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• clock, delay function, timer interrupt, and PWM

  Frank Buss • 10/28/2017 at 04:28 • 3 comments

  The ATtiny9 has an internal oscillator of 8 MHz, which is enabled by default on reset, with a prescaler of 8, resulting in a CPU frequency of 1 MHz. The GNU compiler has a builtin delay function _delay_ms. To use this function, you have to define the CPU frequency with the F_CPU macro and then include util/delay.h. For 1 MHz it can delay up to 4.2 seconds.

  Example for a 1 Hz blinker:

  #define F_CPU 1000000UL
  #include <avr/io.h>
  #include <util/delay.h>
  
  int main(void)
  {
      DDRB |= 1 << PINB2;
      while (1) {
          PORTB &= ~(1 << PINB2);
          _delay_ms(500);
          PORTB |= 1 << PINB2;
          _delay_ms(500);
      }
  }
  

   Internally it uses delay loops, so you can still do other things with the internal timers. 

  The accuracy of the 8 MHz oscillator is guaranteed to be within +/-10% with factory calibration, says the full datasheet, see chapter 17.4.1. Accuracy of Calibrated Internal Oscillator. It can be calibrated to +/-1% manually with the OSCCAL value. Unlike with the PIC microcontrollers, looks like the IDE has no integrated support to do the calibration and to save it in a protected flash location, but there are instructions how to do it, like this one. When I tested it at 20°C room temperature and with 5V power supply, the factory calibration was already better than 1%.
  

  The _delay_ms function uses a delay loop, which has some disadvantages, e.g. if you need an exact time interval, you have to adjust the delay value depending on the rest of the program. 

  A better way to do the delay is by using a timer. The following program uses the 16 bit Timer0 and the capture compare functionality to generate an 1 kHz interrupt. The CPU clock is also changed to 8 MHz, so that you can do more things in the interrupt, if necessary:

  #include <avr/io.h>
  #include <avr/interrupt.h>
  #include <stdint.h>
  
  volatile static uint16_t millis = 0;
  
  // interrupt for compare match
  SIGNAL (TIM0_COMPA_vect)
  {
      if (millis < 500) {
          PORTB &= ~(1 << PINB2);
      } else {
          PORTB |= 1 << PINB2;
      }
      millis++;
      if (millis == 1000) millis = 0;
  }
  
  int main(void)
  {
      // unprotect the CLKPSR register
      CCP = 0xd8;
  
      // set prescaler to 1 for 8 MHz CPU clock
      CLKPSR = 0;
  
      // configure pin B2 as output
      DDRB |= 1 << PINB2;
      
      // count up to 1000 for 1 kHz interrupts
      OCR0A = 1000;
  
      // clear timer on compare match (CTC) mode
      // clock source clk/8
      TCCR0A = 0;
      TCCR0B = (1 << WGM02) | (1 << CS01);
  
      // enable interrupt on OCR0A match
      TIMSK0 = 1 << OCF0A;
  
      // clear all interrupt flags
      TIFR0 = 0xff;
  
      // global interrupt enable
      sei();              
      
      // the rest is done in the interrupt
      while(1) {
      }
  }
  

  This needs about 3 mA (without the LED connected).

  Finally you can use the PWM output for the blinking LED. For this connect it to PB0 and use this code:

  #include <avr/io.h>
  #include <avr/sleep.h>
  
  int main(void)
  {
      // unprotect the CLKPSR register
      CCP = 0xd8;
  
      // set prescaler to 1 for 8 MHz CPU clock
      CLKPSR = 0;
  
      // configure pin B0 as output
      DDRB |= 1 << DDB0;
  
      // 1 HZ PWM frequency (8 MHz / 1024 / 2 - 1)
      OCR0A = 3905;
  
      // toggle OC0A on compare match
      // clear timer on compare match (CTC) mode
      // clock source clk/1024
      TCCR0A = 1 << COM0A0;
      TCCR0B = (1 << WGM02) | (1 << CS00) | (1 << CS02);
  
      // go to sleep for the CPU
      set_sleep_mode(SLEEP_MODE_IDLE);
      sleep_enable();
      sleep_mode();
  }
  

  Enabling the sleep mode at the end reduces the power consumption to about 1.3 mA (without the LED).

  With a different prescaler this could be used as a simple high resolution, fixed frequency square wave generator. For example 1 kHz:

  #include <avr/io.h>
  #include <avr/sleep.h>
  
  int main(void)
  {
      // unprotect the CLKPSR register
      CCP = 0xd8;
  
      // set prescaler to 1 for 8 MHz CPU clock
      CLKPSR = 0;
  
      // configure pin B0 as output
      DDRB |= 1 << DDB0;
  
      // 1 kHZ PWM frequency (8 MHz / 2 / 1000 - 1)
      OCR0A = 3999;
  
      // toggle OC0A on compare match
      // clear timer on compare match (CTC) mode
      // clock source clk/1 (no prescaler)
      TCCR0A = 1 << COM0A0;
   TCCR0B = (...

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