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Tiny Function Generator Sine Wave

7th March 2018

This article describes an update to my Tiny Function Generator program to add a sine wave to the existing seven waveforms it provides:


The Tiny Function Generator has been one of my most popular projects, but several people made the valid comment that it was a pity that it didn't include a sine wave. I realised that this could be added quite simply by providing an array of 256 points, defining one cycle of a sine function.

I could have precalculated the values and included them as constants in the program. However, it seemed more interesting to calculate them in the program.

The program

The program works by first calculating the sine waveform in a 256-byte array, and this is then used by the DDS to generate the sine wave output.

The waveform is stored in the array Sinewave[]:

int8_t Sinewave[256];

Here's the routine CalculateSine() to calculate one cycle of a sine wave:

void CalculateSine () {
  int X=0, Y=8180;
  for (int i=0; i<256; i++) {
    X = X + (Y*4)/163;
    Y = Y - (X*4)/163;
    Sinewave[i] = X>>6;

The for loop uses the Minsky circle algorithm to calculate the 256 points in a sine wave, without needing floating point or trig functions. The value 4/163 is a close approximation to (2*π)/256, where 2*π is the number of radians in a circle, ie one cycle, and 256 is the number of subdivisions of the circle we want.

 An additional Sine() routine then outputs the points from the Timer/Counter0 COMPA interrupt when you select the sine wave:

void Sine () {
  Acc = Acc + Jump;
  OCR1A = Sinewave[Acc>>8] + 128;

The "+ 128" converts the signed 8-bit values in the Sinewave[] array to an unsigned 8-bit number for the OCR1A counter.

There's also a new icon to represent the sine wave.

Other details

The circuit and other details are unchanged from the previous project; see Tiny Function Generator.

Here's the updated program: Tiny Function Generator Sine Wave program.

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