How to stop PWM in single-pulse function?

Using Arduino Programming Questions
1

Hello, I try to create a single pulse function in a "fire and forget" fashion. It's purpose is to control a bi-stable relay.

The code below intends to create a single pulse (duration 50ms on GPIO02 in this example). The pulse width is fine, however the output produces two pulses before it stops. How to resolve?

#define SingleShotOutput 	2 
#define PWMchannel 		1

void IRAM_ATTR pwmstop()
{
  ledcWrite(PWMchannel, 0);
}

void setup() 
{
  ledcAttachPin(SingleShotOutput, PWMchannel);
  pinMode(SingleShotOutput, OUTPUT);
  ledcSetup(PWMchannel, 10, 8); // 10 Hz PWM (100ms), 8-bit resolution
  attachInterrupt(digitalPinToInterrupt(SingleShotOutput), pwmstop, FALLING);
}

void loop() 
{
  ledcWrite(PWMchannel, 128); //50% duty cycle to achieve 50ms on-time
  delay(2000); // Wait a long time to restart the single pulse again
}
2

Which controller (and libraries) are you using?

3

It's a ESP32 Dev module, and i'm just using the code as listed above, no additional library's.

4

On an AVR I'd suspect buffered update of the compare register. For the ESP you can check the data sheet or ask your question in an ESP forum.

5 
#include "sdkconfig.h" // used for log printing
#include "esp_system.h"
#include "freertos/FreeRTOS.h" //freeRTOS items to be used
#include "freertos/task.h"
////
#define evtDoSinglePulse     ( 1 << 0 ) // declare an event
EventGroupHandle_t eg; // variable for the event group handle
// 
esp_timer_handle_t oneshot_timer; //veriable to store the hardware timer handle
//
void IRAM_ATTR oneshot_timer_callback( void* arg )
{
  BaseType_t xHigherPriorityTaskWoken;
  xEventGroupSetBitsFromISR( eg, evtDoSinglePulse, &xHigherPriorityTaskWoken );
} //void IRAM_ATTR oneshot_timer_callback( void* arg )
////
void setup() 
{
  eg = xEventGroupCreate(); // get an event group handle
  //
  gpio_config_t io_cfg = {}; // initialize the gpio configuration structure
  io_cfg.mode = GPIO_MODE_OUTPUT; // set gpio mode
  io_cfg.pin_bit_mask = ( (1ULL << GPIO_NUM_2) ); //bit mask of the pins to set
  gpio_config(&io_cfg); // configure the gpio based upon the parameters as set in the configuration structure
  gpio_set_level( GPIO_NUM_2, LOW);
  // https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/esp_timer.html?highlight=hardware%20timer High Resoultion Timer API
  esp_timer_create_args_t oneshot_timer_args = {}; // initialize High Resoulition Timer (HRT) configuration structure
  oneshot_timer_args.callback = &oneshot_timer_callback; // configure for callback, name of callback function
  esp_timer_create( &oneshot_timer_args, &oneshot_timer ); // assign configuration to the HRT, receive timer handle
//
  xTaskCreatePinnedToCore( fDoTheSInglePulseThing, "fDoTheSInglePulseThing", 5000,  NULL, 5, NULL, 1 );
}
////
void fDoTheSInglePulseThing( void * parameter )
{
  TickType_t xLastWakeTime = xTaskGetTickCount();
  const TickType_t xFrequency = 2000; //delay for mS
  for (;;)
  {
    gpio_set_level( GPIO_NUM_2, HIGH );
    esp_timer_start_once( oneshot_timer, 50000 ); // trigger one shot timer for a 50000us timeout, 50ms
    xEventGroupWaitBits (eg, evtDoSinglePulse, pdTRUE, pdTRUE, portMAX_DELAY ); // event will be triggered by the timer expiring, wait here for the 50000uS
    gpio_set_level( GPIO_NUM_2, LOW );
    xLastWakeTime = xTaskGetTickCount();
    vTaskDelayUntil( &xLastWakeTime, xFrequency );
  }
  vTaskDelete( NULL );
}// end void fDoTheSInglePulseThing

////
void loop() {}

Not tested.
Is one way to do the thing with a ESP32.

I use the HRT, High Resolution Timer, to cause a delay, non-blocking, of 50000us and a task that repeats once every 2000ms.

50000us is a short time to see, so I did not include any PWM code to ignite the LED, just use GPIO High/Low.

6 
void
pulse (
    int  pin)
{
    digitalWrite (pin, HIGH);
    delay (50);
    digitalWrite (pin, LOW);
}
7

Idahowalker & Greg: thanks for your suggestions. However, my search is to find a solution without the use of a timer or delay function as I want to use it in a complex pattern of multiple relays to be operated (which may take 500ms to execute and I do not want the loop to wait for that).

My code is basically suitable, however the extra pulse seems to be a library bug. Considering the 50ms between the two pulses I cannot believe it is related to latency.

8

Whilst experimenting I discovered the following: I changed the line in the interrupt handler from ledcWrite(PWMchannel, 0); into ledcWrite(PWMchannel, 65) and noticed that the duty cycle also changes after two pulses. That brings me to the idea that the code behind ledcwrite is actually a hidden TASK that needs time to schedule and execute. Could this hypothesis be true?

9

if two pulses occur.

just as a quick test did you try

attachInterrupt(digitalPinToInterrupt(SingleShotOutput), pwmstop, RISING);

?

executing a few lines with a timer-interrupt is finished in less than **0,**1 milliseconds.

Another approach would be to setup a timerinterrupt that checks a flag-variable.
if a latency of up to 50 milliseconds is OK this would be simply
if flag is set to true => IOn-pin HIGH
set flag false
on next call flag is false => set IO-pin LOW

at a timer-interrupt-frequency of 20 Hz the pulse would be 50 ms long
on first call of timer-interrupt set IO-pin HIGH
on second call set IO-pin LOW
==> 50 ms pulse.
inside loop do

generatePulse = true;

and you are done

inside the timer-ISR

if (generatePulse) { // wenever flag is true
  generatePulse = false; // reset flag to false
  digitalWrite(myPulsePin,HIGH);
}
else { // whenever flag is false
  digitalWrite(myPulsePin,LOW); 
}

If a latency of 50 ms is too long
you could setup a much higher frequency and then store a timestampe when createPulse is true
and you would have to check for elapsed time >= 50 ms

checking for elapsed time is completely different from a delay()

delay() waits until time is over
an if-condition like

if (millis() - StartOfPulse >= 50)
is executed very fast and very often.
this checking is repeated inside the timer-ISR until condition becomes true

best regards Stefan

10

I am delighted to report that the issue is solved. A very recent update of the ESP32 library solved my issue. So, the code at the top of this topic may serve as an example of a very easy and straightforward one-shot function. Thanks for your support!

11

I have to update my previous post. The solution was not the library update (i tried that as well) but to remove the line "pinMode(SingleShotOutput, OUTPUT);"

12

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