Hi
friends..!! welcome back this time I am here with some discussion on second
part of TIMERS in avr , this time we are
going to discuss about TIMER1. I hope that you have read and understood the
previous posts:
Now
we are aware with the concepts of AVR so we are moving towards TIMER1. TIMER1
is more or less like TIMER0 so without wasting much time we will simply go
through problem statementwe .Once we are done with this, we can proceed to the
CTC and PWM modes of operations in subsequent posts.
Problem Statement
This
time our task is to flash an LED every 2 seconds, i.e. at a frequency of 0.5
Hz. We have an XTAL of 16 MHz.
Using prescaler and interrupt
So
here is the formula.
TIMER COUNT = (REQUIRED DELAY/CLOCK TIME PERIOD ) - 1
Given
that we have a CPU Clock Frequency of 16 MHz. At this frequency, and using a
16-bit timer (MAX = 65535), the maximum delay is 4.096 ms. It’s quite low. Upon
using a prescaler of 8, the timer frequency reduces to 2 MHz, thus giving a
maximum delay of 32.768 ms. Now we need a delay of 2 s. Thus, 2 s ÷ 32.768 ms =
61.035 ≈ 61. This means that the timer should overflow 61 times to give a delay
of approximately 2 s.
Now
it’s time for you to get introduced to the TIMER1 registers (ATMEGA16/32). We
will discuss only those registers and bits which are required as of now. More
will be discussed as and when necessary.
TCCR1B Register
The
Timer/Counter1 Control Register B- TCCR1B Register is as follows.
 |
| TCCR1B Register
|
Right
now, only the highlighted bits concern us. The bit 2:0 – CS12:10 are the
Clock Select Bits of TIMER1. Their selection is as follows.
Clock Select Bits Description
Since
we need a prescaler of 8, we choose the third option (010).
TCNT1 Register
The
Timer/Counter1 - TCNT1 Register is as follows.
 |
| TCNT1 Register
|
It is 16 bits wide since
the TIMER1 is a 16-bit register. TCNT1H represents the HIGH byte whereas
TCNT1L represents the LOW byte. The timer/counter value is stored in
these bytes.
TIMSK Register
The
Timer/Counter Interrupt Mask Register – TIMSK Register is as follows.
 |
| TIMSK Register
|
As
we have discussed earlier, this is a common register for all the timers.
The bits associated with other timers are greyed out. Bits 5:2
correspond to TIMER1. Right now, we are interested in the yellow bit only.
Other bits are related to CTC mode which we will discuss later. Bit 2 –
TOIE1 – Timer/Counter1 Overflow Interrupt Enable bit enables the overflow
interrupt of TIMER1. We enable the overflow interrupt as we are making the
timer overflow 61 times (refer to the methodology section above).
TIFR Register
The
Timer/Counter Interrupt Flag Register – TIFR is as follows.
 |
| TIFR Register
|
Once
again, just like TIMSK, TIFR is also a register common to all the
timers. The greyed out bits correspond to different timers. Only Bits 5:2
are related to TIMER1. Of these, we are interested in Bit 2 – TOV1 – Timer/Counter1
Overflow Flag. This bit is set to ’1′ whenever the timer overflows. It is
cleared (to zero) automatically as soon as the corresponding Interrupt Service
Routine (ISR) is executed. Alternatively, if there is no ISR to execute, we can
clear it by writing ’1′ to it.
Code
Now
its time for coding its too easy like TIMER0, if you have not gone through
TIMER0 please read TIMER0 tutorial first otherwise it will be a bit tough for
you.
/*
* TIMER1.c
*
* Created: 1/28/2013 1:59:29 AM
*
Author: ABHILASH
*/
#include <avr/io.h>
#include <avr/interrupt.h>
// global variable to
count the number of overflows
volatile uint8_t
timer_overflow;
// initialize timer,
interrupt and variable
void timer1_init()
{
// set up timer with prescaler = 8
TCCR1B |= (1 << CS11);
// initialize counter
TCNT1 = 0;
// enable overflow interrupt
TIMSK |= (1 << TOIE1);
// enable global interrupts
sei();
// initialize overflow counter variable
timer_overflow = 0;
}
// TIMER1 overflow
interrupt service routine
// called whenever
TCNT1 overflows
ISR(TIMER1_OVF_vect)
{
// keep a track of number of overflows
timer_overflow++;
// check for number of overflows here itself
// 61 overflows = 2 seconds delay (approx.)
if (timer_overflow >= 61) // NOTE: '>=' used instead of '=='
{
PORTC ^= (1 << 0); // toggles
the led
// no timer reset required here as the timer
// is reset every time it overflows
timer_overflow = 0; // reset
overflow counter
}
}
int main(void)
{
// connect led to pin PC0
DDRC |= (1 << 0);
// initialize timer
timer1_init();
// loop forever
while(1)
{
// do nothing
// comparison is done in the ISR itself
}
}
So
friends it was all about TIMER1 in AVR , in case of any query doubt or critical
view just leave a comment here . in our next tutorial we will discuss about
TIMER2 , till then bye…!!
THNX...:)
