Input PWM duty calculator

Reads an input Pulse Width Modulation (PWM) signal on a given input pin and calculates the effective signal duty by returning an integer in the range of [0-100]%.

This project is written as an esp-idf component and can be pulled into your own project (suggest git submodule).

Usage

Example

  // Handle for instance
  input_pwm_handle_t pwm_in;

  // Example configuration to read PWM from GPIO6
  input_pwm_cfg_t cfg = {
      .gpio=GPIO_NUM_6, 
      .edge_type=PWM_INPUT_DOWN_EDGE_ON,  // We are expecting an inverted PWM signal (low is ON)
      .period_us=2100000  // This PWM signal is expected to have a period <2s (really slow PWM) 
  };
  
  // Now create a pwm reader
  ESP_ERROR_CHECK( input_pwm_new(cfg, &pwm_in) );
  
  // Read duty
  uint8_t input_duty;
  ESP_ERROR_CHECK( input_pwm_get_duty(pwm_in, input_duty) );

  ...
  ...


  // Don't forget to clean resources when done
  input_pwm_del(pwm_in);

How does this work?

A selected input pin input_pwm_cfg_t.gpio is configured to generate interrupts on both rising and falling edges. The interrupt handler then simply keeps track of the signal's time in low or high position as configured via input_pwm_cfg_t.edge_type, along with the signal period.

A separate RTOS task is then notified on each period, so a duty can be calculated outside of the interrupt handler (can't do floating point maths in interrupt handling). This is also where input_pwm_cfg_t.period_us comes in, as it acts as a timeout period so both 0% and 100% can be calculated when no interrupts are in these cases generated. Hence, you must select input_pwm_cfg_t.period_us to be greater than the signal's period, and up to the max latency you are willing to leave with, could be seconds depending on your application.