ATTiny9 blinky

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 = (1 << WGM02) | (1 << CS00);

    // go to sleep for the CPU
    set_sleep_mode(SLEEP_MODE_IDLE);
    sleep_enable();
    sleep_mode();
}