Enable PWM on an ATtiny by programming its registers

1/*
2  Fade for ATtiny85 using only avr-libc
3  Copyright (c) 2022  Hans Peter Müller
4
5  This code recreates Arduino's Fade example on an ATtiny85  using only avr-libc
6  functionality. Without having access to Ardunio's syntactic  sugaring, we need
7  to recreate Arduino's "analogWrite()" function. Under the  hood this function
8  enables Pulse-width-modulation (PWM) by setting the necessary  registers on the
9  ATtiny85. For more information on how PWM works, see these  fantastic
10  tutorials:
11
12  - https://www.re-innovation.co.uk/docs/fast-pwm-on-attiny85/
13  - http://matt16060936.blogspot.com/2012/04/attiny-pwm.html
14
15  You'll also  need Atmel's data sheet to understand how the registers are named
16  and which  functions are controlled by them (see read me document).
17*/
18
19#include <avr/io.h>
20#include  <util/delay.h>
21
22int fadeAmount = 5;
23int brightness = 0;
24
25/*
26  We  enable PWM by setting some registers. This code assumes that a LED is
27  attached  to the lower right pin of the ATtiny85 (pin 0). See schematics.png
28  for further  details on how to setup the circuit.
29
30  DDRB (Data Direction Register Port  B):
31  - DDBO (Data Direction Port B Pin 0) to 1 (output)
32
33  TCCR0A (Timer/Counter  Control Register A):
34  - COM0A1 (Compare Match Output A Mode) to 1 (non-inverting  mode)
35  - WGM0[0,1] (Waveform Generation Mode Register 0 and 1) to 1 (fast PWM)
36
37  TCCR0B (Timer/Counter Control Register B):
38  - CS00 (Clock Select Register  0) to 1 (no prescaling)
39
40  Instead of calling "analogWrite()", we set the  ATtiny's Output Compare
41  Register A (OCR0A) to the current brightness value.  The rest is similar to the
42  original Fade.ino sketch.
43*/
44int main()
45{
46  DDRB = 1 << DDB0;                       
47  TCCR0A = 1 << COM0A1 | 1 << WGM01  | 1 << WGM00;
48  TCCR0B = 1 << CS00;
49
50  while (1) {
51    brightness =  brightness + fadeAmount;
52    if (brightness <= 0 || brightness >= 255) {
53      fadeAmount = -fadeAmount;
54    }
55    OCR0A = brightness;
56    _delay_ms(30);
57  }
58  return 0;
59}