Test firmware for the Atmega328P to communicate with the WS2812B, in line with its relative datasheet Version 5. Two chunks of data are sent, one per pixel, considering the overhead of double buffering of the PWM module.

ws2812b_protocol_pwm_test.c

/*
 * main.c
 * Author : Enrico
 * 
 *
 */ 

#define  F_CPU 16000000UL

#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <avr/io.h>
#include <avr/delay.h>
#include <avr/sfr_defs.h>
#include <stdio.h>

#define BIT_HIGH 242
#define BIT_LOW 249

uint32_t color_data = 0x00005888;
uint32_t buff_data;
uint8_t rx_dummy;
uint8_t  serial_data[55]; // just some bytes more to debug boundaries
volatile  bit_counter = 0;

ISR(TIMER0_OVF_vect)
{

	TCCR0B &= ~((1 << CS00) | (1 << CS01) | (1 << CS02)); // stop timer
	TCNT0 = 0; // reset timer 
	OCR0B = serial_data[bit_counter++]; // update dc here to keep last bit ending with value low
	if (bit_counter <= 49) // compensate for first cycle at 255. 48 cycles in total for 2 chunks, 
						   // +2 for initial low value and final double buffering which introduce latency of one ISR
	{
		TCCR0B |= 1; // continue transfer
	}
	// after 50 pulses pwm pin is low because the pwm cycle is restored to initial value and with no compare match since timer value is 0, hence pin is still low
}

int main(void)
{
	uint16_t i, j;
	uint8_t temp = 0;
	//for now just fixed in a certain way (start at 0x0000)
	temp = MCUCR;
	/* Enable change of Interrupt Vectors */
	MCUCR = temp|(1<<IVCE);
	/* Move interrupts to 0x0002 */
	MCUCR = temp & (~(1<<IVSEL));
	
	// Enable data pin, initial low
	DDRD |= (1 << PIND5);
	PORTD &= ~(1 << PORTD5);
	
	// Fast PWM, no prescaler, invert and output low with max duty cycle
	// Is very important that the PWM will end the cycle with the pwm pin low and start with the pwm pin low, to assert the LED reset
	TCCR0A |= ((1 << WGM00) | (1 << WGM01) | (1 << COM0B1) | (1 << COM0B0));
	TCCR0B |= (1 << WGM02);
	OCR0B = 255; // duty cycle, start low
	// update in ISR after timer times out
	TIMSK0 |= (1 << TOIE0);
	OCR0A = 255; // period
	
	sei();

	// prepare data from the color_data
	// buff so is possible to add more chunks and preserve the shifted data 
	buff_data = color_data;

	for (i = 0; i < 24; i++)
	{
		if (buff_data & 0x01)
		{
			serial_data[i] = BIT_HIGH;
		}
		else
		{
			serial_data[i] = BIT_LOW;	
		}
		buff_data = buff_data >> 1;
	}

	// second chunk to be shifted just to probe if the signal is correctly output from the LED
	buff_data = 0x00505000;
	for (; i < 49; i++) // the additional 49th bit will not be shifted out, but used to put the compare output low and triggering the reset on the WS2812
	{
		if (buff_data & 0x01)
		{
			serial_data[i] = BIT_HIGH;
		}
		else
		{
			serial_data[i] = BIT_LOW;	
		}
		buff_data = buff_data >> 1;
	}
	
	color_data = 0x00800000;
	
	TCCR0B |= 1;
	
    while (1) 
    {
		if (bit_counter > 49)
		{
			OCR0B = 255;
			_delay_ms(50);
			bit_counter = 0;
			buff_data = color_data;
			for (i = 0; i < 24; i++) 
			{
				if (buff_data & 0x01)
				{
					serial_data[i] = BIT_HIGH;
				}
				else
				{
					serial_data[i] = BIT_LOW;	
				}
				buff_data = buff_data >> 1;
			}
			buff_data = color_data >> 1;
			for (; i < 49; i++) 
			{
				if (buff_data & 0x01)
				{
					serial_data[i] = BIT_HIGH;
				}
				else
				{
					serial_data[i] = BIT_LOW;	
				}
				buff_data = buff_data >> 1;
			}
			serial_data[i-1] = 255;
			color_data = color_data >> 1;
			if (color_data == 0)
			{
				color_data = 0x00800000;
			}
			
			TCCR0B |= 1;
		}
    }
}