PWM with pigpio does not trigger alert function regularly

Question

I combine C examples of pigpio for gpioSetAlertFunc and gpioWaveTxSend working with the same pin. My PWM has several pulses, so I expect alert function to print out a message for me several times. Sometimes it happens, but sometimes I run the program, but alert function is not called.

      ```
      int res=gpioWaveTxSend(wave_id, PI_WAVE_MODE_ONE_SHOT);
      ```

...and res gets correct value (no error).

So what might be a reason, that alert function is not called?

I add code snippets to clarify:

      //includes

      static int counter_edge=0;
      void countEdge(int gpio, int level, uint32_t tick)
      {
         counter_edge++;
         printf("GPIO %d became %d at %u\n", gpio, level, tick);
      }

      //the function follows example from pigpio
      int send_pwm(uint32_t pin)
      {
         counter_edge = 0;

         gpioCfgClock(1,0,-1);

         if (gpioInitialise() < 0)
         {
            fprintf(stderr, "pigpio initialisation failed\n");
            return 1;
         }

         //!!! alert function assigned to the pin
         //!!! call countEdge whenever GPIO changes state
         gpioSetAlertFunc(pin, countEdge);

         // Construct and send a 30 microsecond square wave.
         gpioSetMode(pin, PI_OUTPUT);

         const int pulse_size=16;
         gpioPulse_t pulse[pulse_size];
         uint32_t try_send_cnt=0,crc_pwm;

         uint32_t i=0;
         uint32_t interval=10;  //us microseconds
         uint32_t gpio_pin = (1<<pin);
         crc_pwm=0;

         for(i=0;i<pulse_size;i++){
           if(i%2)
           {
             pulse[i].gpioOn = gpio_pin;
             pulse[i].gpioOff = 0;
             pulse[i].usDelay = interval;
           }
           else
           {
             pulse[i].gpioOn = 0;
             pulse[i].gpioOff = gpio_pin;
             pulse[i].usDelay = interval;
           }
         }

         crc_pwm=0;
         for(i=1;i<pulse_size;i++)
           if(pulse[i].gpioOn!=pulse[i-1].gpioOn)
              crc_pwm++;
         // alert function should be triggered crc_pwm times 

         if (wave_id >= 0)
         {
           for(try_send_cnt=0;try_send_cnt<20;try_send_cnt++)
           {
              counter_edge = 0;
              res_pwm=gpioWaveTxSend(wave_id, PI_WAVE_MODE_ONE_SHOT);

              if(counter_edge==crc_pwm)
                  break;
            }

            if(res_pwm==PI_BAD_WAVE_ID||res_pwm==PI_BAD_WAVE_MODE)
              printf("ERROR: failure in PWM\n");

            if(counter_edge!=crc_pwm)
               printf("ERROR: Pwm failed after several attemps (%d           %d)\n",counter_edge,crc_pwm);
            else
               printf("DONE %d %d\n",counter_edge,crc_pwm);
         }
         gpioTerminate();
      }

      //tests in main
      int main(){
         send_pwm(17);
         send_pwm(18);

         return 0;
      }

as a result I observe a message like "GPIO 17 became 1" only sometimes.

Answer 1

From the pigpio website:

Level changes shorter than the sample rate may be missed.

The thread which calls the alert functions is triggered nominally 1000 times per second. The active alert functions will be called once per level change since the last time the thread was activated. i.e. The active alert functions will get all level changes but there will be a latency.

Perhaps you're running your PWM signal at a frequency that is too high. My suggestion is that you start by reducing your PWM frequency with gpioSetPWMfrequency.