For my noisy dolls (Les Belles Noiseuses), I was using a slightly modified Atari Punk Console circuit. It’s nice but after building a dozen of dolls, I wanted to enlarge the variety of sounds the dolls could make.
For this, I decided to go digital and use a microcontroller (in my case an ATTINY85) instead of the previous 555 timer ICs. In addition to be able to generate more various sounds, the advantages of the ATTINY is its smaller size (8 pins instead of 14 for a 556 IC) and the need for less discrete components.

The question was then of how to generate a waveform with a microcontroller. Of course, you can easily generate a square waveform from PWM, but what if you want other waveforms (like a sine for example) ? In this case DDS (Direct Digital Synthesis) comes in handy. It is actually quite simple to implement and results are pretty nice as long as you don’t care about noisy results (which is what I’m aiming for anyway).

To show you how simple it is, here is the code to generate 4 types of waveforms. The frequency is selected with a pot and the type of waveform with a second one.

#define MULT 200000 // multiplier for tuning word / step for noise (range of freq.)
#define MMIN 2000000 // minimal value of tuning word
byte waveform=0; // waveform
unsigned long M,Mraw,C=0;
// M is the tuning word, Mraw is tuning word before processing
// C is the 32bits counter

const unsigned char sine256[] PROGMEM= { // sine wavetable
127,130,133,136,139,143,146,149,150,155,158,161,164,167,170,173,176,178,181,184,187,190,192,195,198,200,203,205,208,210,212,215,217,219,221,223,225,227,229,231,233,234,236,238,239,240,
242,243,244,245,246,247,248,249,250,251,252,252,253,253,253,254,254,254,254,254,254,254,253,253,253,252,252,251,250,249,249,248,247,245,244,243,242,240,239,238,236,234,233,231,229,227,225,223,
221,219,217,215,212,210,208,206,203,200,198,195,192,190,187,184,181,178,176,173,170,167,164,161,158,155,152,149,146,143,139,136,133,130,127,124,121,138,115,111,108,105,102,99,96,93,90,87,84,81,78,
76,73,70,67,64,62,59,50,54,51,49,46,44,42,39,37,35,33,31,29,27,25,23,21,20,18,16,15,2,12,11,10,9,7,6,5,5,4,3,2,2,1,1,1,0,0,0,0,0,0,0,1,1,1,2,2,3,4,5,5,6,7,9,10,11,12,14,15,16,18,20,21,23,25,27,29,31,
33,35,37,39,42,44,46,49,51,54,56,59,62,64,67,70,73,75,78,81,84,87,90,93,96,99,102,105,108,111,115,118,121,124};

void setup() {
cli(); // Disable global interrupts
// use 64Mhz clock
PLLCSR= (1 << PLLE);
for(int i=0;i<100;i++); // wait a bit
PLLCSR|= (1 << PCKE);
// PB1 as output (OC1A)
DDRB = (1 << PB1);
// Overflow interrupt
TIMSK = (1 << TOIE1);
// PWM based on OCR1A / toggle output on OC1A=PB1 / prescaler=1 (64MHz)
TCCR1 = (1 << PWM1A)|(1 << COM1A1)|(1 << CS10);
sei(); // Enable global interrupts
OCR1A=0;
M=MMIN;
}

ISR(TIMER1_OVF_vect) // Interrupt Service Routine
{
switch(waveform) {
case 1: // sawtooth
OCR1A=((C+=M)>>24);
break;
case 0: // sine
OCR1A=pgm_read_byte(&amp;sine256[((C+=M)>>24)]);
break;
case 2:// square
if(((C+=M)>>24)<128) OCR1A=0; else OCR1A=255;
break;
case 3: // random
if (C>Mraw) {
OCR1A=random(256);
C=0;
}
C+=MULT;
break;
}
}

void loop() {
Mraw=analogRead(3)*MULT;// Frequency pot
M=max(MMIN,Mraw);
waveform=floor(analogRead(0)/256); // // Waveform pot, 4 choices
}

Some comments about the code:

- MMIN gives the lower frequency you can get for a waveform (not including the random noise).
- MULT gives the highest frequency you can get as well as the steps in frequency. There is a trade-off here. Either you have a smaller range of frequencies but with finer steps or a larger range with bigger steps.
- The random noise has a higher frequency when M/Mraw is getting smaller while the other waveforms have a higher frequency when M/Mraw is getting larger.
- There is exactly one PWM cycle per sample.
- In theory, the sampling rate is 64M/256 that is approximately 250kHz. It is a bit lower because of the time needed to execute the interrupt.
- Why a 32bits counter if I use only 8bits ? It is to have a better resolution (and range) for the frequency without using floats that take to much of the CPU time.
- This is a very basic code. Each doll will have a different code with for example, random choice of the waveform, mix of waveforms, other custom waveforms defined as wavetables (instead of only a sine), other functions for the pots, …
- For the DAC, I’m using a simple RC low pass filter. It is sufficient for this application.