interrupt pin of attiny85 unstable

Question

I'm trying to learn how to program a MC, so maybe I didn't know much about a MC. I tried to test interrupt pin of ATtiny85 with this simple code

#include <avr/io.h>
#include <avr/interrupt.h>

void setup()
{
  GIMSK = 0b00100000;
  PCMSK = 0b00000001;
  sei();
  DDRB = 0b00010000;
}

void loop()
{
  PORTB = 0b00010000;
  _delay_ms(500);
  PORTB = 0b00000000;
  _delay_ms(500);
}

ISR(PCINT0_vect)
{
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);
  PORTB = 0b00010000;
  _delay_ms(100);
  PORTB = 0b00000000;
  _delay_ms(100);

}

This is just a blink code with an interrupt to make the LED blink faster 10 times. My question is why sometimes the LED blink faster many times with the interrupt pin is not connected to anything (open)? Didn't it have internal pull up or should I use external pull up in my circuit?

Thank you

Answer 1

More than an answer, these are some comments and suggestions on your program, that I hope will help you write cleaner code:

1. Since you seem to be avoiding using the Arduino core library, you may as well write your own main() instead of relying on the one from the core. This way you have a plain C program that can be compiled either within or outside of the Arduino framework.

2. Instead of opaque constants, like 0b00100000, use the bit names. These names are defined in the datasheet and are provided by the avr-libc. They are normally used with the _BV() macro, which expands to (1<<(x)).

3. For those of us who do not have the full datasheet in our head, a comment or two stating what each bit does is helpful.

4. To change only one bit from a port, use the syntax PORTB |= ...; or PORTB &= .... The compiler is smart enough to turn this into single assembly instructions, namely sbi and cbi.

5. Rather than repeating ten times the same lines, use a loop.

Applying these suggestions, your program would like like;

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

ISR(PCINT0_vect)
{
    for (int i = 0; i < 10; i++) {
        PORTB |= _BV(PB4);
        _delay_ms(100);
        PORTB &= ~_BV(PB4);
        _delay_ms(100);
    }
}

int main(void)
{
    PORTB = _BV(PB0);     // internal pullup on PB0
    GIMSK = _BV(PCIE);    // enable pin change interrupt
    PCMSK = _BV(PCINT0);  // sense pin change on PCINT0 = PB0
    DDRB  = _BV(PB4);     // pin PB4 as output
    for (;;) {
        PORTB |= _BV(PB4);
        _delay_ms(500);
        PORTB &= ~_BV(PB4);
        _delay_ms(500);
    }
}

Answer 2

The internal pullup is only activated, if you activate it in your code. To so, you have to write a 1 to the corresponding bit in the PORTB register, while the DDRB register is set for the pin to be an input.

So you the interrupt gets triggered randomly, because the pin was floating.

Since you are currently always write to the whole PORTB, you have to change the last bit to 1 on every PORTB write. Or you can write a 1 to it in setup() and only change the one output bit in PORTB by using

PORTB ^= 0b00010000;

This will toggle only your output pin.