AVR Timer programming Revision as of 17:23, 21 March 2016 by Sandeep (Talk | contribs) (diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Basics

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

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.

Timer/Counter 0

TCNT0
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:

1. The Frequency of the Clock Source with CS02, CS01, CS00 bits.
2. 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)

Timer Counter Control Register 0

TCCR0
D7	D6	D5	D4	D3	D2	D1	D0
FOC0	WGM00	COM01	COM00	WGM01	CS02	CS01	CS00

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

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.

Timer/Counter 0 Flag Register

TIFR
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 every 100ms. To accomplish this, we'll do the following steps.

What is the Max delay Timer 0 overflow generates? Okay, lets calculate. The Explore Ultra AVR dev board comes with a 16MHz on board crystal and the fuse bits are set appropriately. If we use the highest pre-scalar of 1024, calculation shows it can generate a delay of 16milli seconds every time timer zero overflows.

$$Ftimer = CPU Frequency/Prescalar $$ $$Ftimer = 16MHz/1024 = 15.625KHz $$ $$Ttick = 1/ 15.625K = 64 \mu seconds$$ $$Ttotal = 64\mu s X 255 = 16ms$$

Of-course 16ms is not enough, so the next obvious question is:
How many times should the timer overflow to generate a delay of approximately 100msec?

$$ OverFlowCount = 100ms/16ms = 6.25 ≈ 6 $$

Now let's write a simple program which will toggle a port pin (PB0) after the timer 0 overflows 6 times.

1. Load TCNT0 with 0x00
2. 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.
3. Monitor the TOV0 flag in the TIFR0 register to check if the timer has over-flowed, keep a timerOverFlowCount.
4. If timerOverFlowCount >= 6, toggle the led on PD4 and reset the count

Timer 1

Timer/Counter 1

TCNT1H								TCNT1L
D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0

Timer Counter Control Register 1A

TCCR1A
D7	D6	D5	D4	D3	D2	D1	D0
COM1A1	COM1A0	COM1B1	COM1B0	FOC1A	FOC1B	WGM11	WGM10

Timer Counter Control Register 1B

TCCR1B
D7	D6	D5	D4	D3	D2	D1	D0
ICNC1	ICES1	-	WGM13	WGM12	CS12	CS11	CS10

Timer/Counter 1 Flag Register

TIFR
D7	D6	D5	D4	D3	D2	D1	D0
OCF2	TOV2	ICF1	OCF1A	OCF1B	TOV1	OCF0	TOV0