Use Arduino to generate sin wave modulated by gold code - c

Question

I am trying to use Arduino to generate sin wave and gold code is used to determine when the wave will have a phase shift. However, the output is not performed as I expected. Sometimes, it does not occur any phase shift for consequent ten cycles, which should not happen according to our definition of gold code array. Which part of the code could I try to fix the problem?
int gold_code[]={1,-1,-1,-1,-1,-1,-1,1,-1,-1,-1,1,-1,-1,1,1,-1,-1, 1,1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,-1,-1,-1,1,1,-1,-1,1,1,-1,1,-1,1,-1,-1,1,1,1,-1,-1,1,1,1,1,-1,1,1,-1, -1, 1, 1,1,-1,-1,1,-1,-1,1,1,1};
void loop()
{
int n = sizeof(gold_code)/sizeof(gold_code[0]);
byte bsin[128];
int it;
unsigned long tm0;
unsigned int tm;
for(int i=0;i<128;i++)
{
bsin[i] = 8 + (int)(0.5 + 7.*sin( (double)i*3.14159265/64.));
}
int count=0;
int count1=0;
Serial.println(n);
tm0 = micros();
while(true)
{
tm = micros() - tm0;
if(tm > 511)
{
tm0 = tm0+512;
tm -= 512;
count++;
//Serial.println(gold_code[count%n]);
}
tm = (tm >> 2) ;
if(gold_code[count%n]==0){
PORTB = bsin[tm];
}
else{
PORTB = 16-bsin[tm];
}
}
}

Answer 1

The variable count eventually overflows and becomes negative. This, in conjunction with the modulo operation is a sign (pun intended) of a disaster waiting to happen.
Use a different method for limiting the value of count to the bounds of your gold_codearray.
You should expect a significant increase in frequency after removing the modulo operation, so you may need to add some pacing to your loop.
The pacing in your loop is wrong. Variable count increments 4 times as fast as your phase counter.
Also, #Edward Karak raises a valid point. To do a proper phase shift, you should add (or subtract) from tm, not from the sin value.
[EDIT] I was not quite happy with the way the phase shift is handled. It just doesn't feel right to advance the gold counter at the same pace as the phase counter. So I added a separate timer for that. Advances in the gold_code array every 8 microseconds for now, but you can change it to whatever you're supposed to have.
as in:
unsigned char tm0 = 0;
unsigned char tm0_gold = 0;
const unsigned char N = sizeof(gold_code) / sizeof(gold_code[0]);
unsigned char phase = 0;
for(;;)
{
// pacing for a stable frequency
unsigned char mic = micros() & 0xFF;
if (mic - tm0_gold >= 8)
{
tm0_gold = mic;
// compute and do the phase shift
if (++count >= N)
count -= N;
if (gold_code[count] > 0) // you have == 0 in your code, but that doesn't make sense.
phase += 16; // I can't make any sense of what you are trying to do,
// so I'll just add 45° of phase for each positive value
// you'll probably want to make your own test here
}
if (mic - tm0 >= 4)
{
tm0 = mic;
// advance the phase. keep within the LUT bounds
if (++phase >= 128)
phase -= 128;
// output
PORTB = bsin[phase];
}
}
For frequency stability, you will want to move the sine generator to a timer interrupt, after debugging. This will free up your loop() to do some extra control.
I don't quite understand why count increments as fast as the phase counter.
You may want to increment count at a slower pace to reach your goal.