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Timers and counters

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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Reply #15 on Sat 27 Jun 2015 01:12 AM (UTC)
Message
Example of generating 8 MHz signal


After a few similar queries on the forum, I post below a small sketch whose sole purpose is to output 8 MHz clock on one of the pins (the pin varies by board type). This assumes you have a 16 MHz clock. The timer toggles the pin on every clock cycle, and as one unit of "frequency" is an on/off sequence, you get half the frequency on the pin.

You could get a lower frequency by changing OCR1A below. For example, if it was 2 it would toggle at 8 MHz, which means the output frequency would be 4 MHz.


#ifdef __AVR_ATmega2560__
  const byte CLOCKOUT = 11;  // Mega 2560
#else
  const byte CLOCKOUT = 9;   // Uno, Duemilanove, etc.
#endif

void setup ()
  {
  // set up 8 MHz timer on CLOCKOUT (OC1A)
  pinMode (CLOCKOUT, OUTPUT); 
  // set up Timer 1
  TCCR1A = bit (COM1A0);  // toggle OC1A on Compare Match
  TCCR1B = bit (WGM12) | bit (CS10);   // CTC, no prescaling
  OCR1A =  0;       // output every cycle
  }  // end of setup

void loop ()
  {
  // whatever 
  }  // end of loop


Since this is entirely hardware-driven you can be doing other things in your main code (other than using Timer 1, of course).

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Reply #16 on Sat 11 Jul 2015 09:44 PM (UTC)

Amended on Sat 11 Jul 2015 09:45 PM (UTC) by Nick Gammon

Message
Example of timing an interval without interrupts


Sometimes you need to time things when interrupts are off (eg. because you are driving Neopixels, which turn off interrupts because of hardware timing requirements.)

The code below uses Timer 1 to time an interval up to 8.388 seconds. It users a prescaler of 1024, which means each "count" of timer 1 is 1/16e6 * 1024 of a second (0.000064 seconds, or 64 µS). Since it can count up to 65536 we can time 0.000064 * 65536 = 4.194304 seconds. Then the timer overflows, but we can test the "overflow flag" and know the overflow occurred. That lets us time up to 8.388608 seconds before we miss the fact that there was a second overflow.


void startTimer1 ()
  {
  // reset Timer 1
  TCCR1A = 0;
  TCCR1B = 0;
  // zero it
  TCNT1 = 0; 
  TIFR1 |= bit (TOV1);  // clear overflow flag
  // start Timer 1
  TCCR1B =  bit (CS10) | bit (CS12);  //  prescaler of 1024
  }  // end of startTimer1
 
unsigned long getTimer1Reading ()
  {
  unsigned long elapsed = TCNT1;
  if (TIFR1 &  bit (TOV1))
     elapsed += 65536;
  return elapsed;   
  }  // end of getTimer1Reading
 
void setup ()
  {
  Serial.begin (115200);
  Serial.println ();

  }  // end of setup

void loop ()
  {
  Serial.println ("Starting ...");
  Serial.flush ();

  startTimer1 ();
 
  delay (7560);
  float secs = (1.0 / F_CPU * 1024) * getTimer1Reading ();

  Serial.print ("Time taken = ");
  Serial.print (secs);
  Serial.println (" seconds.");

  }  // end of loop


Instead of the delay(7560) in the test, you could have in that place something which turns off interrupts, does something lengthy, and turns them back on again.

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Reply #17 on Fri 07 Aug 2015 02:17 AM (UTC)
Message
Example of doing something jitter-free every x µs



#include <avr/sleep.h>

ISR (TIMER1_COMPA_vect)
  {
  PINB = bit (5); // toggle D13 
  }  // end of TIMER1_COMPA_vect

void setup ()
  {
  pinMode (13, OUTPUT);

  // stop timer 0
  TCCR0A = 0;
  TCCR0B = 0;
  
  // stop timer 1
  TCCR1A = 0;
  TCCR1B = 0;
  
  TCCR1B = bit (WGM12) |  // CTC
           bit (CS10);    // prescaler of 1
          
  // 1e9 / 22050 / 62.5 = 725.6 - round down, then subtract 1
  OCR1A = 724;
  TIMSK1 = bit (OCIE1A);  // interrupt on compare A
  
  set_sleep_mode (SLEEP_MODE_IDLE);  

  }  // end of setup

void loop ()
  {
  sleep_mode ();
  }  // end of loop


The above example was for the Arduino forum, where someone wanted to send something to a DAC every 1/22050 seconds (ie. every 453.5 µs).

The code as posted doesn't actually send anything (it toggles pin 13), but in the ISR is where you would put code to do something more substantial. By sleeping in the main loop we ensure that the ISR is called without jitter, because the processor is idling.

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Reply #18 on Sat 08 Aug 2015 02:47 AM (UTC)

Amended on Sat 08 Aug 2015 05:04 AM (UTC) by Nick Gammon

Message

Below are some charts for working out the bit positions (and names) for the three timers on the Atmega328P (as in the Arduino Uno, Duemilanove, etc.)

The images below are thumbnails, click on them (or RH-click and "save link as") to get a full-sized image.


Creative Commons Licence
These images are licensed under a Creative Commons Attribution 3.0 Australia License.


- Nick Gammon

www.gammon.com.au, www.mushclient.com
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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Reply #19 on Wed 03 Feb 2016 01:05 AM (UTC)
Message
Timer interrupts on the ATtiny85


Following on from a question on Arduino Stack Exchange I developed this short piece of code that calls an interrupt periodically on the ATtiny85.



// ATMEL ATTINY 25/45/85 / ARDUINO
// Pin 1 is /RESET
//
//                  +-\/-+
// Ain0 (D 5) PB5  1|    |8  Vcc
// Ain3 (D 3) PB3  2|    |7  PB2 (D 2) Ain1 
// Ain2 (D 4) PB4  3|    |6  PB1 (D 1) pwm1
//            GND  4|    |5  PB0 (D 0) pwm0
//                  +----+

ISR (TIMER1_COMPA_vect)
  {
  digitalWrite (4, ! digitalRead (4));  //toggle D4 (pin 3 on chip)
  }

void setup() 
 {
  pinMode (4, OUTPUT);  // chip pin 3 

  // Timer 1
  TCCR1 = bit (CTC1);   // Clear Timer/Counter on Compare Match
  TCCR1 |= bit (CS10) | bit (CS13);  // prescaler of 256
  OCR1C = 123;          // what to count to (zero-relative)
  TIMSK = bit (OCIE1A); // interrupt on compare

  }  // end of setup

void loop() 
  { 
  // other code here
  }


You can tweak the time delay by changing both the prescaler and the counter in OCR1C. The prescaler gives you coarse tuning, which gets you into the ballpark of the delay time. The timer then counts up to give you the final delay.

In my case I chose a prescaler of 256 which is or'ing together CS10 and CS13.

The delay is then:


125 ns * 256 * 124 = 3.968 ms


Where 256 is the prescaler and 124 is what we are counting to. This is assuming an 8 MHz clock which gives a clock period of 125 ns.

See the datasheet for what bit settings to use for various prescalers. You can use a prescaler from 1 to 16384, in powers of two (ie. 1, 2, 4, 8, 16, 32 ...).


Flashing more slowly

If you put an LED on pin 3 and want to see the flashing, you need it much slower. For example:


TCCR1 |= bit (CS10) | bit (CS11) | bit (CS12) | bit (CS13);  // prescaler: 16384


That toggles the pin every 254 ms, which is visible.

Warning

On this chip Timer 1 is an 8-bit timer, so you cannot put more than 255 into OCR1C.

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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