View source for AVR Timer programming ← AVR Timer programming

[[Category:AVR Tutorials]]
=Basics=
[[File:Timer Block Diagram.jpg|center|Timer/Counter Unit]]
Timers come in handy when you want to set some time interval like your alarm. This can be very precise to  a few microseconds. 
Timers/Counters are essential part of any modern MCU. Remember it is the same hardware unit inside the MCU that is used either as Timers or Counter. Timers/counters are an independent unit inside a micro-controller.  They basically run independently of what task CPU is performing. Hence they come in very handy, and are primarily used for the following:
*''' Internal Timer''': As an internal timer the unit, ticks on the oscillator frequency. The oscillator frequency can be directly feed to the timer or it can be pre-scaled. In this mode it used generate precise delays. Or as precise time counting machine.
*'''External Counter''': In this mode the unit is used to count events on a specific external pin on a MCU. 
*'''Pulse width Modulation(PWM) Generator''':  PWM is used in speed control of motors and various other applications.

Atmega32 has 3 timer units, timer 0, timer 1 and timer 2 respectively. Let us start our exploration with timer 0. 
=Timer 0 Basics =
[[File:Timer 0.png|center]]
Timer 0 is a 8 bit timer. It basically means it can count from 0 to 2^8 255. The operation of timer 0 is straight forward. The TCNT0 register hold the timer Count and it is incremented on every timer '''"tick"'''. If the timer is turned on it ticks from 0 to 255 and overflows.  If it does so, a Timer OverFlow Flag(TOV) is set. 

You can as well load a count value in TCNT0 and start the timer from a specific count. Another interesting feature is that a value can be set in the Output Compare Register (OCR0), and whenever TCNT0 reaches that value, the Output Compare Flag (OCF0) flag is Set.


{| style="text-align:center;" class="table table-responsive table-bordered" 
!colspan = '8'|TCNT0
|+Timer/Counter 0
|D7||D6||D5||D4||D3||D2||D1||D0
|}

The configuration of the Timer can be set using the TCCR0 register shown below. With this you can basically select two things:
# The Frequency of the Clock Source with CS02, CS01, CS00 bits.
# The mode of the timer. For the first example we will use it in normal mode where it ticks from zero to the highest value(255)

{| class="table table-responsive table-bordered" style="float:center;"
!colspan = '8'|TCCR0
|+ Timer Counter Control Register 0
|D7||D6||D5||D4||D3||D2||D1||D0
|-
|FOC0||WGM00||COM01||COM00||WGM01||CS02||CS01||CS00
|}


{| class="table table-responsive table-bordered" style="float:right;"
!D2||D1||D0||Clock Source
|-
!CS02||CS01||CS00||Freq
|-
|0||0||0|| No Clock (Stopped)
|-
|0||0||1||Clk
|-
|0||1||0||Clk/8
|-
|0||1||1||Clk/64
|-
|1||0||0||Clk/256
|-
|1||0||1||Clk/1024
|-
|1||1||0||Clk/T0-Falling edge
|-
|1||1||1||Clk/T0-Rising Edge
|}


{| class="table table-responsive table-bordered" style="float:right;"
!D6||D3||PWM
|-
!WGM00||WGM01||Mode
|-
|0||0||Normal
|-
|0||1||CTC (Clear timer on compare match)
|-
|1||0||PWM (Phase correct)
|-
|1||1||Fast PWM
|}

The Timer/counter Interrupt  Flag Register(TIFR) holds the two basic flags we need the TOV and OVF. Other bits correspond to the timer interrupts, which we will look at in another tutorial. 

{| class="table table-responsive table-bordered" style="float:right;"
!colspan = '8'|TIFR
|+Timer/Counter 0 Flag Register
|D7||D6||D5||D4||D3||D2||D1||D0
|-
|OCF2||TOV2||ICF1||OCF1A||OCF1B||TOV1||OCF0||TOV0
|}
=Timer 0 Example=
Let us do a simple example to set the timer 0 in normal mode and toggle an LED when it overflows. To accomplish this, we'll do the following steps.

# Load TCNT0 with 0x00
# Set CS00 and CS02 bits in TCCR0 register. This will start the timer at Clk/1024 speed. We will calculate the tick time in just a moment.
# Monitor the TOV0  flag in the TIFR0 register to check if the timer has over-flowed and toggle the LED status.

==How much delay does it generate?==
Okay, lets calculate.  The Explore Ultra AVR dev board comes with a 16MHz on board crystal and the fuse bits are set appropriately.
<html>
$$Ftimer = 16MHz/1024 = 15.625KHz $$
$$Ttick = 1/ 15.625K = 64 micro seconds$$
$$Ttotal = 64us x 255 = 16mSec$$

</html>

=Timer 1=
=Timer 2=