How to Drive RGB LED with PWM

[saiteja]

Hi Nordic Team, i am working on nrg52 to glow a RGB LED with white and Yellow colours. configured the RGB pins to three different Gpio (A2,A3,A4).

for this i am using PWM library example in which i can only configure 2 channel(2 pins) to drive PWM to Gpio pins. if i configure red and green pins using PWM i unable to get yellow color.

i am getting mixed colour of red,green, yellow. at wt fre quency(time period) i should operate PWM to get pure Yellow colour, and also how i can configure 3 pins to drive PWM using PWM library example.

here is my code.

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

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

void pwm_ready(uint32_t pwm_id)    // PWM callback function
{
    ready_flag = true;
}
uint32_t time_period = 2000L;
int main(void)
{

	ret_code_t err_code;
	 printf("VIBRATION:\r\n");
	    /* 2-channel PWM, 200Hz, output on DK LED pins. */
	    app_pwm_config_t pwm1_cfg = APP_PWM_DEFAULT_CONFIG_2CH(time_period, ARDUINO_2_PIN,ARDUINO_A4_PIN);

	    /* Switch the polarity of the second channel. */
	    pwm1_cfg.pin_polarity[1] = APP_PWM_POLARITY_ACTIVE_HIGH;
	    pwm1_cfg.pin_polarity[0] = APP_PWM_POLARITY_ACTIVE_HIGH;
	    /* Initialize and enable PWM. */
	    err_code = app_pwm_init(&pwm_library,&pwm1_cfg,pwm_ready);
	    APP_ERROR_CHECK(err_code);
	    app_pwm_enable(&pwm_library);

	    while(true){

for(int i = 0; i<= 100; i++){

	    app_pwm_channel_duty_set(&pwm_library, 0, i);

	    nrf_delay_ms(100);

}

}

}

[Sigurd]

Hi,

You will need to create a new PWM instance to get a third channel with the PWM library. However, since you are using the nRF52832 I would recommend using the PWM driver instead(nrf_drv_pwm). This uses the PWM peripheral on the chip. Using the PWM library, app_pwm, on nRF52 you are wasting resources like timers and ppi and gpiote channels. Take a look at e.g. this devzone answer where a user configured the PWM driver for a RGB LED.

To get a yellow light you need to drive the red and green pin. You should be able to find the appropriate frequency in the datasheet for the RGB LED you are using.

[saiteja]

hi i implemented PWM driver as you said but i am unable to control the duty cycle using PWM driver. even if i change sequence values in the function

void nrf_drv_pwm_simple_playback(nrf_drv_pwm_t const * const p_instance, nrf_pwm_sequence_t const * p_sequence, uint16_t playback_count, uint32_t flags);

there is nochange. if i set set all sequence values to 0. still tricolor is displaying. how to control the duty cycle of each channel using PWM driver and how to set different values to each channel.

[Sigurd]

If you are using the code in the post I linked to, you can try this:

int main(void)
{
    
    pwm_init();
    
    update_pwm(m_top/2,m_top/2,0);

    while(1)
    {
        nrf_delay_ms(200);
    }

}

This will set the RED(12) and GREEN(14) pins to 50% duty cycle, while the BLUE(15) pin will be 0 V.

The PWM frequency is F(pwm) = base_clock/top_value, so with base clock NRF_PWM_CLK_4MHz and m_top 10000, the frequency will be ~400Hz.

[saiteja]

once i have glow led i am unable to turn it off. i have used these functions

nrf_drv_pwm_stop(&m_pwm0,false);

nrf_drv_pwm_uninit(&m_pwm0);

[Sigurd]

Is the last problem solved, where you able to set a yellow light on the RGB LED?

Could you add in a comment how your "int main(void){...}" function looks like now?

[saiteja]

void stop_LED_glow(){

nrf_drv_pwm_stop(&m_pwm0,false);
nrf_drv_pwm_uninit(&m_pwm0);

}

//here R,G,B are dutycycle

void start_update_pwm(){

R = get_R();

G = get_G();

B = get_B();

for (int i=0; i<100; i++){

	reset_RGB_flag();

	update_pwm(i*R,i*G,i*B);

}

}

int main(void) {

pwm_init();
while(1){
	start_update_pwm();
	nrf_delay_ms(1000);
	stop_LED_glow();
}

}