CN361 Microprocessor Systems Design - Timer

CN361 Microprocessor Systems DesignTimer

Nawin Somyat

Department of Electrical and Computer EngineeringThammasat University

Outline

Course Contents1 Introduction2 Simple Computer3 Micro Architecture4 Peripherals5 Applications

Outline1 Introduction

Basic ComponentsOperation

2 ATmega168 TimersOverviewOutput Compare PinsRelated RegistersOperation ModesNormal ModeCTC ModeOutput Compare Unit

CN361 Microprocessor Systems Design 2 / 70

Timer

In general, a timer is used to measure a period of an event ofinterrest. The measurement is done in terms of number of clockperiods (clock signal used for the timer).The actual time in seconds has to be converted from number ofclock periods by application.Basic components:

1. timer clock source2. timer/counter registers3. control registers4. optional prescaler

A timer can also be used as a counter for counting number ofevents. So the term timer/counter is often seen associated with atimer.


1. Timer Clock Sources

Clock source can be either:InternalExternal

Most controller will use external clock source, e.g. a crystaloscillator. Some controllers provide both internal and externalclock sources. This clock source is usually used for providingsystem clock (clkosc or clksys).System clock is usually used to derive other clock signals, e.g.

CPU clock (clkcpu)I/O clock (clkI/O)ADC clock (clkADC)

Actual clock signals are implementation dependent.


1. Timer Clock Sources

Example of clock sources in ATmega168.


2. Timer/Counter Registers

Each timer has one or more counter registers to keep track ofthe current count of the timer clock period.Register sizes are implementation dependent.

8-bit timer → 8-bit counter register16-bit timer → 2 x 8-bit counter registers (high and low bytes)

Note that some controllers provide separate 16-bit timer whilesome use 2 existing 8-bit timers to give 16-bit timer.


3. Timer Control Registers

Control registers are used to set various aspects of timer operation,e.g.

operating mode, e.g normal, PWM, compare/capturegeneration of interruptselecting clock sourceselecting edge to trigger capture

There may be more than one control register associated with onetimer.


4. Timer Prescaler

Some controllers provide flexibility in timer clock frequencythrough prescaler.

A prescaler is basically a multiplier/divider circuit.It is used to multiply or divide timer clock source frequency(but usually divide) to genearte desired timer clock frequency.

ExampleA timer using system clock with 64 prescaler usually means thistimer will have frequency fsys/64.

Note that a timer may have separate prescaler, not the sameone as system prescaler.


Basic Timer Operation

Basic operation of a timer:counter register starts with initial value (usually 0)every cycle from clktimer, counter register is incremented (ordecremented) by 1when counter register reaches maximum (or minimum) value,overflow occurs in the next clockcounter restarts from initial value


ATmega168 Timers

There are 3 timers in ATmega168.▶ Timer/Counter0 → 8-bit timer with PWM▶ Timer/Counter1 → 16-bit timer with PWM▶ Timer/Counter2 → 8-bit timer with PWM and Asynchronous

Operation

All timers basically have 2 unitscounter unitoutput compare unit


Clock Sources

For Timer/Counter0 and Timer/Counter1:can be clocked by internal or external clock source.Timer/Counter 0 uses pin PD4/T0 as external clock sourceTimer/Counter1 uses pin PD5/T1 as external clock source

For Timer/Counter2:can be clocked by internal synchronous or externalasynchronous clock source.clkT2 is by default equal to the MCU clock, clkI/O.when the AS2 bit in the ASSR Register is written to logic one,the clock source is taken from PB6/TOSC1 andPB7/TOSC2.


External Clock Source Pins

Note that the Arduino board uses external oscillator for systemclock, so the Timer/Counter2 cannot use external clock sourceon Arduino board.


Output Compare Pins

PD6/OC0A is Timer/Counter0 output compare channel A pinPD5/OC0B is Timer/Counter0 output compare channel B pinPB1/OC1A is Timer/Counter1 output compare channel A pinPB2/OC1B is Timer/Counter1 output compare channel B pinPB3/OC2A is Timer/Counter2 output compare channel A pinPD3/OC2B is Timer/Counter2 output compare channel B pin


Block Diagram - 8-bit Timer/Counter


Block Diagram - 16-bit Timer/Counter


Counter Unit - Timer/Counter0

Signal description (internal signals):count - Increment or decrement TCNT0 by 1.direction - Selects between increment and decrement.clear - Clear TCNT0 (set all bits to zero).clkTn - Timer/Counter clock.top - Signalizes that TCNT0 has reached maximum value.bottom - Signalizes that TCNT0 has reached minimum value(zero).


Output Compare Unit - Timer/Counter0


Related Registers

Summary of registers related to timers in ATmega168:

Register Name NoteTCCRnA Timer/Countern Control Register ATCCRnB Timer/Countern Control Register BTCCRnC1 Timer/Countern Control Register CTCNTn Timer/Countern Register TCNT1H, TCNT1LOCRnA Output Compare Register n A OCR1AH, OCR1ALOCRnB Output Compare Register n B OCR1BH, OCR1BLICRn2 Input Capture Register n ICR1H, ICR1LTIMSKn Timer/Countern Interrupt Mask RegisterTIFRn Timer/Countern Interrupt Flag RegisterASSRn3 Asynchronous Status Register n ASSR2GTCCR General Timer/Counter Register

1Only in Timer/Counter12Only in Timer/Counter13Only in Timer/Counter2


Related Registers

8-bit Timer:Timer0 Timer2 NameTCCR0A TCCR2A Timer/Counter Control Register ATCCR0B TCCR2B Timer/Counter Control Register BTCNT0 TCNT2 Timer/Counter RegisterOCR0A OCR2A Output Compare Register AOCR0B OCR2B Output Compare Register BTIMSK0 TIMSK2 Timer/Counter Interrupt Mask RegisterTIFR0 TIFR2 Timer/Counter Interrupt Flag Register

16-bit Timer:Timer1 Name NoteTCCR1A Timer/Counter Control Register ATCCR1B Timer/Counter Control Register BTCCR1C Timer/Counter Control Register CTCNT1 Timer/Counter Register TCNT1H, TCNT1LOCR1A Output Compare Register A OCR1AH, OCR1ALOCR1B Output Compare Register B OCR1BH, OCR1BLICR1 Input Capture Register ICR1H, ICR1LTIMSK1 Timer/Counter Interrupt Mask RegisterTIFR1 Timer/Counter Interrupt Flag Register


Operation Modes

Each timer provides 4 modes of operation:1 Normal mode.2 Clear Timer on Compare Match (CTC) mode.3 Fast PWM mode (single slope PWM).4 Phase Correct PWM mode (double slope PWM).

Two sets of flags in control registers set operation mode.Waveform Generation Mode (WGM) - control countingsequence.Compare Output Mode (COM) - control inversion of outputwith PWM or output level with compare match.


Definitions of Counter Register Values

Datasheet refers to some values for the counter registers (TCNTn):

Symbol Description 8-bit timer 16-bit timer

BOTTOM minimum possible value 0x00 0x0000MAX maximum possible value 0xFF

(decimal 255)0xFFFF(decimal 65535)

TOP highest value that the counterregister will count up to,actual values is dependent onthe mode of operation


Normal Mode

Operation:counter register always counts up (incrementing)no counter clear is performedcounter register counts up to MAX and restarts fromBOTTOMthe Timer Overflow Flag (TOVn) will be set in the same timerclock cycle as the TCNTn becomes zeroan interrupt can be generated with TOVn set


Normal Mode - Timing Diagram for 8-bit Timer

No prescaling:

Prescaling 8 (fclk_I/O/8):


Normal Mode - Timing Diagram for 16-bit Timer

No prescaling:



Normal Mode - Setting Up

General steps:disable interrupt (may not be necessary with default settings)set the WGMn (Waveform Generation Mode) flags forNormal modeset the CSn (Clock Select) flags to select clocksource/prescalerchoose other starting value instead of BOTTOM, ifappropriate, and number of required interrupts for the desiredtime durationset the TOIEn flag to enable timer overflow interruptset the I flag (in SREG) to enable global interrupt


Timer/Counter2 - Registers


Timer/Counter2 - Waveform Generation Mode

Mode WGM22 WGM21 WGM20 Timer/CounterMode of Operation

TOP Update ofOCR2x at

TOV2 FlagSet on

0 0 0 0 Normal 0xFF Immediate MAX1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM2 0 1 0 CTC OCR2A Immediate MAX3 0 1 1 Fast PWM 0xFF BOTTOM MAX4 1 0 0 Reserved – – –5 1 0 1 PWM, Phase Correct OCR2A TOP BOTTOM6 1 1 0 Reserved – – –7 1 1 1 Fast PWM OCR2A BOTTOM TOP

Notes: 1. MAX = 0xFF2. BOTTOM = 0x00


Timer/Counter2 - Clock Source/Prescaler


Timer/Counter2 - Clock Select

CS22 CS21 CS20 Description0 0 0 No clock source (Timer/Counter stopped).0 0 1 clkT2S/1 (No prescaling)0 1 0 clkT2S/8 (From prescaler)0 1 1 clkT2S/32 (From prescaler)1 0 0 clkT2S/64 (From prescaler)1 0 1 clkT2S/128 (From prescaler)1 1 0 clkT2S/256 (From prescaler)1 1 1 clkT2S/1024 (From prescaler)


Example 8.1: Timer/Counter2 - Normal Mode

1 #include <avr/io.h>2 #include <avr/interrupt.h>34 // TIMER2 overflow interrupt service routine5 ISR(TIMER2_OVF_vect) {6 // toggle PB07 PORTB ^= 1<<PB0;8 }9


Example 8.1: Timer/Counter2 - Normal Mode (Cont. 1)

10 int main(void) {11 // output digital pin 8 (PB0)12 DDRB |= 1<<PB0;1314 // select internal I/O clock (default setting)15 ASSR &= ~( 1<<AS2 );1617 // Normal mode ==> WGM22:0 = 0b00018 TCCR2B &= ~( 1<<WGM22 );19 TCCR2A &= ~( 1<<WGM21 | 1<<WGM20 );2021 // prescaler 64 ==> CS22:0 = 0b10022 TCCR2B |= 1<< CS22;23 TCCR2B &= ~( 1<<CS21 | 1<<CS20 );2425 // enable TIMER2 overflow interrupt26 TIMSK2 |= 1<<TOIE2;2728 // enable global interrupt29 sei(); // SREG |= 1<<7;30 while(1) { }31 }


Example 8.1: Timer/Counter2 - Normal Mode (Cont. 2)Working out the frequency of signal at PB0.System clock frequency: fsys = 16 MHz (fixed on Arduino board)Timer2 with prescaler = 64

frequency: fT2 = fsys/64 −→ fT2 = 16 MHz/64 = 250000 Hzperiod: TT2 = 1/fT2 −→ TT2 = 1/250000 Hz = 4 us

In this example, for Timer2 in Normal mode:start count: TCNT2 = 0 (0x00)end count: TCNT2 = 255 (0xFF)1 interrupt takes: 256 x TT2 (counting: 0 → 255 → 0 )1 period of signal at PB0 takes: 2 interrupts = 2 x 256 x TT2

Frequency of:

interrupt: finterrupt = 1

256 x 4 us = 976.5625 Hz

signal at PB0: fat PB0 = 1

2 x 256 x 4 us = 488.28125 Hz



1 #include <avr/io.h>2 #include <avr/interrupt.h>34 // starting count for counter register5 // this will give 250T of Timer2 for 1 interrupt6 int reload_count = 6;78 // TIMER2 overflow interrupt service routine9 ISR(TIMER2_OVF_vect) {

10 // reload counter register11 TCNT2 = reload_count;1213 // toggle PB014 PORTB ^= 1<<PB0;15 }16



17 int main(void) {18 // output digital pin 8 (PB0)19 DDRB |= 1<<PB0;2021 // select internal I/O clock (default setting)22 ASSR &= ~( 1<<AS2 );2324 // Normal mode ==> WGM22:0 = 0b00025 TCCR2B &= ~( 1<<WGM22 );26 TCCR2A &= ~( 1<<WGM21 | 1<<WGM20 );2728 // prescaler 64 ==> CS22:0 = 0b10029 TCCR2B |= 1<< CS22;30 TCCR2B &= ~( 1<<CS21 | 1<<CS20 );3132 // enable TIMER2 overflow interrupt33 TIMSK2 |= 1<<TOIE2;3435 // enable global interrupt36 sei(); // SREG |= 1<<7;37 while(1) { }38 }




In this example, for Timer2 in Normal mode:start count: TCNT2 = 6 (0x06)end count: TCNT2 = 255 (0xFF)1 interrupt takes: 250 x TT2 (counting: 6 → 255 → 0 )1 period of signal at PB0 takes: 2 interrupts = 2 x 250 x TT2

Frequency of:

interrupt: finterrupt = 1

250 x 4 us = 1 kHz


2 x 250 x 4 us = 500 Hz


Timer With Long Period

If an application requires a long period that is not possible in asingle interrupt, you can keep count of the number of interruptsrequired to get the desired period.For example, consider Timer2 with maximum prescaler:

system clock frequency: 16 MHz (fixed on Arduino board)maximum prescaler: 1024Timer2 lowest frequency: 16 MHz/1024 = 15625 HzTimer2 longest period: 1/15625 = 64 us1 interrupt takes: 256 x 64 us (counting: 0 → 255 → 0 )

So, for Timer2, if require a period longer than 16.384 ms, you maykeep count of the number of interrupts in your ISR.or use other timer with higher resolution, i.e. use 16-bit timer


Example 8.3: Timer/Counter2 - Normal Mode withLong Period

Program: Toggle the LED every 1 second.

In this case:desired period of 1 s is longer than 16.384 ms (for Timer2)we will keep count of the number of interrupts



For Timer2:system clock frequency: 16 MHz (fixed on Arduino board)choose prescaler: 256frequency: fT2 = fsys/256 −→ fT2 = 16 MHz/256 = 62500 Hzperiod: TT2 = 1/fT2 −→ TT2 = 1/62500 Hz = 16 uschoose 1 interrupt for every: 100 x TT2 (counting: 156 → 255 → 0)

number of interrupts required for 1 S: 1 S100 x 16 us = 625



1 #include <avr/io.h>2 #include <avr/interrupt.h>34 // starting count for counter register5 // this will give 100T of Timer2 for 1 interrupt6 int reload_count = 156;78 // keep count of the number of interrupts9 int interrupt_count = 0;

1011 // TIMER2 overflow interrupt service routine12 ISR(TIMER2_OVF_vect) {13 // reload counter register14 TCNT2 = reload_count;1516 interrupt_count++;17 if (interrupt_count == 625) {18 interrupt_count = 0;19 // toggle PB020 PORTB ^= 1<<PB0;21 }22 }


Example 8.3: Timer/Counter2 - Normal Mode withLong Period (Cont. 1)

24 int main(void) {25 // output digital pin 8 (PB0)26 DDRB |= 1<<PB0;2728 // select internal I/O clock (default setting)29 ASSR &= ~( 1<<AS2 );3031 // Normal mode ==> WGM22:0 = 0b00032 TCCR2B &= ~( 1<<WGM22 );33 TCCR2A &= ~( 1<<WGM21 | 1<<WGM20 );3435 // prescaler 256 ==> CS22:0 = 0b11036 TCCR2B |= (1<< CS22 | 1<<CS21);37 TCCR2B &= ~( 1<<CS20 );3839 // enable TIMER2 overflow interrupt40 TIMSK2 |= 1<<TOIE2;4142 // enable global interrupt43 sei(); // SREG |= 1<<7;44 while(1) { }45 }


Example 8.3: Timer/Counter2 - Normal Mode withLong Period (Cont. 2)

Working out the frequency of signal at PB0.System clock frequency: fsys = 16 MHz (fixed on Arduino board)Timer2 with prescaler = 256


In this example, for Timer2 in Normal mode:start count: TCNT2 = 6 (0x06)end count: TCNT2 = 255 (0xFF)1 interrupt takes: 100 x TT2 (counting: 156 → 255 → 0 )1 period of signal at PB0 takes: 2 x 625 interrupts

Frequency of:interrupt: finterrupt = 1

100 x 16 us = 625 Hzsignal at PB0: fat PB0 = 1

2 x 625 x 100 x 16 us = 0.5 HzCN361 Microprocessor Systems Design 45 / 70

Timer/Counter1 - Registers


Timer/Counter1 - Waveform Generation Mode

Mode WGM13 WGM12 WGM11 WGM10 Timer/CounterMode of Operation

TOP Update ofOCR1x at

TOV1 FlagSet on

0 0 0 0 0 Normal 0xFFFF Immediate MAX1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM3 0 0 1 1 PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM4 0 1 0 0 CTC OCR1A Immediate MAX5 0 1 0 1 Fast PWM, 8-bit 0x00FF BOTTOM TOP6 0 1 1 0 Fast PWM, 9-bit 0x01FF BOTTOM TOP7 0 1 1 1 Fast PWM, 10-bit 0x03FF BOTTOM TOP8 1 0 0 0 PWM, Phase and Frequency

CorrectICR1 BOTTOM BOTTOM

9 1 0 0 1 PWM, Phase and FrequencyCorrect

OCR1A BOTTOM BOTTOM

10 1 0 1 0 PWM, Phase Correct ICR1 TOP BOTTOM11 1 0 1 1 PWM, Phase Correct OCR1A TOP BOTTOM12 1 1 0 0 CTC ICR1 Immediate MAX13 1 1 0 1 Reserved – – –14 1 1 1 0 Fast PWM ICR1 BOTTOM TOP15 1 1 1 1 Fast PWM OCR1A BOTTOM TOP

Notes: 1. MAX = 0xFFFF2. BOTTOM = 0x0000


Timer/Counter1 - Clock Source/Prescaler

Note that Timer1 shares prescaler with Timer0.


Timer/Counter1 - Clock Select

CS12 CS11 CS10 Description0 0 0 No clock source (Timer/Counter stopped).0 0 1 clkT2S/1 (No prescaling)0 1 0 clkT2S/8 (From prescaler)0 1 1 clkT2S/64 (From prescaler)1 0 0 clkT2S/256 (From prescaler)1 0 1 clkT2S/1024 (From prescaler)1 1 0 External clock source on T1 pin. Clock on falling edge.1 1 1 External clock source on T1 pin. Clock on rising edge.



1 #include <avr/io.h>2 #include <avr/interrupt.h>34 // starting count for counter register5 int reload_count = 3036;67 // TIMER1 overflow interrupt service routine8 ISR(TIMER1_OVF_vect) {9 // reload counter register

10 TCNT1 = reload_count;1112 // toggle PB013 PORTB ^= 1<<PB0;14 }15



16 int main(void) {17 // output digital pin 8 (PB0)18 DDRB |= 1<<PB0;1920 // Normal mode ==> WGM13:0 = 0b000021 TCCR1B &= ~( 1<<WGM13 | 1<<WGM12 );22 TCCR1A &= ~( 1<<WGM11 | 1<<WGM10 );2324 // prescaler 256 ==> CS12:0 = 0b10025 TCCR1B |= 1<< CS12;26 TCCR1B &= ~( 1<<CS11 | 1<<CS10 );2728 // enable TIMER1 overflow interrupt29 TIMSK1 |= 1<<TOIE1;3031 // enable global interrupt32 sei(); // SREG |= 1<<7;33 while(1) { }34 }



frequency: fT1 = fsys/256 → fT1 = 16 MHz/256 = 62500 Hzperiod: TT1 = 1/fT1 → TT1 = 1/62500 Hz = 16 us

In this example, for Timer1 in Normal mode:start count: TCNT1 = 3036 (0x0BDC)end count: TCNT1 = 65535 (0xFFFF)1 interrupt takes: 62500 periods (3036 → 65535 → 0 )1 period of signal at PB0 takes: 2 interrupts = 2 x 62500 x TT2

Frequency of signal at PB0:

fat PB0 = 1

2 x 62500 x TT2= 1

2 x 62500 x 16 us = 0.5 Hz


Clear Timer on Compare Match (CTC) Mode

Operation:counter register always counts up (incrementing)counter register counts up TOP value

TOP value is OCR0A in Timer0TOP value is OCR2A in Timer2TOP value is either OCR1A or ICR1A in Timer1

when counter value (TCNTn) matches TOP value, thecounter register (TCNTn) is cleared to zerothe Output Compare Match Flag (OCFnA) will be set in thesame timer clock cycle as the TCNTn becomes zeroan interrupt can be generated with TOVn set


CTC Mode - Timing Diagram for 8-bit Timer


CTC Mode - Setting Up

General steps:disable interrupt (may not be necessary with default settings)set the WGMn (Waveform Generation Mode) flags for CTCmodeset the CSn (Clock Select) flags to select clocksource/prescalerchoose value for TOP (assign the value to either OCRnA orICR1A)set the OCFn flag to enable output compare match interruptset the I flag (in SREG) to enable global interrupt

As in Normal mode, the TOVn Flag is set in the same timer clockcycle that the counter counts from MAX to BOTTOM.


Example 8.5: Timer/Counter2 - CTC Mode

1 #include <avr/io.h>2 #include <avr/interrupt.h>34 // TIMER2 compare match channel A interrupt service routine5 ISR(TIMER2_COMPA_vect) {6 // toggle PB07 PORTB ^= 1<<PB0;8 }9


Example 8.5: Timer/Counter2 - CTC Mode (Cont. 1)

10 int main(void) {11 // output digital pin 8 (PB0)12 DDRB |= 1<<PB0;1314 // select internal I/O clock (default setting)15 ASSR &= ~( 1<<AS2 );1617 // CTC mode ==> WGM22:0 = 0b01018 TCCR2B &= ~( 1<<WGM22 );19 TCCR2A |= 1<<WGM21;20 TCCR2A &= ~( 1<<WGM20 );2122 // prescaler 64 ==> CS22:0 = 0b10023 TCCR2B |= 1<< CS22;24 TCCR2B &= ~( 1<<CS21 | 1<<CS20 );2526 // set output compare register (OCR2A) - interrupt every 100T27 OCR2A = 99;2829 // enable TIMER2 output compare match interrupt30 TIMSK2 |= 1<<OCIE2A;3132 // enable global interrupt33 sei(); // SREG |= 1<<7;34 while(1) { }35 }


Example 8.5: Timer/Counter2 - CTC Mode (Cont. 2)

Working out the frequency of signal at PB0.System clock frequency: fsys = 16 MHz (fixed on Arduino board)Timer1 with prescaler = 64


In this example, for Timer2 in CTC mode:start count: TCNT2 = 0 (0x00)end count: TCNT2 = OCR2A = 99 (0x63)1 interrupt takes 100 periods (0 → 99 → 0 )1 period of signal at PB0 takes 2 interrupts: 2 x 100 x TT2

Frequency of:interrupt: 1

100 x 4 us = 2500 Hz


2 x 100 x 4 us = 1250 HzCN361 Microprocessor Systems Design 61 / 70

Using Output Compare Unit

On compare match between Counter register (TCNTn) andOutput Compare register (OCRnx), the output compare unit cangenerate signal on the Output Compare pin (OCnx).

Each timer controls 2 output compare pins (channel A and B)

Timer/Counter0 controls: PD6/OC0A and PD5/OC0BTimer/Counter1 controls: PB1/OC1A and PB2/OC1BTimer/Counter2 controls: PB3/OC2A and PD3/OC2B


Compare Match Output Unit


Compare Match Output Unit

The Waveform Generator uses the Compare Output Mode flags(COMnx) to define the state of the Output Compare pins (OCnx).

To use the Output Compare pins (OCnx), the DDR bit for theOCnx pin must be set as output before the OCnx value is visibleon the pin.

The Waveform Generator uses the COMnx bits differently innormal, CTC, and PWM modes.


Timer/Counter2 - Compare Output Mode

COM2A1 COM2A0 Description0 0 Normal port operation, OC2A disconnected.0 1 Toggle OC2A on Compare Match1 0 Clear OC2A on Compare Match1 1 Set OC2A on Compare Match

Compare Output Mode, non-PWM Mode

COM2B1 COM2B0 Description0 0 Normal port operation, OC2B disconnected.0 1 Toggle OC2B on Compare Match1 0 Clear OC2B on Compare Match1 1 Set OC2B on Compare Match



Example 8.6: Timer/Counter2 - Use OC2A in CTC

Program: Generate square wave on OC2A with frequency of 1 kHz.

Timer2 with prescaler = 64frequency: fT2 = fsys/64 −→ fT2 = 16 MHz/64 = 250000 Hzperiod: TT2 = 1/fT2 −→ TT2 = 1/250000 Hz = 4 us

Use toggle output on compare match.desired output period: 1/1000 = 1 msfor toggle output mode, should toggle for every 0.5 msrequired number of periods: 0.5 ms / 4 us = 125value for OCR2A: 124 (counting: 0 → 124 → 0)

Note that interrupt will not be used.



1 #include <avr/io.h>2 #include <avr/interrupt.h>34 int main(void) {5 // OC2A pin, digital pin 11 (PB3)6 DDRB |= 1<<PB3;78 // set compare ouput mode to toggle ==> COM2A1:0 = 0b019 TCCR2A &= ~( 1<<COM2A1 );

10 TCCR2A |= 1<<COM2A0;1112 // select internal I/O clock (default setting)13 ASSR &= ~( 1<<AS2 );1415 // CTC mode ==> WGM22:0 = 0b01016 TCCR2B &= ~( 1<<WGM22 );17 TCCR2A |= 1<<WGM21;18 TCCR2A &= ~( 1<<WGM20 );1920 // prescaler 64 ==> CS22:0 = 0b10021 TCCR2B |= 1<< CS22;22 TCCR2B &= ~( 1<<CS21 | 1<<CS20 );2324 // set output compare register (OCR2A)25 OCR2A = 124;26 while(1) { }27 }


Timer/Counter1 - Compare Output Mode

COM1A1/COM1B1 COM1A0/COM1B0 Description0 0 Normal port operation, OC1A/OC1B

disconnected.0 1 Toggle OC1A/OC1B on Compare

Match.1 0 Clear OC1A/OC1B on Compare Match

(Set output to low level).1 1 Set OC1A/OC1B on Compare Match

(Set output to high level).




Program: Generate square wave on OC1A with frequency of 50 Hz.

Timer1 with prescaler = 64frequency: fT1 = fsys/64 −→ fT1 = 16 MHz/64 = 250000 Hzperiod: TT1 = 1/fT1 −→ TT1 = 1/250000 Hz = 4 us

Use toggle output on compare match.desired output period: 1/50 = 20 msfor toggle output mode, should toggle for every 10 msrequired number of periods: 10 ms / 4 us = 2500value for OCR1A: 2499 (counting: 0 → 2499 → 0)

Note that interrupt will not be used.



1 #include <avr/io.h>2 #include <avr/interrupt.h>34 int main(void) {5 // OC1A pin, digital pin 9 (PB1)6 DDRB |= 1<<PB1;7 // set compare ouput mode to toggle ==> COM1A1:0 = 0b018 TCCR1A &= ~( 1<<COM1A1 );9 TCCR1A |= 1<<COM1A0;

10 // CTC mode (TOP is OCR1A) ==> WGM13:0 = 0b010011 TCCR1B &= ~( 1<<WGM13 );12 TCCR1B |= 1<<WGM12;13 TCCR1A &= ~( 1<<WGM11 | 1<<WGM10);14 // prescaler 64 ==> CS12:0 = 0b01115 TCCR1B &= ~( 1<< CS12 );16 TCCR1B |= (1<<CS11 | 1<<CS10 );17 // set output compare register (OCR1A)18 OCR1A = 2499;19 while(1) { }20 }


CN361 Microprocessor Systems Design - Timer

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