Inverting PWM in nRF52840 pinn

[refer_pin]

Hello,

I want to output in one pin 400kHz and inverse of 400kHz in the other pin. I have somehow managed to output the the 400kHz but haven't managed to output its inverse.

Here is my code:

#include <stdio.h>
#include <string.h>
#include "nrf_drv_pwm.h"
#include "app_util_platform.h"
#include "app_error.h"
#include "boards.h"
#include "bsp.h"
#include "app_timer.h"
#include "nrf_drv_clock.h"
#include "nrf_delay.h" 
#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"


#define m_top     20 
#define m_step    1 

#define  led_external  26 
#define  led_reversed  27

static nrfx_pwm_t m_pwm0 = NRFX_PWM_INSTANCE(0);


uint16_t step = m_top / m_step;

static nrf_pwm_values_common_t sequence_values[m_step * 2]; 	

uint16_t value = 0;



static void pwm_common_init(void)
{

    for(int i = 0; i<m_step; i++)
    {
		
      value += step; 
      sequence_values[i] = value;
      sequence_values[m_step + i] = m_top - value; 
    }

    nrfx_pwm_config_t const config0 = 
    {
      .output_pins = 
      {
          led_external,
          led_reversed| NRFX_PWM_PIN_INVERTED,
          NRFX_PWM_PIN_NOT_USED, 
          NRFX_PWM_PIN_NOT_USED 
      },
      .irq_priority = APP_IRQ_PRIORITY_LOWEST, 
      .base_clock   = NRF_PWM_CLK_16MHz, 
      .count_mode   = NRF_PWM_MODE_UP, 
      .top_value    = m_top, 
      .load_mode    = NRF_PWM_LOAD_COMMON, 
      .step_mode    = NRF_PWM_STEP_AUTO

    };
    APP_ERROR_CHECK(nrfx_pwm_init(&m_pwm0, &config0, NULL));
}
static void pwm_play(void)
{
  nrf_pwm_sequence_t const seq0 = 
  {
    .values  = sequence_values, 
    .length           = NRF_PWM_VALUES_LENGTH(sequence_values), 
    .repeats          = 0, 
    .end_delay        = 0 
  
  };
  (void)nrfx_pwm_simple_playback(&m_pwm0, &seq0, 1, NRFX_PWM_FLAG_LOOP);

  }



static void log_init(void)
{
  ret_code_t err_code = NRF_LOG_INIT(NULL);
  APP_ERROR_CHECK(err_code);

  NRF_LOG_DEFAULT_BACKENDS_INIT();

  }





int main(void)
{
log_init();
bsp_board_init(BSP_INIT_LEDS);
pwm_common_init();
pwm_play();

  NRF_LOG_INFO("PWM application started!!");
   
}

I was looking into the previous answers but I wasn't able to still apply their solution into my program. From what I know so far is that "NRFX_PWM_PIN_INVERTED" only works in idle state and that "seq_values->channel_0 = duty_cycle | 0x8000" is a possible answer but I am not sure how to integrate that in my program. 

Asking for help/advice how to integrate 400kHz output in pin 0.26 and inverse 400kHz frequency in pin 0.27 of nRF52840 board thru modifying my code?

[Charlie]

Hi, 

What you have experienced is true.Please see the solution from a similar case: Invert PWM PIN, PWM, PWM intialize .

[refer_pin]

Charlie said:

Please see the solution from a similar case: Invert PWM PIN, PWM, PWM intialize .

Yeah, that's where I got the "seq_values->channel_0 = duty_cycle | 0x8000" idea but I dont know how to add that to my program

[hmolesworth]

Try this, every sample for the inverted pin has to have most significant bit set:

      sequence_values[m_step + i] = (m_top - value) | 0x8000; // Inverted output

[refer_pin]

Wouldn't that invert both pwm frequency coming out from pin 0.26 and pin 0.27?

[hmolesworth]

Sorry, I wasn't being very clear. To get differential (or inverse or antiphase or full-bridge, all same thing) output don't use NRF_PWM_LOAD_COMMON but instead GROUPED or WAVEFORM or SINGLE as these each use a separate value for each pin. This requires a bigger buffer of course; the inverted value has the "| 0x8000" whereas the non-inverted output does not. Have a look at section 47.2 Decoder with Easy DMA

[refer_pin]

hmolesworth said:

GROUPED or WAVEFORM or SINGLE

Which one of them should I use? and how could I integrate them to my code? what changes will I make?

[hmolesworth]

Try this, not tested:

With "step" as the number of discrete PWM changes, Change
static nrf_pwm_values_common_t sequence_values[m_step * 2]; 	
to
static nrf_pwm_values_common_t sequence_values[step * 2]; 	

static void pwm_common_init(void)
{
Change
    for(int i = 0; i<m_step; i++)
to
    for(int i = 0; i<step*2; i+=2)
    {
Change
      value += step; 
      sequence_values[i] = value;
      sequence_values[i+1] = value;
to
      value += m_step; 
      sequence_values[i]   = value;
      sequence_values[i+1] = value | 0x8000; 
    }

    nrfx_pwm_config_t const config0 = 
    {
      .output_pins = 
      {
Change
          led_external,
          led_reversed| NRFX_PWM_PIN_INVERTED,
          NRFX_PWM_PIN_NOT_USED, 
          NRFX_PWM_PIN_NOT_USED 
To
          led_external,
          NRFX_PWM_PIN_NOT_USED, 
          led_reversed| NRFX_PWM_PIN_INVERTED,
          NRFX_PWM_PIN_NOT_USED 
      },
      .irq_priority = APP_IRQ_PRIORITY_LOWEST, 
      .base_clock   = NRF_PWM_CLK_16MHz, 
      .count_mode   = NRF_PWM_MODE_UP, 
      .top_value    = m_top, 
Change
      .load_mode    = NRF_PWM_LOAD_COMMON, 
To
      .load_mode    = NRF_PWM_LOAD_GROUPED, 
      .step_mode    = NRF_PWM_STEP_AUTO

    };
    APP_ERROR_CHECK(nrfx_pwm_init(&m_pwm0, &config0, NULL));
}

Not totally sure what waveform you want, but these changes allow a PWM output and inverted PWM output ramping from low duty cycle to max duty cycle. The pins order changes because of the mapping for the different modes; GROUPED uses 2 entries per step controlling 2 pins each.

In the code you posted it looks like you used m_step erroneously instead of step

[refer_pin]

Thank you so far it's working..a bit better, I can see the inverted PWM. Is there a possible way to get constant 50% duty cycle? The program is somehow making the pwm "glitchy"

[Charlie]

Hi,

I made one example that shows how to get 50% with GROUPED mode. Suggest you refer to nrf52840 ps section PWM and nRF52 nRF5 SDK example pwm_driver to learn how to use PWM peripheral on nRF52 devices. 

#include <stdio.h>
#include <string.h>
#include "nrf_drv_pwm.h"
#include "app_util_platform.h"
#include "app_error.h"
#include "boards.h"
#include "bsp.h"
#include "app_timer.h"
#include "nrf_drv_clock.h"
#include "nrf_delay.h" 
#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"


#define m_top     20 
#define m_step    1 

#define  led_external  26 
#define  led_reversed  27|NRFX_PWM_PIN_INVERTED

static nrfx_pwm_t m_pwm0 = NRFX_PWM_INSTANCE(0);


uint16_t step = m_top / m_step;

static nrf_pwm_values_grouped_t sequence_values[]=    {
        { 0x8000, 0 },
        { 0, 0x8000 }
    };	


uint16_t value = 0;


static void pwm_common_init(void)
{
    nrfx_pwm_config_t const config0 = 
    {
      .output_pins = 
      {
          led_external,
          NRFX_PWM_PIN_NOT_USED, 
          led_reversed| NRFX_PWM_PIN_INVERTED,
          NRFX_PWM_PIN_NOT_USED 
      },
      .irq_priority = APP_IRQ_PRIORITY_LOWEST, 
      .base_clock   = NRF_PWM_CLK_16MHz, 
      .count_mode   = NRF_PWM_MODE_UP, 
      .top_value    = m_top, 
      .load_mode    = NRF_PWM_LOAD_GROUPED, 
      .step_mode    = NRF_PWM_STEP_AUTO

    };
    APP_ERROR_CHECK(nrfx_pwm_init(&m_pwm0, &config0, NULL));
}
static void pwm_play(void)
{
  nrf_pwm_sequence_t const seq0 = 
  {
    .values.p_grouped  = sequence_values, 
    .length           = NRF_PWM_VALUES_LENGTH(sequence_values), 
    .repeats          = 0, 
    .end_delay        = 0 
  
  };
  (void)nrfx_pwm_simple_playback(&m_pwm0, &seq0, 1, NRFX_PWM_FLAG_LOOP);

  }



static void log_init(void)
{
  ret_code_t err_code = NRF_LOG_INIT(NULL);
  APP_ERROR_CHECK(err_code);

  NRF_LOG_DEFAULT_BACKENDS_INIT();

  }





int main(void)
{
log_init();
bsp_board_init(BSP_INIT_LEDS);
pwm_common_init();
pwm_play();

  NRF_LOG_INFO("PWM application started!!");
   
}