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Timer_Interrupt_Notes.ino
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/*
These notes put together by Joel Murphy for Pulse Sensor Amped, 2015
The code that this section is attached to uses a timer interrupt
to sample the Pulse Sensor with consistent and regular timing.
The code is setup to read Pulse Sensor signal at 500Hz (every 2mS).
The reasoning for this can be found here:
http://pulsesensor.com/pages/pulse-sensor-amped-arduino-v1dot1
There are issues with using different timers to control the Pulse Sensor sample rate.
Sometimes, user will need to switch timers for access to other code libraries.
Also, some other hardware may have different timer setup requirements. This page
will cover those different needs and reveal the necessary settings. There are two
part of the code that will be discussed. The interruptSetup() routine, and
the interrupt function call. Depending on your needs, or the Arduino variant that you use,
check below for the correct settings.
******************************************************************************************
ARDUINO UNO, Pro 328-5V/16MHZ, Pro-Mini 328-5V/16MHz (or any board with ATmega328P running at 16MHz)
>> Timer2
Pulse Sensor Arduino UNO uses Timer2 by default.
Use of Timer2 interferes with PWM on pins 3 and 11.
There is also a conflict with the Tone library, so if you want tones, use Timer1 below.
void interruptSetup(){
// Initializes Timer2 to throw an interrupt every 2mS.
TCCR2A = 0x02; // DISABLE PWM ON DIGITAL PINS 3 AND 11, AND GO INTO CTC MODE
TCCR2B = 0x06; // DON'T FORCE COMPARE, 256 PRESCALER
OCR2A = 0X7C; // SET THE TOP OF THE COUNT TO 124 FOR 500Hz SAMPLE RATE
TIMSK2 = 0x02; // ENABLE INTERRUPT ON MATCH BETWEEN TIMER2 AND OCR2A
sei(); // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED
}
use the following interrupt vector with Timer2
ISR(TIMER2_COMPA_vect)
>> Timer1
Use of Timer1 interferes with PWM on pins 9 and 10.
The Servo library also uses Timer1, so if you want servos, use Timer2 above.
void interruptSetup(){
// Initializes Timer1 to throw an interrupt every 2mS.
TCCR1A = 0x00; // DISABLE OUTPUTS AND PWM ON DIGITAL PINS 9 & 10
TCCR1B = 0x11; // GO INTO 'PHASE AND FREQUENCY CORRECT' MODE, NO PRESCALER
TCCR1C = 0x00; // DON'T FORCE COMPARE
TIMSK1 = 0x01; // ENABLE OVERFLOW INTERRUPT (TOIE1)
ICR1 = 16000; // TRIGGER TIMER INTERRUPT EVERY 2mS
sei(); // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED
}
Use the following ISR vector for the Timer1 setup above
ISR(TIMER1_OVF_vect)
>> Timer0
DON'T USE TIMER0! Timer0 is used for counting delay(), millis(), and micros().
Messing with Timer0 is highly unadvised!
******************************************************************************************
ARDUINO Fio, Lilypad, ProMini328-3V/8MHz (or any board with ATmega328P running at 8MHz)
>> Timer2
Pulse Sensor Arduino UNO uses Timer2 by default.
Use of Timer2 interferes with PWM on pins 3 and 11.
There is also a conflict with the Tone library, so if you want tones, use Timer1 below.
void interruptSetup(){
// Initializes Timer2 to throw an interrupt every 2mS.
TCCR2A = 0x02; // DISABLE PWM ON DIGITAL PINS 3 AND 11, AND GO INTO CTC MODE
TCCR2B = 0x05; // DON'T FORCE COMPARE, 128 PRESCALER
OCR2A = 0X7C; // SET THE TOP OF THE COUNT TO 124 FOR 500Hz SAMPLE RATE
TIMSK2 = 0x02; // ENABLE INTERRUPT ON MATCH BETWEEN TIMER2 AND OCR2A
sei(); // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED
}
use the following interrupt vector with Timer2
ISR(TIMER2_COMPA_vect)
>> Timer1
Use of Timer1 interferes with PWM on pins 9 and 10.
The Servo library also uses Timer1, so if you want servos, use Timer2 above.
void interruptSetup(){
// Initializes Timer1 to throw an interrupt every 2mS.
TCCR1A = 0x00; // DISABLE OUTPUTS AND PWM ON DIGITAL PINS 9 & 10
TCCR1B = 0x11; // GO INTO 'PHASE AND FREQUENCY CORRECT' MODE, NO PRESCALER
TCCR1C = 0x00; // DON'T FORCE COMPARE
TIMSK1 = 0x01; // ENABLE OVERFLOW INTERRUPT (TOIE1)
ICR1 = 8000; // TRIGGER TIMER INTERRUPT EVERY 2mS
sei(); // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED
}
Use the following ISR vector for the Timer1 setup above
ISR(TIMER1_OVF_vect)
>> Timer0
DON'T USE TIMER0! Timer0 is used for counting delay(), millis(), and micros().
Messing with Timer0 is highly unadvised!
******************************************************************************************
ARDUINO Leonardo (or any board with ATmega32u4 running at 16MHz)
>> Timer1
Use of Timer1 interferes with PWM on pins 9 and 10.
void interruptSetup(){
TCCR1A = 0x00;
TCCR1B = 0x0C; // prescaler = 256
OCR1A = 0x7C; // count to 124
TIMSK1 = 0x02;
sei();
}
The only other thing you will need is the correct ISR vector in the next step.
ISR(TIMER1_COMPA_vect)
******************************************************************************************
ADAFRUIT Flora, ARDUINO Fio v3 (or any other board with ATmega32u4 running at 8MHz)
>> Timer1
Use of Timer1 interferes with PWM on pins 9 and 10.
void interruptSetup(){
TCCR1A = 0x00;
TCCR1B = 0x0C; // prescaler = 256
OCR1A = 0x3E; // count to 62
TIMSK1 = 0x02;
sei();
}
The only other thing you will need is the correct ISR vector in the next step.
ISR(TIMER1_COMPA_vect)
******************************************************************************************
ADAFRUIT Gemma (or any other board with ATtiny85 running at 8MHz)
NOTE: Gemma does not do serial communication!
Comment out or remove the Serial code in the Arduino sketch!
Timer1
Use of Timer1 breaks PWM output on pin D1
void interruptSetup(){
TCCR1 = 0x88; // Clear Timer on Compare, Set Prescaler to 128 TEST VALUE
GTCCR &= 0x81; // Disable PWM, don't connect pins to events
OCR1C = 0x7C; // Set the top of the count to 124 TEST VALUE
OCR1A = 0x7C; // Set the timer to interrupt after counting to TEST VALUE
bitSet(TIMSK,6); // Enable interrupt on match between TCNT1 and OCR1A
sei(); // Enable global interrupts
}
The only other thing you will need is the correct ISR vector in the next step.
ISR(TIMER1_COMPA_vect)
******************************************************************************************
******************************************************************************************
******************************************************************************************
******************************************************************************************
******************************************************************************************
******************************************************************************************
******************************************************************************************
*/