Pulse-Width Modulation - Experimenting with Raspberry Pi - page 191

The reverse of open is the pwm_close() call. Here the thread is instructed to stop
( stopf=1 ), and if there is a thread running, a join with the thread is performed. The join
causes the caller to block until the thread itself has ended. Then the PWM structure is freed,
completing the close operation.

void
pwm_close(PWM *pwm) {
pwm->stopf = 1;
if ( pwm->thread )
pthread_join(pwm->thread,0);
pwm->thread = 0;
free(pwm);
}

The software PWM signal starts when the ratio is established by a call to pwm_ratio() :

void
pwm_ratio(PWM *pwm,unsigned n,unsigned m) {
pwm->n = n <= m ? n : m;
pwm->m = m;
if ( !pwm->thread )
pthread_create(&pwm->thread,0,soft_pwm,pwm);
else
pwm->chgf = 1;
}

This call establishes the values for N and M. Then if no thread is currently running,
one is created with the thread's ID saved in the PWM structure. If the thread is already
running, we simply point out to the thread that the N
M

values have changed so that it can

adapt to it at the cycle's end.
The function soft_pwm() is the software PWM engine itself. The pthread_create()
call passes the PWM structure into the call as a void *arg , which is used by the function to
access the PWM structure. The entire procedure is an outer and inner loop. The outer loop
runs as long as the stopf flag is zero. Then the floating-point period variables fperiod ,
percent , and ontime are calculated.
From there, the inner loop continues until either the chgf or stopf flag variables
become nonzero. If the stopf becomes nonzero, both loops are exited. Once the thread
function exits, the thread ends. The thread resources are reclaimed in the pwm_close()
call when it joins.

static void *
soft_pw(void *arg) {
PWM *pwm = (PWM *)arg;
double fperiod, percent, ontime;