Direct digital synthesis (Karplus-Strong plucked string algorithm) on the Atmel ATtiny85 AVR microcontroller using the MCP4725 12-bit DAC over I2C using USI-based TWI as described in Application Note AVR310

attiny85_synth.c

/**
 * Author: Joe Futrelle
 * License: CC0 - Free as in freedom
 */
/**
 * ATtiny85 direct digital synthesis
 * using MCP4725 12-bit I2C DAC
 * TWI is handled using Atmel's USI TWI implementation
 * which is provided as part of Application Note AVR310
 * http://www.atmel.com/Images/Atmel-2561-Using-the-USI-Module-as-a-I2C-Master_AP-Note_AVR310.pdf
 */
/**
 * For details of the MCP4725 TWI protocol see its datasheet at
 * http://ww1.microchip.com/downloads/en/DeviceDoc/22039d.pdf
 * Breakout board from Adafruit recommended for prototyping
 */
#include <avr/io.h>
#include <stdlib.h>
#include "USI_TWI_Master.h"

// DAC address (may vary, see datasheet)
#define DAC_ADDR 0x62
// DAC "write but don't write to EEPROM" command
#define DAC_WRITE 0x40

// Use unsigned 16-bit values for DAC 12-bit values
#define DAC_T uint16_t

// TWI message buffer; only need 4 bytes
uint8_t buf[4];

// call before using dac_out
void dac_init() {
  // prepare to send a bunch of write commands
  buf[0] = DAC_ADDR << 1;
  buf[1] = DAC_WRITE;
}

void dac_out(DAC_T v) {
  // must call dac_init first to populate first two bytes
  // of message buffer
  // construct 2-byte representation of 12-bit value
  buf[2] = v >> 4;
  buf[3] = ((v & 0xF) << 4);
  // now send the write command over USI TWI
  USI_TWI_Start_Transceiver_With_Data(buf, 4);
}

/**
 * Random note player using Karplus-Strong plucked string
 * algorithm
 * http://en.wikipedia.org/wiki/Karplus%E2%80%93Strong_string_synthesis
 */
// using signed sample type
#define SAMPLE_T int16_t
// zero is 0xFFF >> 1 in unsigned representation
#define ZERO 2047

// sample buffer
#define BUF_SIZE 64
SAMPLE_T wav[BUF_SIZE];
// minimum wavelength
#define MIN_WL 5
// noise burst length
#define NOISE_L 24

size_t ks_wptr; // position in wavetable
SAMPLE_T ks_prev; // previous sample

void ks_init(size_t wl) {
  // initialize wavetable to short noise burst
  size_t i;
  for(i = 0; i < wl; i++) {
    if(i < NOISE_L) { // short burst of noise
      wav[i] = (SAMPLE_T)(rand() & 0x0FFF) - ZERO;
    } else {
      wav[i] = 0;
    }
  }
  // initialize wavetable pointer
  ks_wptr = 0;
  // initialize filter
  SAMPLE_T ks_prev = wav[wl-1];
}

SAMPLE_T ks_iter(size_t wl) {
  // retrieve sample value from buffer
  SAMPLE_T v = wav[ks_wptr];
  // now filter the sample
  wav[ks_wptr] = (ks_prev / 2) + (v / 2);
  ks_prev = v;
  // recur
  ks_wptr = (ks_wptr + 1) % wl;
  // return output sample
  return v;
}

int main(void)
{
  // initialize
  USI_TWI_Master_Initialise();
  dac_init();

  for(;;) {
    // silence
    dac_out(ZERO);
    // choose wavelength
    size_t wl = (rand() % (BUF_SIZE - MIN_WL)) + MIN_WL;
    // initialize Karplus-Strong algorithm with this wavelength
    ks_init(wl);
    // choose number of iterations
    uint16_t iter = rand() & 0x0FFF;
    size_t i = 0; // position in note
    for(i = 0; i < iter; i++) {
      dac_out(ks_iter(wl) + ZERO);
    }
  }
  // never reached
  return 0;
}