sometimes PWM signal is reversed .

[Kyu]

Hi Nordics!

I am using app_pwm.c file in SDK 8.1, and when I use it with softdevice(advertising or data transfer event), sometimes pwm signal is reversed.

I guess, reversed signal's reason is the BLE interrupt. Because this problem is not occurred, when I don't use softdevice ( advertising or data transfer event)

And I need your advice for overcome this problem.

These are my development environment.

---

SDK 8.1

SD 8.0

nRF51822 AA G0

custom board

Timer1- 2channels

Timer2-1 channel.

3 PWM channels - Red led, Green led, Blue led.

---

first, I used app_pwm library. (100Hz )

/* */

static void pwm_init(void)

{

    ret_code_t err_code;
    
    /* 2-channel PWM, 100Hz, output on LED pins. */
    app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_2CH(10000L, GPIO_LED_RED, GPIO_LED_GREEN);
    app_pwm_config_t pwm2_cfg = APP_PWM_DEFAULT_CONFIG_1CH(10000L, GPIO_LED_BLUE);
    
    /* Set the polarity of the channel. */
    pwm1_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_HIGH;
    pwm1_cfg.pin_polarity[1] = APP_PWM_POLARITY_ACTIVE_HIGH;
    pwm2_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_HIGH;
        
    /* Initialize and enable PWM. */
    err_code = app_pwm_init(&PWM1,&pwm1_cfg,pwm_1_ready_callback);
    APP_ERROR_CHECK(err_code);
    err_code = app_pwm_init(&PWM2,&pwm2_cfg,pwm_2_ready_callback);
    APP_ERROR_CHECK(err_code);
    
    app_pwm_enable(&PWM1);
    app_pwm_enable(&PWM2);
}



void pwm_1_ready_callback(uint32_t pwm_id)   

{
    pwm_1_ready_flag = true;
}

void pwm_2_ready_callback(uint32_t pwm_id)   
{
    pwm_2_ready_flag = true;
}





second, I set the pwm chanel duty by using below code. 

(in this function, I didn't wait callback. I just keep trying until PWM is ready)

static void color_setting(uint8_t red, uint8_t green, uint8_t blue)
{
    
    color_red=red;
    color_green=green;
    color_blue=blue;
    
    pwm_1_ready_flag = false;
    pwm_2_ready_flag = false;
    
    /* Set the duty cycle - keep trying until PWM is ready... */    
    while (app_pwm_channel_duty_set(&PWM1, 0, (uint32_t)(sin_table[color_red]/2.55)) == NRF_ERROR_BUSY);
    while (app_pwm_channel_duty_set(&PWM1, 1, (uint32_t)(sin_table[color_green]/2.55)) == NRF_ERROR_BUSY);
    while (app_pwm_channel_duty_set(&PWM2, 0, (uint32_t)(sin_table[color_blue]/2.55)) == NRF_ERROR_BUSY);

    /* ... or wait for callback. */
   //    while(!pwm_1_ready_flag);


}

sin_table's value is from 0 to 255. so I divided it by 2.55

And this color_setting function is called in app_timer call_back function.

my app timer's interval is 40ms.

In other words, PWM values is changed in every 40ms, and it repeat 0~100 value.

This "PWM signal reversed" issue is not appeared, when I don't use softDevice.

Is there any idea for avoiding this problem??

This problem is critical issue on my project.

Please help me.

Thanks.

[Susheel Nuguru]

This is a known issue on nRF51 chips with PWM. There is actually no easy way to make sure that the signals are not inverted when using softdevice. The engineers are trying their best to try to avoid it in the driver. Hopefully it will be addressed in the future SDK releases.

[gohak]

I would also like to add that i do see this problem. In addition, after a while of transitioning my 2 channel PWMs at different duty cycles, and then I set only the duty cycle for 1 side of the channel while the other is set to zero, it seems like both PWM channel seems to be running even when my software explicitly setting one of the channel to be 0.

Regards, William