Nordic SDK: PWM duty cycle of 0 on LED

[tyler]

I am using PWM on a BLE Nano using Nordic SDK. I was having problems with the PWM and decided I would set the duty cycle to 0 to see if the program was working right:

#include <stdbool.h>  
#include <stdint.h>
#include "nrf.h"
#include "app_error.h"
#include "bsp.h"
#include "nrf_delay.h"
#include "app_pwm.h"

#define TIMER1_INSTANCE_INDEX      (TIMER0_ENABLED)


APP_PWM_INSTANCE(PWM1,1);                   // Create the instance "PWM1" using TIMER1.

static volatile bool ready_flag;            // A flag indicating PWM status.

void pwm_ready_callback(uint32_t pwm_id)    // PWM callback function
 {
ready_flag = true;
}

int main(void)
{
ret_code_t err_code;

app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_1CH(5000L, BSP_LED_0);
int dutyCycle = 0;
app_pwm_channel_duty_set(&PWM1, 0, dutyCycle);

pwm1_cfg.pin_polarity[1] = APP_PWM_POLARITY_ACTIVE_HIGH;

/* Initialize and enable PWM. */
err_code = app_pwm_init(&PWM1,&pwm1_cfg,pwm_ready_callback);
APP_ERROR_CHECK(err_code);
app_pwm_enable(&PWM1);

}```   

After doing this I hooked an LED up to BSP_LED_0 and was surprised to find that it lit up. What could cause an LED to light up when the duty cycle is 0? Am I doing something wrong?

[Daniel Wang]

Try to call app_pwm_channel_duty_set() after app_pwm_enable().

Also, you dont have a while (true){} to stop the main/program from exiting?

[Sigurd]

You should also either use the ready_flag or wait for the PWM to be ready when setting the duty cycle, in order to make sure that the duty cycle is actually set.

[tyler]

@daniel_wang @Sigurd: I added a while(true){}, moved the app_pwm_channel_duty_set() after the app_pwm_enable(), and check the ready_flag before setting the duty cycle. After doing all this I am getting the exact same behavior from my device. You can see my new code here: gist.github.com/.../e2f1042de352ac749744270a0b75bb34

. What am I doing wrong? How can I go about fixing it?

[Sigurd]

You are trying to enable the PWM before you initialize it.

Try this code instead:

#include <stdbool.h>
#include <stdint.h>
#include "nrf.h"
#include "app_error.h"
#include "bsp.h"
#include "nrf_delay.h"
#include "app_pwm.h"

#define TIMER1_INSTANCE_INDEX      (TIMER0_ENABLED)


APP_PWM_INSTANCE(PWM1,1);                   // Create the instance "PWM1" using TIMER1.

static volatile bool ready_flag;            // A flag indicating PWM status.


int main(void)
{
    ret_code_t err_code;
    
    app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_1CH(2000L, BSP_LED_0);

    pwm1_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_HIGH;



    /* Initialize and enable PWM. */
    err_code = app_pwm_init(&PWM1,&pwm1_cfg,NULL);
    
    APP_ERROR_CHECK(err_code);
    app_pwm_enable(&PWM1);

    
    while(true)
    {
        int dutyCycle = 50;
        /* Set the duty cycle - keep trying until PWM is ready... */
        while(app_pwm_channel_duty_set(&PWM1, 0, dutyCycle) == NRF_ERROR_BUSY);
                
        // enter into sleep mode
        __SEV();
        __WFE();
        __WFE();
    }
    
}

This will give you 500 Hz PWM on the BSP_LED_0 pin(P0.21 on the nRF51-DK)

[tyler]

@sigurd: Ok I ran your code and it seemed like everything was fine. Then I changed the dutyCycle to 0 and the behavior stayed the exact same. Shouldn't a duty cycle of 0 set the led to be completely off? Why would the LED not change at all when switching the duty cycle to 0? BTW I am using a BLE Nano device... Idk if that helps at all, but here is the pin out chart: i.stack.imgur.com/TOiKn.jpg and I am using po_28