PWM driving a Buzzer

[vn2000]

I am using SDK V15 with an nRF52DK and Keil compiler.

I have difficulty finding an example to use a GPIO to drive a Piezo Buzzer. The requirement is very simple: use a GPIO to generate a sound frequency of about 18 Khz to the Buzzer.

The PWM examples in the SDK are so convoluted and difficult to understand. I looked at other examples in the forum and many are outdated, some cannot be compiled, others are compiled but crashed, etc..

I would appreciate if someone can provide a simple example to drive a Buzzer at 18 Khz.

[Edvin]

Hello,

I suggest you take a look at the pwm_library example instead of the pwm_driver. It is a bit easier, and more suited to generate a steady PWM signal. The pwm_driver example is intended for automatic manipulation of the PMW duty cycles, if you want to e.g. play a short sound clip, or do a LED blink pattern.

Best regards,

Edvin

[vn2000]

I was using the pwm_library and manage to have the demo2 barely working,

The buzzer barely makes a sound but not at the frequency at 18 Khz as I wish.

I looked at the value of the pre-scaler of PWM_CLK in nrf52_bitfields.h and they are not as flexible. The values keep being divided by 2 as follows:

#define PWM_PRESCALER_PRESCALER_DIV_1 (0UL) /*!< Divide by 1 (16MHz) */
#define PWM_PRESCALER_PRESCALER_DIV_2 (1UL) /*!< Divide by 2 ( 8MHz) */
#define PWM_PRESCALER_PRESCALER_DIV_4 (2UL) /*!< Divide by 4 ( 4MHz) */
#define PWM_PRESCALER_PRESCALER_DIV_8 (3UL) /*!< Divide by 8 ( 2MHz) */
#define PWM_PRESCALER_PRESCALER_DIV_16 (4UL) /*!< Divide by 16 ( 1MHz) */
#define PWM_PRESCALER_PRESCALER_DIV_32 (5UL) /*!< Divide by 32 ( 500kHz) */
#define PWM_PRESCALER_PRESCALER_DIV_64 (6UL) /*!< Divide by 64 ( 250kHz) */
#define PWM_PRESCALER_PRESCALER_DIV_128 (7UL) /*!< Divide by 128 ( 125kHz) */

I tried to add more prescalers:

#define PWM_PRESCALER_PRESCALER_DIV_256 (8UL) /*!< Divide by 256 ( 62.5kHz) barely working*/
#define PWM_PRESCALER_PRESCALER_DIV_512 (9UL) /*!< Divide by 512 ( 31.25kHz) not working*/
#define PWM_PRESCALER_PRESCALER_DIV_1024 (10UL) /*!< Divide by 1024 ( 15.625kHz) not working*/

Of the above, only PWM_PRESCALER_PRESCALER_DIV_256 is barely working.

And 62.5 Khz is not even in the audio frequency.range.

Is there a way to readjust the above definitions in a more flexible way to get the audio frequency at 18 Khz? 

[Edvin]

The app_pwm module, which is used in the pwm_library example doesn't use the PWM peripheral, but the TIMER and GPIOTE to toggle the pins. Hence, the PWM_PRESCALER is not used in this project. 

Have you tried to analyze the PWM output to see that it has the PWM period and PWM duty cycle that you expect?

BR,

Edvin

[vn2000]

Hi Edwin,

Last week I was busy working on the PCB. Now I got back and was able to use examples from the app_pwm module thanks to your suggestion.

However I have one more question: After I drive the buzzer for a few seconds then stop. But even after it stopped I can still see pulses coming out from that GPIO, using an oscilloscope. What do I need to do to let that GPIO completely quiet (either high or low). Do I need to reconfigure that GPIO?

[Edvin]

What does the pulses look like, and how did you stop it? 

Either try to set the duty cycle to 0% or 100%, depending on whether you want it high or low (it is also dependent on what your active state on the PWM channel is set to, high or low). 

Can you show me what it looks like? Alternatively, you can disable the PWM instance using app_pwm_disable(). 

BR,

Edvin