ADC to PWM using an ATtiny13a for driving a plasma speaker.

ATtiny85-ADC-to-PWM.c

#include <avr/io.h>

int main(void)
{
	// Setup ADC2 as an input, using an internal voltage reference, with left bit alignment in ADCH|ADCL
	ADMUX = (1 << REFS0) | (1 << MUX1) | (1 << ADLAR);
	// Enable the ADC, Start the conversion, Autotrigger, and /16 prescale
	ADCSRA = (1 << ADEN) | (1 << ADSC) | (1 << ADATE) | (1 << ADPS2);

	// Setup fast PWM output on OCB0
	OCR0B = 0x80; //Initialize OCR0B to result in 1/2 duty cycle.
	DDRB = (1 << PB1); // Set the direction of PortB pin 1 (pin 6 on pdip) as output
	// Use OCR0B as the PWM output, run in Fast PWM Mode with a 0xFF top
	TCCR0A = (1 << COM0B1) | (1 << WGM01) | (1 << WGM00);
	TCCR0B = (1 << CS00); // Run at full internal clock speed, no prescaling

	unsigned char floor = 0x08;
	while(1)
	{
		unsigned char adc_input = ADCH;
		if (adc_input > 0xFF - floor)
			OCR0B = 0xFF;
		else
			OCR0B = adc_input + floor;
	}
}