www.open-tager.ru

#include <avr/interrupt.h>
#include <avr/sleep.h>
#include <stdint.h>

typedef uint32_t   u32;
typedef uint16_t   u16;
typedef uint8_t      u8;
typedef int32_t      s32;
typedef int16_t      s16;
typedef int8_t      s8;

#define DACP OCR0A
#define DACN OCR0B

__attribute__((naked)) int main(void)
{
//   CLKPR = 0b10000000; // 16 MHz from the high freq PLL clock
//   CLKPR = 0b00000000;
   // Configure ports

   DDRB  = 0b11110111; // OC0AB are L/R PWM output. Drive unused pins low (helps noise).
   PORTB = 0b00000000; // No activate pull-ups
//   DDRB  = 0b00000011;

   // ADC
   ADMUX  = 0b00010011; // PB3, AVcc, 10 bit, 2.56VRef
   ADCSRA = 0b11001101; // CPU/32 sampling rate; Sampling freq: 16 MHz / 32 / 13 = 39 kHz

   // Power reduction
   PRR   = 0b00001010; // Clock-gate USI and Counter1.
   MCUCR = 0b00100000;


   // PWM
   TCCR0A = 0b10100011; // Fast PWM
   TCCR0B = 0b00000001; // Increment TCNT0 at CPU; Sampling freq: 16MHz / 256 = 64 kHz (= MAX)
   //TIMSK  = 0b10; // Interrupt on overflow

   sei();

   while(1) {
      sleep_cpu();
   }
}

#if 0

inline s16 min(s16 a, s16 b) {
   return (a<b)?a:b;
}

inline s16 max(s16 a, s16 b) {
   return (a>b)?a:b;
}

// 0% duty cycle at idle. Pulses too narrow at low volume.
ISR(ADC_vect) {
   static u8 last_error;

   u16 adc      = ADC;

   s16 val      = (s16)(adc >> 1) - 256;

   u8  error   = adc & 1;
   u8  dither   = error & last_error;

   last_error   = error - dither;

   val += dither;
   val  = min(max(val, -255),255);

   DACN = max(val, 0);
   DACP = max(-val, 0);
}

#else
// 50% duty cycle at idle. Slightly less efficient, but better sounding.
ISR(ADC_vect) {
//ISR(TIMER0_OVF_vect) {
   static u8 last_error;

   u16 adc      = ADC;
   s8 val1      = (u8)(adc >> 2) - 128;
   u8 val2      = (u8)(adc & 0b10) >> 1;
   u8 error   = adc & 1;
   u8 dither   = error & last_error;
   last_error   = error - dither;

   u8 pos = 128 + val1 + val2;
   u8 neg = 128 - val1 - dither;

   DACP = pos;
   DACN = neg;

   if(val1 == 127)
      DACP = 255;

   if(val1 == -128)
      DACN = 255;

}

#endif