troubles with bcm2835 hardware PWM

Question

I try to use hardware PWM on RPI 3b+ (Linux 11 bullseye) by using bcm2835.h (ver. 1.71) with small C code given below

#include <bcm2835.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int main(int argc, char **argv)
{
    if (!bcm2835_init()) return 1;

    bcm2835_gpio_fsel(18, BCM2835_GPIO_FSEL_ALT5);
    bcm2835_pwm_set_clock(2);
    bcm2835_pwm_set_mode(0, 1, 1);
    bcm2835_pwm_set_range(0, 2);
    bcm2835_pwm_set_data(0, 1);
    bcm2835_close();

    return 0;
}

It was compiled, built and run with no errors, but oscilloscope connected to corresponding pin shows that output signal has duty cycle approx ~ 99%, not 50%, as expected. Changing corresponding argument in bcm2835_pwm_set_data() has no effect, duty cycle does not change. Changing arguments in bcm2835_pwm_set_clock() and bcm2835_pwm_set_range(0, 2) result in frequency changing, but duty cycle does not varying! Why it could be?

Answer 1

This is a very old library and little used. See https://raspberrypi.stackexchange.com/a/117592/8697 for comparison of libraries.

AFAIK you need to use sudo to access hardware PWM - it applies to most libraries attempting hardware PWM. It may work if you set suid on the compiled binary.

It is possible with pigpio (using the daemon and socket or pipe interface); WiringPi (deprecated) and my pi-gpio (both require sudo) and in python with Pi.GPIO (also requiring sudo).

For interest the following is one of my test programs using pi-gpio.

/*
This is a hardware PWM test program
It generates 2 PWM outputs @10kHz with differing duty cycles

 cc -Wall -o hwPWMtest -lpi-gpio -lpthread -lm -lcrypt -lrt hwPWMtest.c
 2021-10-16
*/

#include <stdio.h>
#include <string.h>

#include <pi-gpio.h>

#define PWM0 12 // this is physical pin 32
#define PWM1 13 // this is physical pin 33

int main(int argc, char *argv[]) {
  rpi_info info;
  // Pi3 & earlier have 19.2MHz clock
  // The following gives a precise 10kHz signal
  int DIVIDER = 15;
  int RANGE = 128;
  setup();

  get_rpi_info(&info);

  if (strcmp(info.processor, "BCM2711") == 0) {
    // Pi4 has 54MHz clock
    DIVIDER = 45;
    RANGE = 120;
  }

  pwmSetGpio(PWM0);
  pwmSetGpio(PWM1);

  pwmSetMode(PWM_MODE_MS); // use a fixed frequency
  pwmSetClock(DIVIDER);    // gives a precise 10kHz signal

  pwmSetRange(PWM0, RANGE);
  pwmWrite(PWM0, RANGE / 4); // duty cycle of 25%

  pwmSetRange(PWM1, RANGE);
  pwmWrite(PWM1, RANGE / 2); // duty cycle of 50%
    cleanup();
  return 0; // PWM output stays on after exit
}