ATtiny85 PWM lower frequency than expected

Question

I am new to programming MCUs and am trying to use PWM on an ATtiny85. I have looked around online for various tutorials and have managed to get it working with the code below. My problem is that I expect the PWM frequency to be 8MHz/256 = 31.25kHz, but it is actually 3.9kHz, which would suggest that a pre-scaler value of 8 is being used. I have looked at the datasheet but still can't figure it out.

#define F_CPU 8000000
#include <avr/io.h>

int main(void)
{
  //Table 11-3 Compare Output Mode, Fast PWM mode
  //Table 11-5 Waveform Generation Mode Bit Description
  //COM0A0 - non-inverting
  TCCR0A |= (0<<WGM02)|(1<<WGM01)|(1<<WGM00)|(1<<COM0A1);
  //Table 11-6 Clock Select Bit Description
  TCCR0B |= (0<<CS02)|(0<<CS01)|(1<<CS00); //pre-scale factor = 1

  OCR0A=128; // 128/256 = 0.5 duty cycle

  //make PWM pin output
  DDRB |= (1<<DDB0);

  while(1){}

  return 0;
}

I am programming the MCU using a Raspberry Pi with avrdude and avr-gcc as per this instructable: http://www.instructables.com/id/Programming-the-ATtiny85-from-Raspberry-Pi/

Any help or suggestions would be greatly appreciated. Thanks :)

The line TCCR0B |= (0<<CS02)|(0<<CS01)|(1<<CS00); //pre-scale factor = 1 doesn't do what you seem to think it does. It does set the CS00 bit, but has absolutely no effect on the CS01 and CS02 bits: you are ORing these values into the register (|=), which can only change 0s to 1s, not vice versa. — jasonharper
@jasonharper Thanks for pointing that out. I understand what you are saying, but I can't see why it would cause my problem since the CS01 and CS02 bits should be zero by default. — spacenut1

spacenut1 spacenut1 · Accepted Answer · 2018-02-24T11:13:52

I discovered that by default the fuses on the ATtiny85 are set to divide the 8MHz clock by 8 which accounts for the apparent pre-scale factor of 8 that I encountered. I changed the fuses according to this fuse calculator and it worked perfectly. It is strange that none of the tutorials I read mentioned this, but hopefully my struggles can help someone else who has the same problem.

ATtiny85 PWM lower frequency than expected

1 Answers