Hiii…..friends as we discussed in previous articles
that AVR MCU provides us facility of using some inbuilt advance features, TIMER
is one of then and in this tutorial we are going to discuss about the TIMERs in
AVR. Timer is a very useful and most widely used feature of AVR MCU it will be
used in various applications and projects so one must have knowledge of TIMERs.
Have a look…..
INTRODUCTION
The TIMERs in AVR will be discussed in OVERFLOW
MODE (TIMER0) and OUTPUT COMPARE MODE (TIMER1) in this tutorial we will discuss
OVERFLOW MODE (TIMER0) only.
TIMER…??
A TIMER is nothing but a register . its size may bee
8 bit or 16 bit, which is called resolution of the timer. A 8 bit register can
store numbers 0-255 , similarly a 16 bit register can store numbers 0-65535.
The value of these registers can be increased or decreased without the CPU
intervention, at a rate defined by the user. this rate can be can be equal to
or less than the clock frequency of the CPU. The frequency at which timer
register increases or decreases is known as Timer frequency.
We can use the TIMER registers to calculate the
TIMER efficiently because the increment of register value by 1 means one clock
has completed. So when the timer OVERFLOWS means the register value reaches to
its maximum , we can inform CPU sending an interrupt. After reaching maximum
value register again resets to 0. In overflow condition we must have to write a
INTRUPPT SERVICE ROUTINE(ISR) to handle the event.
USING TIMER IN
OVERFLOW MODE(TIMER0)
In ATMEGA 16 and 32 , there are three timers are available ,
of which simplest is TIMER0, with 8 bit resolution.
THE PRESCALER
The
Prescaler is a mechanism for generating clock for timer by the CPU clock. As
you know that CPU has a clock source such as a external crystal of internal
oscillator. Normally these have the frequency like 1 MHz,8 MHz, 12 MHz or
16MHz(MAX). The Prescaler is used to divide this clock frequency and produce a
clock for TIMER. The Prescaler can be used to get the following clock for
timer. No Clock (Timer Stop). No Prescaling (Clock = FCPU) FCPU/8 FCPU/64
FCPU/256 FCPU/1024
TIMER0
REGISTERS.
Here is the description of some
register of TIMER0 , which we are going to use-
TCCR0 – TIMER COUNTER CONTROL
REGISTER
This register is used configure the timer.
 |
As you can see there are 8 Bits in
this register each used for certain purpose. For this tutorial I will only
focus on the last three bits CS02 CS01 CS00 They are the CLOCK SELECT bits.
They are used to set up the Prescaler for timer.
TCNT0 – TIMER COUNTER 0
This
register is the main counter in the TIMER0. The timer clock counts only this
register as 1. It means tomer clock will increase the value of this 8 bit
register by 1 every timer clock pulse.
 |
TIMSK- TIMER INTRUPPT MASK REGISTER
This register is used to
activate/deactivate interrupts related with timers. This register controls the
interrupts of all the three timers. The last bit BIT0 Controls the interrupts of TIMER0.
TOIE0 : This
bit when set to “1” enables the OVERFLOW interrupt.
PROGRAMMING
PRESCALER- 1024
CLOCK FREQUENCY- 16 MHz
Clock input of TIMER0 = 16MHz/1024 =
15625 Hz Frequency of Overflow = 15625 /256 = 61.0352 Hz if we increment a
variable “count” every Overflow when “count reach 61” approx one second has
elapse. If count reaches 61 we will
toggle PB0 which is connected to LED and reset “count= 0”.
SETTING
UP THE TIMER0
//
Prescaler = FCPU/1024
TCCR0|=(1<<CS02)|(1<<CS00);
//Enable Overflow
Interrupt Enable
TIMSK|=(1<<TOIE0);
//Initialize Counter
TCNT0=0;
OR
//
Prescaler = FCPU/1024
TCCR0|=TCCR0|00000101;
//Enable Overflow
Interrupt Enable
TIMSK|=TIMSK|00000001;
//Initialize Counter
TCNT0=0;
Now the
timer is set and firing Overflow interrupts at 61.0352 Hz
The
ISR
ISR(TIMER0_OVF_vect)
{
//This is the interrupt service routine for TIMER0 OVERFLOW
Interrupt.
//CPU automatically call this when TIMER0 overflows.
//Increment our variable
count++;
if(count==61)
{
PORTC=~PORTC;
//Invert the Value of PORTC
count=0;
}
}
COPMLETE
SOURCE CODE
/* PROGRAM TO BLINK LE AT 0.5Hz CONNECTED AT PB0 */
#include <avr/io.h>
#include <avr/interrupt.h>
volatile uint8_t
count;
void main()
{
// Prescaler = FCPU/1024
TCCR0|=(1<<CS02)|(1<<CS00);
//Enable Overflow Interrupt Enable
TIMSK|=(1<<TOIE0);
//Initialize Counter
TCNT0=0;
//Initialize our varriable
count=0;
//PB0 as out put
DDRB|=0x01;
//Enable Global Interrupts
sei();
//Infinite loop
while(1);
}
ISR(TIMER0_OVF_vect)
{
//This is the interrupt service routine for
TIMER0 OVERFLOW Interrupt.
//CPU automatically call this when TIMER0
overflows.
//Increment our variable
count++;
if(count==61)
{
PORTB=~PORTB; //Invert the Value of PORTB
count=0;
}
}
Now just compile the source code and burn the hex
file generated in the ATMEGA 16, then implement the circuit as shown below on a
breadboard or on your development board whatever you want to use, and you will
find your LED blinking at a rate of 0.5 Hz.
So we have
studied the basic of TIMERs in AVR in next tutorial we will discuss about TIMER
IN OUTPUT COMPARE MODE(TIMER1) .
I hope you found this article useful in case of any
doubt, query, or critical view just leave a comment.
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