why does my debouncer not work? - c

Question

I'm trying to write a debouncer that will only return a valid argument (>0) if it has been debounced (-1 four bouncing).
I've come up with this so far but it always returns -1, why is that I'm wondering:
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#define BOUNCETIME 500000000 //(500ms)
#define SetTime(x) clock_gettime(CLOCK_REALTIME, (x)); // set time
static struct timespec tset;
struct timespec tnow;
int DeBounce(unsigned int arg)
{
static int val = -1;
long long nsec = 0;
if (val < 0) {
val = arg;
SetTime(&tset);
return arg;
} else {
SetTime(&tnow);
if (tnow.tv_nsec < tset.tv_nsec)
nsec = tnow.tv_nsec + 1000000000;
else
nsec = tnow.tv_nsec;
if (tnow.tv_nsec - tset.tv_nsec > BOUNCETIME) {
printf("arg okay\n");
val = -1;
return arg;
}
else
printf("bounce, ignore!\n");
return -1;
}
}
int main (void)
{
printf("#1 %d\n",DeBounce(0));
usleep(1);
printf("#2 %d\n",DeBounce(1));
usleep(200);
printf("#3 %d\n",DeBounce(1));
sleep(1);
printf("#4 %d\n",DeBounce(1));
}
the ouput I get is:
$ ./debounce
#1 0
bounce, ignore!
#2 -1
bounce, ignore!
#3 -1
bounce, ignore!
#4 -1
$

Answer 1

usleep(600); is 600 microseconds. But your debounce period is 500 milliseconds.
Furthermore tnow.tv_nsec - tset.tv_nsec is not correct as tv_nsec is not the full time value but only the number of nanoseconds past the second. The correct way to calculate elapsed time in nanoseconds is something like this:
(tnow.tv_sec * 1.0e-9 + tnow.tv_nsec) - (tset.tv_sec * 1.0e-9 + tset.tv_nsec)