.

In the inverting mode the value of the OC0 pin is just the reverse of that in the above figure.

2.

Phase Correct PWM Mode

This mode is very similar to the Fast PWM mode except that whenever the value of the timer reaches its maximum value then instead of clearing the value of the timer it simply starts counting down.

 

Overview of timers

SETTING TIMERS IN CVAVR :